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Community and Q&A

Roof Replacement and Proper Ventilation

jmvar | Posted in Green Building Techniques on

Hello, I live in Maryland (zone 4?) in a home we purchased in 2017 that had recently had the roof replaced.  Last year we had a leak during a very heavy rainstorm and realized that when the roof was replaced the sheathing was not replaced and it should have been.

We would like to install solar panels and want to address the sheathing that is soft is certain spots before we move forward installing solar.  I believe the reason that the sheathing is soft is due to improper venting below the sheathing.

You can see in the pics that I have highlighted specific areas.  The area at the top (yellow) closest to the ridge has a very low attic above it with blown insulation and no baffles.  The area below that (orange) is a vaulted ceiling in the living room and kitchen.  The area below that (blue) is a dinning room that I think used to be an open car port and had an addition put on it.  The double blue line represents where I think that the eave was blocked when the the addition was done.  The red area is an open entry area with an entry door into the kitchen.

The roof is a 3-12 pitch and has a ridge vent at the top of the yellow section.  all of the eves have vents but as I said, I think that further up they are blocked.

As we call roofers to come and provide estimates, I’d like to understand what would be the best way, if at all possible to vent this roof?

To add another level of complexity, we are considering doing an addition to this side of the house in the long term future (5-10) years.

Thank you for your input in advance.

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  1. Expert Member
    Akos | | #1

    Low slope roofs are very hard to vent properly. If the soft spots were not from water leaks, your roof might already have issues with leaky ceilings and insufficient venting.

    About the only bandaid you can try for the blocked off section is something like a low profile off ridge vent. These are typically used for venting bellow dormers, but some of these can be installed in low slop roof, so you'll have do some digging in spec sheets. Of course, no amount of venting can fix a leaky ceiling, so if any of the sections bellow have something like T&G with no air barrier. That is a much harder problem to fix.

    1. jmvar | | #2

      Hi Akos, thank you for the reply. I have been looking at smartvent type solutions from DCI, owens corning, and other manufacturers but they all require a 4-12 pitch.

      I did find this product from quarix that can be used on a 3-12 pitch:

      What if I place this just above where the venting is blocked off and have it vent out at the ridge vent?

      I have a drywall ceiling with can lights where the ceiling is cathedraled. After some reading on this wonderful site I now know that is a problem and I need to either remove these or seal them up which I will do.

      1. Expert Member
        Akos | | #3

        That should work, but you need to place it both above and bellow where it is blocked off. You still need an exhaust vent of the roof section bellow as well.

        Canned lights do have to go, the simplest is to replace it with slim LED ones. These have a gasket and a pretty strong spring clamp, they seal pretty well.

        1. jmvar | | #10

          Well....I called Quarrix and they advised against using their intake vents because I will be putting them under the solar panels. When I pressed the rep about specifically why I shouldn't use them he couldn't talk about specifics, he just said don't do it, the conditions are not what the product was designed for.

          So I ask you experts for your input. My choices are to either put the intake vents under the solar panels and have them vent out of the ridge vent or to not vent the rood at all.

          1. Expert Member
            Akos | | #11

            I can't see what solar panels have to do with roof vents. Maybe if the panels are too close and are sealed around the edge, it would effect airflow.

            There are many manufactures that make similar shingle vents like:


            You can try giving them a call. The google search term is "shingle over intake vent".

  2. jmvar | | #4

    I have spoken with several roofers and I think I am going to replace all the sheathing. It is very painful because the shingles were probably installed less than 5 years ago....on comprimised sheathing that needed to be replaced.

    I am going to take this opportunity to insulate at the roof level with solid foam and I am still researching my options and becoming an informed consumer. Are there any resources anyone can recommend to learn about insulating at the roof? Solar panels will be going on top of the roof. 2/3rds of the roof is over a cathedral ceiling, and the other 1/3 is over an attic insulated with blown insulation on the side you see in the pics in my first post. The other side is over an attic with blown insulation.

  3. Expert Member
    Akos | | #5

    This is probably your best resource:

    The one issue you have is your attic insulation is on the floor not against the rafters. If you seal up the vented attic, you end up with this in between space between the foam and blown in insulation which will create problems in colder climates.

    You can do foam overt the whole roof and keep the attics vented, but than you loose the R value benefit of the rigid insulation in those section. You are spending a lot of money on foam+2nd roof deck for not much benefit.

    In your case, probably the simpler approach is to pull back the roof deck, spray the ceiling in the cathedral section with closed cell SPF and install a new deck. Keep the other attic sections as is and vented.

