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Community and Q&A

Flat roof

User avatar
Armando Cobo | Posted in General Questions on

I’m designing a large, high-performing house with “flat” roof in Dallas, TX. In NM most flat roofs are done with .25/12 slope Built-ups or Modified Bitumen roof, but here since there is more rain fall, I’ll like to use .5/12 slope and higher-end roofing material. I would like to use mid-R30s insulation. My options so far are:
1. Fiber type: TPO, KEE or PVC with A) 1” polyiso above the roof decking and 8” OC foam under roof decking, or B) 5” polyiso above the roof decking.
2. 5.5” Sprayed CC Foam roof.
In NM most flat roofs are done with .25/12 slope Built-ups or Modified Bitumen roof, but here since there is more rain fall, I’ll like to use .5/12 slope and higher-end roofing material.
After reading about different methods and brands, I’m more confused on which system is better. I would like to ask your thoughts from experienced folks. Thanks.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Armando,
    Your list of acronyms under the heading "Fiber type" has me confused. I'm not sure what you mean by PTO -- perhaps you intended to type "TPO"? TPO refers to thermoplastic polyolefin, which is a type of single-ply roofing membrane. It is not a "fiber type" (which implies some type of insulation).

    I have found references to KEE in several roofing articles -- DuPont makes a PVC roofing with a product called "Elvaloy KEE" and a roofing journal reports that "ASTM ... has assigned a number to the new Standard Specification for KEE Sheet Roofing." So I assume that KEE is either a type of single-ply roofing or an additive used in single-ply roofing.

    I know what PVC is. It's polyvinyl chloride, and PVC is used to make single-ply roofing. It doesn't seem to me that KEE and PVC are "fiber types."

    Concerning insulation approaches: you list three approaches. These are:
    -- 1 inch polyiso above the roof decking and 8 inches open-cell spray foam under the roof decking (about R-35);
    -- 5 inches polyiso above the roof decking (about R-32);
    -- 6.5 inches closed-cell spray foam roof -- presumably, on top of the roof sheathing (about R-42).

    Obviously, you need to price out these various options. The last option provides the highest R-value and the best approach to addressing thermal bridging. It also includes roofing -- all you need is a coating to protect the foam from UV -- so you save the cost of your single-ply membrane.

    All of these approaches will work.

    For more information on this topic, see Insulating Low-Slope Residential Roofs.

  2. User avatar
    Armando Cobo | | #2

    Thanks Dan. I've edited my typos. My understanding is all are called fiber roofing because they have a fiber layer, or two, intertwined between layers of polymers. Hey, this could be TX talk, I guess… ;-))
    My issue is that every company or rep. talks beauties about their product and blasts their competition (not that I’m surprised)… however it’s always the same problem but with the other product. All fiber type representatives blast foam roofing, and vice versa. So really it’s hard to know who’s telling the truth and who’s not. The PVC guy says the TPO comes unglued, the TPO guys says the PVC does.
    All these roofs are about similar price down here, so I’m looking for the best long time performance, less degradation, cracking and maintenance. I’m hoping that someone who’ve used some of these roof types for a while would write about their experiences.

  3. User avatar
    Dana Dorsett | | #3

    Cost is one thing, but unless the 5.5" of ccSPF is blown with a very-low global warming potential (GWP) agent, it could be doing a world of lifecycle damage to the climate, whereas any polyiso & open cell combination would not. Most 2 & 3lb closed cell is currently blown with HFC245fa, which has ~1000x CO2 GWP. Honeywell & DuPont have both released sub-10x CO2 blowing agents for closed cell polyurethane in recent years/months, but it will take time for those to become widely adopted.

    Honeywell has made an attempt to compare the lifecycle global warming potential of their newer blowing agent (trade name Solstice) to other common building materials, outlined here:

    Note, they didn't run the comparison with foam blown with their HFC245fa variant (trade name Enovate), since the bar-charts would then not even fit on the page, since it has something on the order of 500-800x greater carbon footprint than Solstice blown goods.

    Polyiso is blown with pentane @ 7x CO2 GWP, and, open cell foam is blown with H2O (water), with even lower GWP. You can stack up 50" of polyiso and still not be anywhere near the lifecycle damage of 5.5" of 3lb closed cell blown with HFC245fa.

    An inch of polyiso is a bit silly - almost useless as a thermal break against rafters/trusses, and may even be hard to find. Most roofing iso starts at 2" thickness and goes up from there, and you'd want 2 layers with seams overlapped to thermally break against any shifting at the seams. If you did 4" above the roof deck you'd hit about R22 when derated for temperature under hot roofing) above the roof, with double-layered 3" you'd be at R33 (derated). Wintertime and shoulder season performance would be higher. SFAIK no 2lb roofing would hit R32 @ 5", though there are lower density versions suitable for walls that could hit that range. Figure on about R5.8-6 /inch nominal @ 75F average temp through the foam and R5.5/inch when derated for very high or very low outdoor temp.

  4. Ronnie Allen | | #4

    TPO and PVC used welded seams and are stronger than the field. TPO is usually installed and more available in wider sheets so there are less seams to weld. I wouldn't worry about the seams when they are done properly for either system.

    I would look at EPDM pricing as well. I know it is black but lets compare. Lay a sheet of .045 thick EPDM outside in the sun and it will be there in great shape in 20 years. You could take that same sheet that has been laying there for 20 years and put it on new roof and it will last another 20 years. There is no other roof material that can do that.

    TPO and PVC has the scrim sheet somewhere in the middle of the sheet and one mfg might brag that their sheet is "x" amount of mills over scrim. The higher the "x" supposedly the better the sheet. Why? The mfgs know the sheet sheet will mil down from exposure, limiting the life of the sheet to 10 to 20 years.

    You may be looking at TPO or PVC for its reflectivity. Just remember the white sheet can get very dirty quick and you may be up there cleaning it to keep the reflectance you want and you don't want to be up there with a power washer.

    For energy savings you are usually better putting your money in insulation and air sealing, which brings us to foam roofs. The idea behind them are great but they usually don't work like they sound. When the top coating of the roof is penetrated the foam will become compromised. You would need to do monthly inspections to make sure there are no flaws in the coatings. Flaws could include falling tree branches stabbed into the roof or even damage from critters or birds.

    For residential work for single ply membranes I would stick with minimum 0.060 thick sheets, no matter what the material you use. My opinion is EPDM is the clear winner as far as maintenance, ease of repair, life cycle cost.

    I am not a big fan of real thick insulation above the deck because of the long screws. Just my opinion here but I would think about using 2 staggered layers of 1.5" above the deck. This will keep the screws cost down. (Maybe use 2" but check the pricing, could be a significant jump when figuring in screws)

    Maybe the "fiber type" was meant to be "Fibertite". "FiberTite" being a roof manufacturer who also hails the "KEE" technology. All mfgs believe their product is the best and usually you will deal with a roofer/ builder that is married to that brand and couldn't offer you another product or mfg if they wanted to. I would get several quotes and check the residential warranty for the product as it can be different from the commercial.

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