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How to build a gabled roof on a rigid insulation roof?

Leon Stepanian | Posted in Green Building Techniques on

Hello to all at this forum. Back in 2013 I posted on my garage build here….

https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/28085/best-way-build-cathedral-roof-garage#comment-58463

Since a few months now I had started doing some work on my home to increase the insulation value.

I am including a photo of how it is today just before we add the roof sheathing, but I am sort of stuck on how to include a gable roof on each end of the house.

I have been taking time lapse videos and regular videos during the complete process. This is much harder then building a new garage where everything is really under your control. Retrofitting a home is chock full of those endless and time consuming details.

I have included a photo of my original house and one photo of where I am now. What prompted the change is many fold. The shingles were going bad (due for a BP refund), the windows, front door and patio door needed to be replaced, the house siding was pretty worn down, but the greatest problem stems from the actual design of the home itself. As you can see the front gable roof is directly over an area that protrudes out from the main wall and this type of design does not permit having a working soffit along that area. This causes a major dead air space in the attic that gets so hot during the summer that the rooms in that area become very uncomfortable or air conditioning is required continuously at maximum.

There is also that my new garage now has a new brick and future siding look that does not match the house. .So these are good reasons to take the plunge so to say.

My garage is perfect. During the dead of winter at -30 Deg.F, I shut off all power to the garage for one whole day and the internal temperature dropped 2 degrees. I did not try this with my house but I am sure it would have been -30 after one whole day without electricity.

The other main reason for these changes is when I starting looking into HVAC alternatives for the home as is, that is with very limited insulation in the attic given a 4.25/12 pitch of the roof left no head room for increasing insulation depth at the eaves. Most HVAC guys wanted to install a central unit in the attic as it was. When I asked them about how the equipment will fair trying to air condition the home during the summer while it is in a super hot attic, or, how it will heat my home during the winter while the attic is dead cold. They all said the unit is insulated but I found out it is no better then R10. So these guys wanted me to dish out a good 24 grand while providing meager efficiency. So it was time to make a move once the studying was done. A year later and this is where I am today. hahaha 30 pounds lighter to boot.

The 2nd photo is a collection of shots of where I am at right now. (please excuse the photo quality)
The third is a diagram I made to better explain what I did.

With the roof now covered by 8 inches of rigid insulation and held down by real 2 by 4s and 12″ screws (we found a way to increase our odds at landing on the trusses below), I am curious if there is a good way to include a 24 inch long gable roof at each end. Since the roof is now configured to provide 42 air channels of 2″ by 20″ each, I cannot build the gable lookouts in the regular manner since this will block the air movement in 8 of these air channels and for me, that seems unacceptable.

I guess I am just over worried about the gable ends not being strong enough to meet its task. The only way I can see to do this is to add 2 x 12s along the gable wall held with some bolts into the first roof trusses (gable truss) so it will reach right flush with the top of the 8″ insulation on the roof. This would provide an anchoring for the lookouts but maybe going out 24″ would be to long.

I have been at this for the last 3 months what with rain delays and all. During the initial “demolition” I had a 20 cubic meter container full to the top, weighed out at 5.7 tons when it left. Because of the high level of labor intensive work involved, I did most of the work myself especially those finer details, and had hired my son and a few friends on some days for more specific labor intensive tasks. Such a project is definitely not for the faint of heart and at 58 years young, this project has really pushed me to my limits in terms of physical demands. But I had no choice since the costs to delegate the whole project to a general contractor would have been prohibitive and anyways, there are no contractors that would have done what I did. hahahaha

Just as in my garage build where I set a goal to prepare all the foundations (footing, wall, slab and driveway) so that it receives only one concrete pour (done successfully), my challenge for the house was to make sure all the wood framing of the house itself is covered by insulation so as to provide a full thermal break. This has been successful and the house is now what many would call very close to the Persistent method.