  4. jmvar | | #6

    Akos, your spray foam suggestion is very interesting.

    As it is right now there is not much venting if any in the small area that is the attic. It does have a ridge vent, but I am almost positive the eve vent up to that ridge vent passages are blocked to the attic. Would my solution be to place the quarrix inlet vents where the attic area begins (where orange and red line meet in my pic) and rely on the ridge vent as the exhaust?

  5. Expert Member
    Akos | | #7

    Provided there is a high and low vent, it should work. Fixing the leaking pot lights bellow is still a must though.

    1. jmvar | | #8

      Akos, if I end up going with the spray foam option, there will inevitably be an air gap in places between the spray foam and the roof deck since it is being sprayed from outside on the drywall that is the interior ceiling. Will that air space cause problems?

      1. Expert Member
        Akos | | #9

        It is always best to add a bit of venting for this gap. The only moisture source there will be from night time condensation, so it is nowhere near as much air interior air leaks, but still something. Shingles over 15lb felt are 0.6perm, so it would allow for a bit of drying. Doing nothing will most likely work but a bit of venting is guaranteed no issues.

  6. Jon_R | | #12

    Note that code doesn't require high and low vents. If you only have one, then you just need more vent area (150:1 instead of 300:1).

    1. jmvar | | #13

      @Jon R, can you explain the more vent area comment? Does this mean that if I don't have enough intake vents I can make up for it with more exit vents? What if I only have so much ridge to for the ridge vent?

  7. jmvar | | #14

    I have had several roofers come out several have mentioned that They put ice and water shield on all roofs that are a 3-12 pitch or less. I am sitting at about 2.5-12.

    Is putting ice and water shield on the entire roof standard?

    1. Expert Member
      BILL WICHERS | | #15

      Usually you only put ice and water shield out at the eaves to protect against snow melt water from getting under the shingles and into the roof and walls of the structure. It’s usually two roll widths’ (one really isn’t enough) worth of ice and water shield up from the edge of the roof.

      Putting ice and water shield on the entire roof isn’t common, but is occasionally done. With a very low-pitch roof, the ice and water shield gives an extra layer of protection. Shingles work with gravity to keep water out of the roof assembly, but with a very shallow pitch roof and the right conditions water can sometimes “back up” enough to get up under the shingles and then inside. Ice and water shield is a fully adhered membrane (no gaps that aren’t completely sealed), so it’s a waterproof barrier to guard against water getting into the structure that manages to sneak past the layer of shingles.

      You have some other options though besides shingles and ice and water shield. You can used roll roofing, which is like a huge sheet that is one giant shingle. Other options are torch down, which is a commercial style roof membrane. These both look very different from a regular shingled roof though, so you may have some aesthetics to think about.


  8. jmvar | | #16

    Thanks for the reply Bill.

    Doesn't the sheathing underneath need to breath? Would ice and water shield over the whole roof affect that?

    1. jmvar | | #17

      I did a search prior to asking this question here on GBA but got over 100 hits and read through some but couldn't find my specific question discussed. As I was searching through google results I found this GBA thread. It get contentious but has some good insight.

      Due to my ventilation challenges under my sheathing I am inclined to only use ice and water shield at the eves. If anyone has any updated info to share it would be appreciated.

  9. jmvar | | #18

    Well after may consultations with roofers that I found on the Owens Corning, GAFF, and other vent manufactuer websites, none of them wanted to put in a ridge vent or the low profile intake exhaust vents in my roof because they said that the pitch was too low and there could be leaks from wind driven rain and snow.

    I remeasured my pitch and it is actually 4.25-12 which meets or exceeds the 3-12 pitch required by most ridge and low profile vent manufacturers.

    I found one roofer willing to install these vents and work is scheduled to begin this week.

    I have one last question, if you could please take a look at the plan for the vents and let me know what you think. I am nervous about having so many vents but I am not sure there is anything else I could do besides not vent at all.

    In the picture you will see where the solar panels will go, the vents are marked in blue for the eve vents that already exist, red for the low profile vents to be installed, and green for the ridge vent that exists but will be replaced with a new one.

  10. Expert Member
    Akos | | #19

    That does feel like a lot of venting. I would calculate the amount free vent area you need for each section and size the vents for that. The upper attic probably does not need a full length intake vent. Most likely the partial eave vent on the south side and eave vent on the north side would be enough free area.

    If there isn't a lot of structure between the cathedral roof and attic sections, you might be able to combine the intake and exhaust vent into a single vent by making a couple of vertical cuts into the sheathing.

    The lower attic section can also be gable vented, looks like you should have enough space on the siding for an exhaust vent.

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