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Green Building Blog

What’s Wrong With These Roof Details?

The latest entry in our ongoing series, ‘What’s Wrong With This Picture?’

Image 1 of 2
What's wrong with this picture? Click on the photo to enlarge it.
Image Credit: Building America
What's wrong with this picture? Click on the photo to enlarge it.
Image Credit: Building America
Building America logo
Image Credit: Building America

Readers are invited to identify as many errors they can spot in the attached photo of a multifamily building in Minneapolis, Minnesota.

This is the latest photo in our ongoing series, “What’s Wrong With This Picture?” (To see three previous photos in the series, click the links in the box below.)

The photo comes from Garrett Mosiman and Pat Huelman at the University of Minnesota. (Their work is funded by the Building America program.)

There are (at least) five problems related to energy efficiency and building durability that can be spotted in this photo. This is a north-facing façade of a townhouse development in Minneapolis (Climate Zone 6). The building has asphalt shingle roofing and vinyl siding (with a few fiber-cement shingle-style panels added as architectural accents).

This photo was taken in mid-March 2013; outdoor temperatures had been well below freezing for many weeks.

Mosiman and Huelman will answer the question next week. In the meantime, post your comments below. (Click here to read the answers to this puzzler.)

One interesting note: when we’ve published “What’s Wrong With This Picture?” articles in the past, GBA readers have always been able to identify more errors than were noticed by the photographers who submitted the photo.


  1. Boro | | #1

    Top 5 Problems
    1. The Architect is still alive
    2. The builder has any street credibility
    3. A bank would actually lend money for this design
    4. The owners of these units probably have no idea how much rot & repair they're in for
    5. The Architect is still alive!

  2. Expert Member
    Dana Dorsett | | #2

    Save the architects...
    ...just encourage them to go into a different line of work! :-)

    The gables over the set-back porches guarantee that snow accumulates up against the siding, they've combined gable vents with ridge-vents, short-circuiting the soffit venting paths, the double-gable dormer valley is incapable of soffit-to-ridge venting on the valley side pitches, where the dormer ridge meets the upper roof the ridge vent is guaranteed to be buried for months in sloughing snow from the higher roof, probably leaks/drips.

    Who ever invented the double-gable valley look deserves a special place in hell, along with those who repeat the form. It's guaranteed to accumulate everything from leaves to lichens to snow and sheds water poorly- a constant maintenance issue. The practicality & functionality of shed-dormers should trump whatever classical-proportioned gables aesthetics the architect may be going for in these situations.

  3. GBA Editor
    Martin Holladay | | #3

    Response to Robert Swinburne
    You know that because... architects never make mistakes, right?

  4. Robert Swinburne | | #4

    no architect because
    Mostly because its really, really ugly.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Robert Swinburne
    I suspect your syllogism may contain a fundamental error, and that your conclusion could (perhaps) be disproved with a single counter-example. But I digress from the matter at hand.

  6. Robert Swinburne | | #6

    there was no architect
    probably designed by someone who took drafting in high school (sarcasm + generalization)

  7. Robert Swinburne | | #7

    @ Martin
    If I were to review my own portfolio, I could probably provide the counter-example to disprove my own conclusion.

  8. GBA Editor
    Martin Holladay | | #8

    Gotta love the siding design
    Someone went to a lot of trouble to specify the five types of siding: namely, fake cedar shingles and four colors of vinyl siding. I'm not sure whether a high-school student could come up with such a sophisticated exterior palette.

    And note the subtle aesthetic effect made by specifying mismatched gable vents -- with one rectangular vent and two round vents.

  9. mfredericks | | #9

    common problem?
    I can appreciate the attempt at articulating the facade with some variety in materials, especially in higher density townhomes like these which can sometimes look just as bad if they're all identical. However the roofing issues are inexcusable. I'm amazed at how common this double gable mistake appears around my area. There are 2 brand new buildings under construction in our neighborhood, which both have a single point of drainage where two gables meet up.

    Another issue, not mentioned yet could be a poor air-sealing job in the attic considering how much ice damming has occurred on almost every roof edge.