Leon Stepanian
Terrebonne, Quebec, Canada
Zone 6

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Leon,
    Too much information.

    Let me cut to the chase: Are you interested in advice on building a roof overhang at the rakes? I think that's what you mean by "gable roof."

    (The whole thing is already a gable roof -- but I'm guessing that you are having trouble with the rake overhangs.)

  2. GBA Editor
    Martin Holladay | | #2

    Leon,
    To clarify the terminology: Assuming that we are talking about a roof with no hips or valleys, a typical shed roof (or one side of a gable roof) usually has four sides: the ridge, the eave, and two rakes.

  3. GBA Editor
    Martin Holladay | | #3

    Leon,
    Here are some photos and diagrams showing rake overhangs.

    The basic concept is that the rake overhang is supported by blocking (installed 90 degrees to the rake). When the roof is loaded with snow, the rake overhang is supported by the blocking; it can't sag because the blocking can't hinge. (Where the blocking is toenailed to the wall sheathing or rafter, a crack can't open at the top of the joint because the roof sheathing is well nailed to the blocking.)

    .

  4. Leon Stepanian | | #4

    Hello Martin;

    Thanks for your prompt reply. Yes the rake overhangs and the images you showed are right on. Also, sorry for overwriting but I thought readers would be interested in some background.

    From your pictures, I made the following diagram as this is exactly how my home is now insulated at the roof/wall junction and I have added what I will do to produce the rake overhangs.

    Thanks again for your input.

    Leon

  5. Expert Member
    Malcolm Taylor | | #5

    Leon,
    If you are strapping the roof with 2"x4"s over the 8" rigid insulation, why not run the new 2"x4" on edge overhang you are proposing back to the next 2"x4" strapping that is 24" back onto the roof and block in between? This means you aren't relying on the sheathing for rigidity.
    That's generally how you frame overhangs over 12" deep on gable ends, whether the walls are free-framed or trussed. You can find a diagram of a similar overhang at the link I've provided:
    http://www.resourcesnorth.org/woodframe/eng/training/l-4-3.htm

  6. Leon Stepanian | | #6

    Malcolm,

    Thank you for your comments. My last diagram is surely not complete because it is only showing the lookouts that are between the overhang at the ridge and the facia board.

    I took a grab showing the ridge lookout before it was nailed into place. They are 10 foot length with 2 foot overhang. So my ridge is solid and my eave lookouts are solid as well. Those are real 2" by 4" and not the standard 1.5" by 3.5". I am just trying to make sure I don't make a mistake since this is not the standard frame on frame as per your great link.

    Leon

  7. GBA Editor
    Martin Holladay | | #7

    Leon,
    I don't like the detail that you posted in Comment #4.

    It's kind of too bad that you got so far into this project without deciding on a detail for your rake overhang.

    Here's my problem with your suggested solution: Your 2x12 is cantilevered upward without much attachment to the house -- it's just attached at the bottom to a 2x4 truss chord.

    I'd prefer to see a 2x6 or 2x8 in this location, with the top of the 2x6 or 2x8 in the same plane as the top of your roof trusses. It would have been good to cut back your (wimpy) existing roof sheathing, and to install new 1/2-inch sheathing to tie the rake overhang back to a couple of truss bays.

    If you follow my plan, you would install 2x6 or 2x8 blocking to establish the rake overhang. Then you would install the 1/2-inch sheathing described above to tie it all together. Once this was sheathed, you would install 8 inches of rigid foam over the rake overhang, just like the rest of the roof. (I know that the overhang doesn't have to be insulated, but continuing the foam makes things simpler to build.)

    You end up with a thicker rake overhang with this approach, but it will be much sturdier than the way you envision doing it.

  8. Leon Stepanian | | #8

    Martin;

    I really like what you are saying as this is deep down my thought as well. Your approach is unfortunately not possible so I thought about another option to secure the top end of the 2 x 12.

