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Community and Q&A

Flat Roof Construction & Insulation.

Codhod | Posted in General Questions on

Hello Everyone! 

Im working  on a 120 year old house. The  back half of the house was torn down and I am currently framing it back up. The roof is a 1/12 pitch 2×10’s 16” O/C. 5/8” sheathing. 

I live in zone 5, South Dakota. Not ideal for flat roof but to appease the historical board I have to maintain the existing Flat roof. 

The rest of the house has a 8/12 pitch hip roof with asphalt shingles. 

my question is what insulation route should I go? Materials?

Originally I was going to just Epdm the sheathing and closed cell foam and batt the interior with the max I can get in the 2×10 rafters. 

The roof will be a hot roof with no ventilation. 

Im currently getting bids on the roof and have a contractor recommended for me to stack iso boards on the sheathing and then just doing the batting on the inside. he would then use a pvc rubber membrane. 

does this work for a stack? Rubber, iso boards, sheathing, batting, sheetrock? 

epdm or PVC for the membrane? He is recommending pvc as it’s not Black and won’t pull in the heat. 

I have another contractor suggesting EPDM directly to the Sheathing. 

open to any other suggestions as well. 


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  1. seabornman | | #1

    If you can put enough polyiso on the deck to keep the deck from condensing the moist air that makes it through the fiberglass below, that would be the way to go. You can find that thickness for your climate zone somewhere on this site or buried in building code. If you just put fiberglass below, you will have condensation problems.

  2. Expert Member
    Akos | | #2

    Installing rigid above your roof deck would greatly increase the assembly R value as it reduces the thermal bridging of the rafters. If you can get this done for less money than the flash and batt option, it is well worth it.

    In zone 5 you need 40% of your R value to be rigid insulation, so in this case with 2x10 rafters with R30 batts between the rafters, this means at least R20 rigid above the deck.

    It is also a good idea to air seal the roof deck before the rigid goes down, this can be done by taping the seams with a decent sheathing tape before the rigid is installed. Also try to use two layers of rigid insulation with the seams staggered. Besides the insulation benefit of staggering, it also reduces the stress on your roof membrane and reduces the chance of seams splitting.

  3. onslow | | #3


    Plus one on Joel's observation about just batts.

    I have multiple near flat roofs with 8 1/4" EPS nail base and PVC membrane on top, R-19 batts under for vapor open to inside for moisture control. The EPS thickness keeps the sheathing well above any dew point I would see here despite being in CZ6 dry.

    For one roof, the available "thickness" for insulation was constrained by a door threshold that allows use of the roof as an upper deck. For that one area we went with a combo of polyiso and 4" EPS nail base to get nearly the same above deck R value. Whether the combo is really keeping the polyiso protected enough to avoid the cold R drop is open to question. If you are really tight on how thick you can go with above deck insulation then the all polyiso method will give you a better number on paper.

    I would recommend taking any membrane up the adjoining wall several inches if at all possible. This will be especially important if your main roof drops its snow load onto the flat roof below. Regardless of other roof positions, taking the membrane up the wall will protect a weak point of flat roofs, the junction with side walls. While it unlikely that you could get an ice dam tall enough to back up water I go for better safe than sorry.

    Being in a hysterical protection zone, I would advise that you look to controlling the over hang's visual thickness when adding insulation above sheathing. Best to avoid any committees suddenly deciding that it doesn't look like the old roof.

    I would recommend PVC/fabric material despite the higher cost. I used the IBS system. Fire rating provided by two layers of fiberglass mat. It is both mechanically fastened and heat fused. The flashing materials are PVC coated stock bent to suit. The membrane is heat fused to it. In my case I used a contrasting color to the wall finish to match my window frames. All this is over the synthetic roofing paper protecting the nail base. It does have a lifetime warranty. Whose is open to question.

    My membrane is not adhered over the field like many EPDM or TPO methods which does allow it to lift during very high winds. So far so good, and replacement if ever needed may prove simpler. Anti lift vents are used on shopping center sized jobs I believe. It might also be instructive to do a search on the posting about wrinkles in an adhered membrane that appeared in GBA few weeks ago. JLC also had an article about failed TPO around skylights in the last few months. The materials desire to expand and contract may be at odds with being constrained by the adhesive and result in stresses at corners.

    If you are planning to do the roofing at this time of year, South Dakota might be a bit chilly for good results with adhesives.

    1. Codhod | | #4

      Thanks for the ideas and expertise!

      I don’t have anything keeping the roof from going higher other than the fact that if I maintain the current 6” subfascia and add 4” of foam board im at a 10” fascia which seems quite large and may not be the best look? Currently my rafter tales are level cut to 6” I could maybe take an inch or two smaller but then im not leaving much meat on the tails

      Do you just have the sub fascia stick up 4” above the sheathing for a place to nail the finish fascia and drip cap, gutters etc.?

    2. Codhod | | #5

      What can I use to protect the sheathing from rain / snow until I get the insulation and membrane installed? Is it a bad idea to put ice water shield down on the sheathing?

  4. Codhod | | #6

    Anyone have any input on if I should put the Ice and water shield type product down on the sheathing before the iso board and pvc membrane are installed?

    Im mostly concerned with the lag time between sheathing and actually putting the iso and membrane down. Id like to protect the sheathing from rain/snow. It could be a couple weeks to a couple months all depending on weather and roofer timing.

    I also have to deal with the transition from roof pitch to roof pitch so atleast it’s not raining in the living room..

    Any suggestions on what to protect my sheathing and temporarily sealing up the roof until membrane is installed if I shouldn’t use ice water shield?

    Thanks again!


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