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Question on insulation for a low-slope roof terrace

user-7034824 | Posted in General Questions on

Please see the attached PDF. I am doing a remodel design and we will be adding a Roof terrace over living area. This is in the Atlanta Georgia area. The roof terrace will be almost 1800 sq. ft. The roofing contractor is proposing putting 4″ of Rigid foam on top of the roof decking them laying dens-deck on top of that then applying TPO. This is going to be a fairly high trafficked area with party’s etc on the Roof. Will the Rigid foam with Dens Deck make a decent walking surface, or will the foam compress and cause issues (it will only be sloped at 1/4″/foot).

I am not so well versed in Flat Roof/Roof Terrace design and have read a lot about insulating, etc, but it seems the more I read, the more confused I become! My original design called for air impermeable closed cell spray foam under the Deck and something like TufDek PVC for the walking surface, but the home owner now seems to be backing away from that, I think in part based on information the contractor is giving him. It could also be a cost issue – I know spray foam is expensive – but so far the homeowner hasn’t given any reason. I also know TufDeck is also probably kind expensive, so I also gave an option for Rubber Roof pavers over EPDM or TPO

I have something different in mind vs. what the Roofer wants to do and will post the drawings for how I want the Roof terrace to be built soon.

Thanks for looking!


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  1. GBA Editor
    Martin Holladay | | #1

    It looks like you forgot to attach the pdf.

    TPO is a roofing membrane. It is not a walking surface, and it certainly isn't a patio surface.

    Once you have installed your roofing, you have to design and install the patio. You can use pavers, or sleepers covered by a wood deck. But you don't want the homeowners to be walking on the roofing.

    As evidence that it's not a good idea to walk on TPO roofing, you might be interested to know that several companies sell walkway pads for TPO commercial roofs. These walkway pads allow HVAC repair contractors to access rooftop equipment without harming the roofing. Here is a link:

  2. user-7034824 | | #2

    Thanks Martin. I am aware that TPO is not a walking surface. That is what the Roofer said he would install, and I am assuming (as I speced on the original design) that the roofer would use the Rubber pavers as I had originally called out. My question is on the insulation and using all that (Pavers over Dens-Deck over TPO over Rigid foam. Will the Foam support all that, or will it compress and cause other issues (ponding).

    I haven't had a chance to post the drawing yet, but will soon.

    Thanks for your Reply!

  3. user-7034824 | | #3

    OK, Here is a Roughed out Detail. Please tell me if I am going wrong with this.

  4. GBA Editor
    Martin Holladay | | #4

    There are some odd details shown.

    1. The new 2x10s can't rest on rigid foam. The 2x10s will dig into the soft foam. If you want 2x10s there -- and I'm now sure why you do -- you'll have to install a layer of OSB or plywood between the rigid foam and the bottom of the 2x10s.

    2. It looks like you are planning to install a 3/4-inch deck (unspecified) directly over the TPO roofing. I'm not sure what that means -- some type of attractive boards to create a deck, perhaps? If so, you forgot two layers: (a) some method of establishing an air space between the roofing and the 3/4-inch deck, and (b) something to fasten the deck boards to.

    Or maybe you are talking about a layer of OSB or plywood. I'm not sure what this layer is, or what (if anything) it is fastened to.

  5. user-7034824 | | #5

    My Bad on the wording. It should have been TPO OVER the Deck, not below! The 2x10's don't rest on the Rigid foam. The 2x10's Pitch from the 2x4 plate and will be supported by a wall on the other side. The 2x10's will support the deck board (glued/screwed to the "rafters") on which the TPO will be adhered (above the Deck ;-). Here is the PDF with the Rigid Foam removed to show that the 2x10's aren't resting on the foam but are above the tapered layer. As indicated in the PDF I will have (2) two 2" (4" total) layers of foam below the terrace decking and the tapered layer above the "Roof" decking sitting on the TJI's. My initial idea was to fill the entire cavity with Rigid Foam, but that would be a lot on Foam insulation, thus why I asked on the PDF if I could fill in between the top and bottom layers of Rigid with Cellulose. This will be a un-vented assembly so no air gaps. Here is the PDF without the Rigid for clarity.

  6. user-7034824 | | #6

    Wanted to add: The tapered layer above the habitable living area runs under the sloped 2x10's and will be continuous. I know there will be some thermal bridging from where the terrace deck attaches to the 2x10's, but the 2x10's never touch the deck over the living area, so no heat/cold will be transferred into the deck above the living area...

  7. Jon_Lawrence | | #7


    I will share with you my experience with a recently completed 1,000 sf low-slope roof. There will be a patio, solar panels and garden on this roof. The roof is roughly 30'x36' with a 1/4" slope along the longer end. I needed a minimum of 4" of polyiso above the roof deck. We built the base layers and taper entirely from 2lb polyiso, so the depth of the insulation at the far end was 13.5". We used fasteners for the base layers and then adhesive for the tapered layers, 1/2" Densdeck and 75mil fleece-backed TPO membrane. The Densdeck gives a very solid walking surface, has a very high compressive strength and should act as a baseplate to spread the load over a greater surface area of the foam. I have not noticed any compression and it has received a lot of loads since being installed, including 600 lb pallets of nail base and currently 20" of heavy wet snow. While the TPO is very strong, GAF recommended we add a sacrificial layer under the ballasts for the solar panels and under the garden base. I have not spec'd the walking surface for the patio portion yet, but it will need to incorporate a pedestal design to take out the slope. Most roofing supply house stock the materials we used, but the TPO and some of the fasteners were special order with a 1-week lead time. My crew was able to install everything in a day.

  8. user-7034824 | | #8

    Thank you Jonathan, That is very helpful. Since this is the way the roofer already wants to go, and is much simpler than what I am proposing, I think I will go with this. I was just kind of uncomfortable with a walk surface on foam, but if the densdeck spreads things out enough to not have issues with the foam then that works for me. Like I said originally, I don't have a lot of experience in this area, so I was trying to remove any loads from the foam, This alleviates my concerns! Many thanks!


  9. GBA Editor
    Martin Holladay | | #9

    The method suggested by your roofer (and described by Jonathan Lawrence) is better than the method shown in either of your two sketches.

    In other words, you want a thick layer of continuous rigid foam above your roof sheathing, followed by DensDeck. You don't want to use the cut-and-cobble insulation method shown in your illustrations.

  10. jberks | | #10

    I did something similar on my build.

    I didn't pitch the insulation, but had the roof built with a pitch, and it made for a more simple purchase of insulation. I did 2 layers of 3" polyiso (all adhered). I did a 1/8 asphalt cover board. I don't remember why I didn't do dendsdeck, but I actually do regret it slightly. after doing my roof membrane (liquid applied polyurethane) you can feel the coverboard and foam move slightly under your foot when you walk on it. But, I will be putting down a green roof in the spring, so I think I'll be fine.

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