  10. GBA Editor
    Martin Holladay | | #10

    Summarizing so far
    1. The gables over the porch roofs trap snow against siding. (Dana Dorsett)

    2. Gable vents plus ridge vents provide a short circuit that undermines the effectiveness of the soffit vents. (Dana Dorsett)

    3. The double-gable dormer valley is incapable of soffit-to-ridge venting on the valley side pitches. (Dana Dorsett)

    4. Intersection of gable ridge vents and main roof slope is likely to be buried in snow and is a potential leak site. (Dana Dorsett)

    5. The double-gable valley is guaranteed to accumulate everything from leaves to lichens to snow and sheds water poorly - a constant maintenance issue. (Dana Dorsett)

    6. Ice dams are a sign of air-sealing errors in attic. (Mark Fredericks)

  11. user-1050854 | | #11

    5 Problems in Pictures
    1. Dana had two. The lower gable forcing show against the siding and the valley between the upper gables with no where for the water to go.

    2. The ice indicates non-effective air sealing at the ceiling plane.
    3. It also indicates ineffective insulation in the attic areas.
    4. I think the vinyl siding is a problem in itself.

    The venting as listed by Dana is not a problem. Soffit vents are not required by good building practice. We don't know from the image if the gable vents are real or just decorative. Thus any issues with these must have further investigation. Second the idea of short-circuiting ventilation is far from accepted. Lower and upper vents work, they do not have to be extreme lower and upper. The purpose is to remove moisture. If you are not dumping moisture in with exfiltration and ventilation fan ducts, you don't have much to move.

    Locally we have a product that can be inserted into the shingle layers at a point appropriate for the construction. It allows the upper course of shingles to be held up to allow ventilation air movement. It is ideal for houses built in the 30's - 50's with no soffits. It is ideal for providing ventilation midway up a roof line for a conditioned attic behind a knee wall, yet the cathedral ceiling or clipped ceiling is in need of air movement against the roof deck.

  12. Expert Member
    Dana Dorsett | | #12

    Soffit vents bigger than higher vents ARE good practice.
    The lower you put the venting in the attic space, the lower the stack effect forces on the whole house, provided you the cross sectional area of the soffit venting is substantially bigger than vents higher up. (JoeL probably has test data on this.)

    Inserting a gable vent to a soffit-ridge venting approach lowers the stack effect through the attic portion, reducing net stack effect benefit of the soffit-ridge venting, but offset somewhat by wind-pressure flows. There's no real benefit, only down side to adding a gable vent, but where (as in the case of the double gable) there is a dramatically reduced soffit area, there may be some benefit, especially if it isn't dead-ended at the dormer, and wind can drive air to through attic spaces out other vents on the other side. Suffice to say, it doesn't look well thought out, but maybe...

    The shingle-layered roof vents are cool & all, but I wonder how well they really work under a persistent seasonal foot of snow?

    The notion that the gable vents might be only for decoration (butt-ugly as they are, and not even the same on every gable, round on the red sided parts, rectangular elsewhere), and thus may be moot, is a bit like saying the whole picture might just be the architects computer rendering, and not the real building. Unless there's evidence to the contrary, the presumption in my mind that they're there for more than mere visual effect.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Design comments
    I find it amusing that comments on GBA about a project's poor appearance seem to be limited to those that perform poorly by posters standards, while they remain mute on the almost uniformly awful architectural design of most houses featured on the site - as witnessed by the Canadian "Greenest House" blogged which has so far elicited no reaction to its looks.

  14. GBA Editor
    Martin Holladay | | #14

    Response to Malcolm Taylor
    I guess you are talking about this house. I'll give you my reaction to its appearance:

    1. The Canadian house is much more attractive than the building on this page.

    2. The roof slopes are more functional and elegant by far than the house on this page, but I give it a few demerits for the north roof that transitions from a steep slope to a shallow slope. That's one of my pet peeves.

    3. I'm not partial to two-siding houses -- I much prefer simple designs with a single siding type -- so the Canadian house gets a few demerits for siding inconsistency. But it sure beats the five-siding house on this page.

    4. I'm guessing that you don't like visible PV modules or solar thermal panels -- but that type of equipment doesn't bother me at all. Taste differs, after all.

    5. I like the fact that the foundation extends about 18" to 24" above grade; that's attractive. It gives the house some elegance, to my eye, compared to a squat house that wallows in the mud.

    6. Both the Canadian house and the Minnesota building on this page get demerits for the picture-frame window trim. Whatever happened to using 5/4 stock for the head, so that the head trim is proud of the jamb trim, and for choosing a narrower sill detail?