    I added a red line on the following drawing showing how I could put a steel strip of metal from the top of the 2 x 12, over the first 2 x 4 then anchored onto the second 2 x 4.

    I don't think I can do any more then this since for me at this stage there is no turning back. Yes I should have planned this part of the project more thoroughly but had so many other details that it just fell into the "we'll solve it when we get there" category.

    What I did not want was any break in the total coverage of the insulation around the house framing and did not want any blockage in the air channels. No exceptions, so that's why I even covered over the existing house eave lookouts. Right now, every nick and cranny is covered and is behind 3" or 8" of insulation, all sealed.

    I think a good metal strip will be enough to prevent the 2 x 12 from tipping outward, but don't forget, the rake beam 2 x 6 will also be well supported at the ends, plus the 5/8" final roof sheathing will tie all of those together.

    I really appreciate your input as this has just given me good solutions to work with. If you have other ideas, please let me know. I will start to build up one side of the rakes today so will have more info later. Back on a hot insulated roof.

    Leon

  9. GBA Editor
    Martin Holladay | | #9

    Leon,
    Your current detail is an improvement. If you beef up the blocking on the rake -- switching from 2x4s to 2x6s or 2x8s -- you'll have a lower likelihood that the entire overhang will hinge downward.

  10. Leon Stepanian | | #10

    Martin;

    Hmmmmmm. I could go with maximum 2 x 6 and then use a 2 x 8 as the rake beam. That should do it. I'll have to source the metal strips but this will not delay the work. Thanks again. You are the man.

    Leon

  11. GBA Editor
    Martin Holladay | | #11

    Leon,
    As they say in Yerevan - khntrem.

  12. Leon Stepanian | | #12

    Martin,

    Nice one.

    I have redone the diagram which is what I will do as a final revision. All the wood is in and cut. Even found a great strapping. Because my lookouts start from not perfectly aligned toppers, I will let the lookouts run wild and snap a line to plumb them up. Should be a breeze. I'll continue time lapsing the work.

    Leon

  13. Leon Stepanian | | #13

    Here is an update.

    Managed to finish the Eastern rake. I made some changes to the plan. I dropped the 2x12 by adding a 1 1/2" by 1 1/2" trim on top of the 2x12 in order to lower the base so I could get more wood involved where the bolts are secured. Also decided not to recess the bolts in order to have full wood thickness and then put a plywood over the 2x12 to finish the look and hide the bolts.

    Images below. You can also see that we are starting to finish the front and front/sides of the house with SureTouch 2" insulation and brick veneer, Scaffolding is now on the West side so that should be done in a few days.

    Leon

  14. GBA Editor
    Martin Holladay | | #14

    Leon,
    Thanks for the updated photos. I'm not an engineer, but that looks like good work. I always smile when I see a generous rake overhang -- it's a sign of a thoughtful designer.

  15. Leon Stepanian | | #15

    Another update... I finished the west rake several days ago as shown in the first image. I had completely forgotten about our west window which is the only one that we did not replace since it was a fresh opening we did only about 5 years ago and we really like the window. But now I have to extent the outer edge.

    Also about the siding I will be putting on is similar to Canexel (mine is http://www.kwpproducts.com/products/prestige/) engineered siding that resembles wood grain finish. I used 6" lag screws so my 1 x 4 furrings are well secured through 3" of insulation and a 3/8" fiber sheathing before it lands on the framing studs. The 1x4s are 3/4" thick. So hear is my present quandary. In all the instructions of all these types of manufacturers all they "officially" indicate is to use long enough nails to land at least 1" to 1 1/2" into the studs, only taking into account the sheathing and furring thicknesses of a standard built home situation. Surely these guys know that their siding is being installed on thick wall insulation.

    So is there an official stance on siding. If only nailed or screwed onto the furring, does this counter any building code, or, do I have to use again 6" wood screws to again land on the studs? That would be a pain and add so much more screws going through the insulation.