    7. The entryway features are more inviting on the Canadian house than the Minnesota building.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    I only use the Canadian house as one example.There are never any comments on either the poor floor plans or exterior elevations of the projects featured here, so it seems to me ironic that they suddenly become a topic of condemnation when discussing projects that fail in other more practical ways.

  16. GBA Editor
    Martin Holladay | | #16

    Response to Malcolm Taylor
    I agree with you completely that aesthetics and design matter. Green building will never be widely accepted if green builders promote ugly buildings. I therefore invite you to post comments of any kind addressing the design of houses that appear on this site.

    I do remember one house we featured that generated comments on its appearance -- and it wasn't a house with performance problems. You can read the comments here (beginning at comment #5):
    A Net-Zero-Energy House for $125 a Square Foot.

  17. GBA Editor
    Martin Holladay | | #17

    Another response to Malcolm
    Here is another house we featured that generated negative comments on its appearance: 1970s Home Goes Net Zero.

  18. nvman | | #18

    Never mind the winter, summer is a problem too
    Is that a single downspout at the intersection of the two gable dormers?
    There is so much ice that i can't make out the detail but I am guessing that there can't be much eavestrough at that intersection.
    How is that going to handle the rainwater?
    It is going to be absolutely overloaded in almost any rainstorm.

  19. talldave | | #19

    Ugly Houses
    I have to chime in on the ugly house issue. Some of it comes from trying to do too much on a small building, and some comes from not attempting to make the building look nice at all. If you want to know why energy efficient houses never took off in the 70's, you can look at the cover of any Popular Science that has a dorky house on the cover. As a general rule, people don't want to live in an ugly house, and they want their homes to look like a home. It's very important to make a house look like a place you'd like to call home, and that it will be around for a long time.

    In the example above, it seems you have a designer that is just trying to glue "pretty" onto a house and figuring that "more gables" means "more pretty." I looked at the house that Martin referenced in post #14, and while there are nice siding colors, I can't really use the words elegant or attractive to describe it. It has lots of different materials trying to disguise the fact that it's basically a two-story shoebox. It doesn't fool anyone with an eye for buildings. However, a large enough portion of the home-buying public responds to these foolish tricks, so these practices get perpetuated.

    It's hard for designers and builders to trust in the simple beauty of a well built house. They thought gables were the answer above. In the Canadian house, I don't understand why the two roofs have two different roof pitches, since it doesn't add anything to the house. They both have solar panels on them, and one would think they would want them both to be set at the same angle. The windows also cut into the trim, which always looks like a mistake was made and modified during construction. (And I know that the picture was probably taken early in the morning at the height of summer, but I get a kick out of seeing a picture of solar panels mounted in the shade of the building they are serving!)

    Martin says that he's not bothered by the panels on the roof, but every time I see them I think of Al Franken on SNL back in the 70's with his satellite dish hat. I rarely see panels installed that look like any care was taken regarding their appearance. There's notches taken out of the upper panels, and two different types of panels on the lower roof, above the main entry. It's visually sloppy.

    All that being said, it's easy to criticize, but very difficult to really design a small, simple, elegant, energy-efficient house. So many things have to be considered and come together nicely. Architects can't just shake them out of their sleeves. It takes time, time costs money, and cost is a huge consideration in buildings. It's not surprising designers rely on cheap tricks to dress up buildings.

  20. GBA Editor
    Martin Holladay | | #20

    Response to David Wytmar
    I agree with most of your criticisms of the Canadian house, several of which I also highlighted. Like you, I gave it demerits for its inelegant mixture of two siding types. Like you, I gave the house demerits for its jumbled roof pitches. Like you, I gave it demerits for its inelegant window trim.

    As for the gables on the Minnesota house -- you're preaching to the choir. Not a single GBA reader has risen to the defense of those hideous gables.

  21. user-659915 | | #21

    Design is not just what it
    Design is not just what it looks like and feels like. Design is how it works.

    ~ Steve Jobs

  22. Scotchy | | #22

    glazing and eaves...
    How about:

    1. There's a lot of glazing area for a north facing wall exposing it to heat loss.
    2. The eaves are tiny, exposing the walls to weather and evaporative cooling.

  23. GBA Editor
    Martin Holladay | | #23

    Adding to my previous summary
    I don't think I'm going to count John Nicholas's suggestion that the choice of vinyl siding is an error. So, continuing my list that was started in Comment #10:

    7. Ice dams are evidence of insufficient attic insulation. (John Nicholas)

    8. The downspout at the bottom of the valley between the double gables is too small to handle a significant rain storm. (Aaron Gatzke)

    9. The area of north-facing glazing is too large. (This point is perhaps arguable.) (Scott Simpson)

    10. The roof overhangs are insufficient for proper moisture management. (Scott Simpson)

  24. homedesign | | #24

    Cheap, Dark Colored Shingles
    This was also "One" of the reasons that Joe Lstiburek had to Re-Build his roof after barely 15 years.

  25. GBA Editor
    Martin Holladay | | #25

    Response to John Brooks
    I agree that many brands of asphalt shingles last about 15 years.

    While specifying asphalt shingles (or specifying vinyl siding) may strike some GBA readers as short-sighted (or a poor decision), it really isn't an error -- at least not the type of error that we're trying to pinpoint in this online game.

  26. homedesign | | #26

    Exhaust vent thru the roof
    Probably not a good idea for a building with other Ice Dam Issues

  27. homedesign | | #27

    Shingle Color Choice can contribute to failure
    Especially if the "Fashionable" colors are only available in cheap not-so-thick shingles
    I believe a heavier not-so-dark shingle would fair better.

  28. GBA Editor
    Martin Holladay | | #28

    Response to John Brooks
    "Exhaust vent through roof": OK -- that one counts. I'll add it to the list of errors when I make my next summary.

  29. GeorgeH3 | | #29

    My guesses
    I'm not an architect, a professional contractor or work in the construction industry - I'm just a DIYer but I saw these issues:
    1. Two gables drain into each other (creates a valley) which can cause water drainage issues
    2. The roof has ridge vents and gable-end vents which can cause venting issues
    3. It looks like the insulation in the attic space isn't optimum because parts of the roof are clear of snow whereas others aren't...I guess it could be due to solar evaporation but you can see an area below one of the roof-mounted vents that is clear which would indicate hot air coming out of the vent.
    4. The design allows massive ice dams to form - can cause roof structure rotting
    5. The guy walking down the steps looks shady!


  30. Aaron Birkland | | #30

    A great book to read is Traditional Construction Patterns by Stephen A. Mouzon. From the Introduction:

    Today, buildings that are strictly an "expression of our age" often express and reinforce the most negative aspects of modern society, such as disjointedness, separation, confusion, and despair

    It is instead a collection of misshapen pieces of traditional architecture stitched together with no clear vision of what the resulting creature should be. Most of us probably can't explain exactly what is what is wrong with this sort of building because there are just enough traditional pieces thrown in to make it look vaguely traditional at first glance, with a head, arms, leg, and a body. But in both cases, we immediately know that something isn't right.

  31. user-1075855 | | #31

    For the double gable valley
    For the double gable valley and the gable over the porch, if there is a slope in those valleys, why would snow and other accumulation even be a problem? Sorry if this is a stupid question; I don't know much on roofs.

  32. GBA Editor
    Martin Holladay | | #32

    Response to J.S.
    When snow builds up in a wide, shallow valley, it gradually turns to ice. (The same process occurs in mountain valleys; glaciers are the result.) As more snow continues to fall, followed in some cases by rain, water can back up under the shingles. This is one type of ice dam.

  33. homedesign | | #33

    An Excellent Video about Ice Dams
    Here's a video that was never promoted at GBA.(as far as I know)
    I think it's Excellent and On-Topic.

  34. Joe_Bob | | #34

    Don't think the roof shape is the main problem
    My take is that this is a straightforward case of too much interior heat loss through the attic and not enough ventilation to keep the roof plane at outdoor temperatures...and not having so much to do with the shape of the roof. I think this building would have ice dams on it no matter what shape the roof were.

    Look at the portion of the main roof over the porch on the right. There is the circle around the roof vent where the snow has melted away. Then there is a thick ice dam on the eave right below it, which isn't even an area where drainage is concentrated, as with the valley between the double gable. You can also see the shadow of the roof rafters where the wood has conducted heat through the roof.

    Ergo, we can plainly see that insulation is inadequate and/or air-sealing is poorly done. Add under-ventilation to that and I think there you have 95% of the problem. If the roof deck were staying cold enough there wouldn't be that much melting and re-freezing. Especially so since this is the north side of the building.