    I also added a photo of the house front with the Suretouch insulation and veneer. Kept things sweet and simple. Also started putting up the roof sheathing, did half the roof but not fast enough to protect it from the rain which is expected all weekend. I'll have to give it a few days to dry before I put on the underlayment and edge membranes and finish the other half.

    Just got a quote from a soffit, facia and gutter contractor for both the home and garage. For the home it calls for double facia and soffits so double the expense. Man you have to really think about doing what I am doing. I could do it myself and save the labor but I am getting kind of tight for time and have enough work load. I'll make a decision in the next few days.

  16. GBA Editor
    Martin Holladay | | #16

    Leon,
    The paragraphs below come from the following article: Fastening Furring Strips to a Foam-Sheathed Wall:

    Okay, now your furring strips are firmly secured to your building. The next question is: how do you attach your siding to the furring strips? After all, once you have installed vertical 1x4 strapping over foam sheathing, your siding nails are embedded in only ¾ in. of wood. Is that enough?

    Table R703.4 in the 2006 IRC (“Weather-Resistant Siding Attachment and Minimum Thickness”) specifies the required minimum fastener lengths for attaching siding. Moreover, in Footnotes n, p, and y — footnotes that apply to some, but not all, types of siding — the IRC notes that fasteners must “penetrate framing 1 1/2 inches.”

    Confusingly, however, Table R703.4 provides no guidance to builders installing siding on furring strips over foam sheathing. The table anticipates several scenarios, including “fiberboard sheathing into stud,” “gypsum sheathing into stud,” “foam plastic [presumably without furring strips] into stud,” and “direct to studs.” Each of these scenarios deserves its own column in the table. However, there is no column for “furring strips over foam sheathing.”

    Until recently, many siding manufacturers recommended that siding nails penetrate 1 in. or 1¼ in. into wood. That’s beginning to change, however. The Vinyl Siding Institute requires only ¾ in. of fastener penetration for vinyl siding; James Hardie Corp. accepts only 7/16 in. of penetration for fiber-cement lap siding.

    According to building scientist Joseph Lstiburek, if you have any doubts arising from the fact that your siding nails penetrate into only ¾ in. of wood, just switch from smooth-shank nails to ring-shank nails. Unless you’re building near the coast in south Florida, ring-shank nails will be more than adequate, even when penetration into wood is only ¾ in.

  17. Leon Stepanian | | #17

    Hi Martin and thanks for your great portrayal of the problem and for the link to your article. I found the pdf on the FastenMaster Website. I am using their HeadLok 6" screws. My set-up is furring 3/4", Formular 200 insulation 3", wall sheathing 3/8" so there is 4 1/8" before landing on the studs by 1 7/8" plus an easy 1/8" more when the screw head slightly penetrates the furring. This provides a good 2" of stud penetration,

    As per the indications my wall studs are at 16" OC, siding weight is about 2lbs per sq.ft. so very negligible. My wind designation is B. My screws are placed at an average of 24" apart so according to their table #4 my Connection Allowable Design Wind Pressure (PSF) is 49, so if I understand this correctly, on table #4 if I look at the B column, I should be good for 140 mph winds.

    I found another document attached herein in which there is a table showing that #8 wood screws having a 1" penetration will be ok, so if I extend this and use 1 1/2" long #10 deck screws to attach the siding to the 3/4" furring, I should be more then OK, even better then using ring shank nails. I did a few trials and my siding handles the #10 deck screw very well without pre-drilling any pilot holes.

    So my plan will be to slightly increase the number of 6" screws holding the furring to the studs, just to be extra sure, then I will use 2" #10 deck screws to secure the tongue and groove siding to the furring. The extra 1/2" length will be required because I have a feeling I will require some shimming between the furring and the siding to keep the boards straight and plum, given the secured furrings are slightly embedded into the insulation thus creating some slight waviness that is not wanted in the final siding appearance.

    Thanks again.

    Leon

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