  35. David_Gregory_CZ3_CA | | #35

    Roofs - Minnesota and Canada
    I would guess that all the problems already mentioned are exacerbated by the orientation: The west side of the gable roofs are going to have particularly bad freeze-thaw cycles, as afternoon sun and heat will melt snow, sending it running for the shadows. If it were south-facing, might be a little better? (but still no excuse for creating a 'glacier valley'). Open to being told I'm wrong.

    Roof exhaust vent likely to not vent well after a heavy snow; besides that I've seen some that have become detached...or are never attached properly to begin with! (bathroom fans venting directly into ceiling plenums, walls, attics...>.<)

    The gables over the entries are an attempt to keep snow from falling on people exiting, and the stairs. What would readers recommend? The best you can do is make them narrower, to leave more room for snow and water to exit at the corners; or make them simple shed roofs, bring them out further, and shift the stair landing to the side (a bit awkward, but should work).

    The Canada house has a big no-no, if I recall a certain GBA writer(and former roofer)'s advice: The pitch changes from steeper to shallower, and on the north side; this inflection point will likely gather water/snow/ice, right?

  36. dickrussell | | #36

    Keeping snow from falling
    On David's comment: "The gables over the entries are an attempt to keep snow from falling on people exiting, and the stairs. What would readers recommend?"

    It has been my observation that a properly designed and built house, with a lot of insulation on the attic floor and near zero air leakage into the attic from below (non-cathedral ceiling) or with a tight and very well insulated roof structure (cathedral), is far, far less likely to shed snow over the side. This of course does not apply to metal roofs. On my own roof, the snow just sits there, piled up right to the edge without budging, and there are no icicles or ice dams.

  37. user-1055972 | | #37

    double trouble gable
    I guess we could say there is some roof deck warming going on. However, the monster ice on the twin gable valley and the valley to the corner of the left hand house will always be problematic icewise under certain conditions because a little melt (under heavy snow or solar insolation and ambient temps just under freezing) on all that roof real estate converges on a point.

  38. user-659915 | | #38

    These front elevations and their roofs were clearly designed according to the KICS system purposely to appeal to low-information buyers. 'Keep it complicated, stupid.' It's been asked, how would you do it different? To which the only response would be the old local's reply to a tourist's request for directions - 'Oh if I were to be going there I wouldn't be starting from here.'

    Some comments on the Canadian house:
    • I suspect the lower front roof may have been kicked up to provide enough slope length to accommodate the panels without exceeding the preferred overhang to the windows below.
    • In doing so the designers broke the sensible esthetic rule from 'A Pattern Language' that lower roofs should flatten, pagoda style, rather than increase in pitch, in order to lead the eye pleasantly out to the horizon. To reverse this principle creates an unpleasant broke-back effect (and can lead to functionally compromised relationships with upper story windows, as here).
    • I suspect there may have been some misinterpretation in the comments above of the rear roof in this picture. What we see is clearly a horizontal eave, not a low-slope shed roof extension (which I know Martin despises). The rear roof is probably a north-facing gable. Or possibly a hip.
    • Assuming the image has not been reversed the photograph was clearly taken in the late afternoon, not in the morning.
    • The lilacs are lovely.

  39. kevin_in_denver | | #39

    Cold Downspouts
    David Gregory touched on this:

    In snowy, cold places I have a rule never to feed an east or west facing roof plane into a north facing downspout. The downspout will eventually split and be torn off the gutter. Even on a perfectly designed and built cold roof. Knowing this rule, the building designer has to start completely over.

    It's a difficult rule to follow all the time. Oversize gutters and downs made of heavier gauge metal last longer but will still fail.

    Note if the entire cold roof faces north, then north facing downs are OK, and the gutter is OK too.

  40. user-573411 | | #40

    ...the real reason for the particular issue in this photo was the shear amount of very heavy, very wet snow, combined with several rain events, and then subsequent snow that fell during the timeframe this picture was taken. I realize most posters will feel this is a 'poster child' for inadequate insulation or ventilation, but the reality is the Minneapolis/St. Paul metro area suffered through a 'perfect storm' of winter's worst during early 2013. My opinion is that no amount of insulation or ventilation would have eliminated the majority of the ice dams and icicles we experienced. The ONLY solution to eliminate them this year was to REMOVE THE SNOW. The clearest part of the image that shows this is how the snow has sagged over the framing members and melted away from the exhaust of the bathfan. You can even see the individual shingles through the snow. Temperatures are clearly at, or above, freezing, at least during the day. The snow is a very good absorber of the meltwater coming from above, and the insulation and ventilation in the building's attic is actually promoting the creation of the ice dams as cold air (from the cooler north side of the home and nightime, below freezing air) is directed up the underside of the roof deck, with the snow also insulating the ice dams from any potential melting from the limited sun that may strike them during the day.
    We built and currently maintain a townhome project that suffered from similar 'complexity' in its elevations (dictated by an exuberant City Council hoping to 'add character' to what, by it's nature, would be a development with repetitive features). The ice damming and icicles we experienced were equal to those in the picture. The elevations are problematic, but the real issue is a lack of maintenance; in this case the expectation that the Roof can do all the work by itself. Two days after raking the snow in the drifted valleys and eaves down to the hard pack, the dams on the roofs of our project were completely gone. The remaining snow above that point melted off harmlessly.
    Icicles are obviously a symptom of other problems, and cannot be ignored - Insulate, Ventilate, AND remove the snow when it builds up, especially along the eaves and valleys and where icicles and ice dams have occurred in the past.
    BTW - We also expect ice to build up at some point and, as a matter of practice, triple the amount of ice and water protection code requires. Water is the universal solvent in this world, and any builder or designer who chooses to ignore this fact, and the damage it can cause, has no business working in the housing business.

  41. GBA Editor
    Martin Holladay | | #41

    Response to Steven Behnke
    I have heard many people give the advice you provide -- namely, that building owners in cold climates should routinely remove snow from their roofs, even if the building is only a few years old. I think it's nuts.

    A new building (even one in northern Vermont or Minneapolis) can be designed to function well without requiring the owners to shovel the roof. Unfortunately, so few buildings are built with attention to airtightness, adequate insulation, and effective ventilation (where appropriate) that most people have never experienced how a well-designed and well-built building performs. As a result, they conclude that there is no solution to these problems besides shoveling their roofs.

    For more information on ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

  42. GBA Editor
    Martin Holladay | | #42

    Adding to my previous summary
    Continuing my list that was started in Comments #10 and #23:

    11. Bath exhaust fan termination through roof contributes to ice dams. (John Brooks)

    12. The double-gable design leading to a narrow valley concentrates water gathered from a large area to a single outlet, guaranteeing ice buildup and drainage problems. (Robert Grindrod)

    13. A north-facing downspout should never be used to drain an east-facing or west-facing roof plane. (Kevin Disckson)

  43. user-573411 | | #43

    Roofs in cold climates.

    I feel you took the image out of context. this hasn't been a 'Normal' winter here.
    Unfortunately (again) the idea that Ventilation and Insulation will eliminate Ice damming ignores basic physics... (I'm mostly speaking about the climate we have in Southern/Central Minnesota, though I'm sure it happens in other areas. We get many days, and nights, through the winter where we have temps above and below freezing - take a look at our pothole problems in our roads)

    When you isolate the roof from home heat loss, the roof will be at the mercy of air temp. When you introduce air that is above freezing to the bottom side of the roof deck, you will melt the snow (at the roof deck); and this happens frequently. When that water moves down the roof to the (colder) eave, that water will freeze. Instant ice dams. And in my opinion, this is caused by ventilation and insulation.
    There's an image on this website that even demonstrates that even the bloom of heat(loss) off the building walls can be directed into the soffit venting, allowing the roof deck to warm, but leaving the eaves cold. The temperature balance can be that tight at many times during early and late winter here.
    It's very frustrating that the idea that Roofs can do all the work on their own is the industry norm; It's equally frustrating that the 'one-size-fits-all' theory of Insulate & Ventilate is presented as a cure all to ice dams and snow drifts. They are simply part of a system, that needs to include a human component. When the snow has a moisture content double or triple what is normal (about 0.1" water to 1" of snow), man needs to step in. When the snow on a North-facing slope is building up beyond what the sun can deal with man needs to step in.

    Structures are machines, that need to be maintained.

    The image you presented is not a representative picture of this happening; for all we know the ice dams were removed the next day.

    To me this does not represent a 'flaw' in construction as much as a build-up of natural and man-made issues. And yes, the multiple gables, and other 'snow catchers' just exacerbate the problem.

    Speaking directly to the picture and trying to use it as an example, one point that needs to be addressed as to the ventilation of the roof in the area between the double gables (and yes, I know I'm softening the gist some of my points above) is that as a multiple-unit structure, ventilation in the soffits (or anywhere on the roof for that matter) is not allowed within 4-feet of the party wall. This seriously hampers roof ventilation, and the ability of the roof to perform well on it's own, regardless of the amount of insulation at the wall (remember these are gable ends, and presumably have full depth insulation right up against the sheathing. And looking at the relationship between the windows and the fascia, there appears to be a full (12") energy heel there as well). Yet the largest icicle is right at that point. The mechanics of this are likely Gravity, Temperature and Moisture content, combined with the (Code-Required) lack of ventilation.

    I need to mention that our buildings have HERS indexes in the low 50's and that we have airchange numbers between 1.3 and 1.5. While not industry leading ,these are Really good numbers.
    There are many years when the project I have spoken off did not need snow removal to maintain the roofs - Since 2006 we have cleared dams twice. This suggests that the damming is not solely a direct result of ventilation/insulation issues, but is more likely to be due to specific climactic issues.
    One last point is that our project also happens to be adjacent to a snow-making area, and as such is frequently subjected snow in excess of the adjacent community. Even so, as I mentioned we have only cleared the dams twice in 6 years of winters.

    I'm not arguing against ventilation or insulation, only against the idea that they will cure all that ails a building (when we're looking at ice dams).

  44. GBA Editor
    Martin Holladay | | #44

    Response to Steven Behnke
    You are right that in some weather conditions, there will be ice formation at the eaves, even when a building has been constructed correctly. However, such conditions are rare, and should not result in the need to shovel a roof or remove ice.

    You repeatedly express skepticism about the standard pair of recommendations to prevent ice dams, which you call "insulate and ventilate." I am also a skeptic about that pair of recommendations, as I explained in the article I linked to. I am especially skeptical about the value of ventilation (although ventilation is sometimes useful to minimize ice dams).

    The measure you never mentioned -- and the one that roofers and insulators often forget -- is air sealing. Improper air sealing is responsible for 90% of ice dam problems. Proper air sealing of the thermal envelope (usually, the attic floor) should always be the first step to correcting an ice dam problem.

    However, in the case of the house in the photo, we also have many contributing design issues.

  45. user-573411 | | #45

    A couple follow-ups
    I still don't buy the idea that lack of airsealing would cause the ice dams in that picture. In our case, and ACH of 1.3 to 1.5 indicates a really good job of air-sealing. yet we had ice damming on North- facing roofs, South-facing roof and, significantly, unheated, ventilated roofs.

    I am suggesting that this year was an anomaly in this area (though one that seems to be happening with greater frequency.) The period mentioned, March 2013, included the beginnings of significant warm spells, as well as several excessively wet and large March snows.

    All of the construction elements pointed out have value, and all must be done to best practice standards for the building to perform. But the importance of maintenance should not be understated.

  46. homedesign | | #46

    90 percent
    Martin wrote: "Improper air sealing is responsible for 90% of ice dam problems"

    I would not be surprised if this is true....

    Is this your opinion or does "90%" come from a study?

  47. GBA Editor
    Martin Holladay | | #47

    Response to John Brooks
    You caught me. That was a guess.

  48. homedesign | | #48

    Curious about Ice Dams
    I'm not from a Cold Climate ...
    just wondering about barns and unheated buildings
    Is it possible or common to see Ice Dams on an unheated building in a High Snow load climate?

  49. GBA Editor
    Martin Holladay | | #49

    Response to John Brooks
    It's rare, but it happens. The sun can cause melting, and water can freeze at the eaves -- especially if the top part of the roof is in the sun and the eaves are in the shade. It can also happen in valleys, when the sun strikes the top of the roof.

    Clearly, you can't totally eliminate ice. But if you get the details right, the small amount of ice that sometimes forms on a roof won't cause problems.

  50. kevin_in_denver | | #50

    Response to Steven Behnke
    You said,

    "When you isolate the roof from home heat loss, the roof will be at the mercy of air temp. When you introduce air that is above freezing to the bottom side of the roof deck, you will melt the snow (at the roof deck); and this happens frequently. When that water moves down the roof to the (colder) eave, that water will freeze. Instant ice dams."

    This is not true. If the ambient air is above freezing, the eaves, gutters and downs also will be. Sun melting or warm attics due to house air leakage are the most common problems.

    I also agree with Martin that roof snow removal is nuts. It's also suicidal:

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