Really Built-Up Membrane Roofing Systems

Archive of Older Topics Archive - Product Discussions #4
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I am trying to educate myself more about membrane roofing systems. Although I always feel a bit insecure in this area, it seems like the run-of-the-mill architect is thoroughly confused. Many of the roof consultants that I run into aren’t that much better.

I am involved in a project that is a renovation going for LEED certification. The building is older (probably at least 60 years old). We will be tearing off the existing membrane roofing, putting down new insulation (probably lightweight insulating concrete) and installing a membrane roofing system. For a number of reasons, the architect wants to limit the slope to 1/8-inch per foot. I have voiced my objections, but we have crossed that bridge and the architect has gotten the building department to waive the code requirement for 1/4-inch per foot.

Initially, the architect wanted to use a TPO system, but the owner’s representative is insisting on a 3-ply APP modified bit system and suggested a particular system, but when I looked into it, the manufacturer would not warrant the system on the slope desired. Evidently the owner’s rep was not aware of the 1/8-inch slope (he recently joined the team). He is now suggesting a 3-ply system mod bit system with a fleece-backed TPO membrane adhered to it.

I have run into this suggestion before, and it really strikes me as guilding the lily. I can get a 2-ply mod bit system with a 20-year warranty on this slope without excluding ponding water so getting a 3-ply system with such a warranty is not an issue either. Intuitively, it seems like getting a good roof system and putting a another good roof membrane over it would enhance performance, but I wonder how much. I have heard of some roof consultants recommending this approach. I know that mixing systems is not always a good idea, but are there drawbacks in this case?

While there are times that “Less is More”, I also know that many times, “More is Better.” At what point does “More” become merely “More” without adding value to the project.

Feedback and comments would be greatly appreciated.

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Another issue, Peter, is using an adhered roofing system (whether with hot bitumen or torched) on lightweight insulating concrete, because of the time if takes for moisture in the insulating concrete to evaporate, and the affect this can have on the adhesion of the membrane. Roofing manufacturers may have restrictions on the use of their system over this substrate.

3

Let’s see, an area that receives approximately 54 inches of rainfall per year (a little more than an inch per week) and the building department “waived” the roof slope requirement?

The building official technically can’t “waive” a requirement without some kind of equivalency or proof that the code requirement is impractical for the specific situtation. I’d be interested in hearing what the architect’s rationale was for reducing the slope.

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The previous edition of the Massachusetts building code allowed lower slopes if the manufacturer provided a warranty at the intended slope. I guess you could say it’s a bit like that.

Code issue aside, I think that putting a single ply membrane on top of a perfectly good mod bit roof makes less than no sense to me. What does this accomplish that roof which is properly designed, carefully detailed, (I know it will be specified properly since Peter is writing it), and carefully observed by an independent roof monitor, will get you?

I’ve always been leery of using torch-applied systems due to the very real risk of fire.

5

Is the TPO top membrane required for extra reflectivity for LEED points? I cant think of any other reason to put a roof on the top of a perfectly good new roof.

Johns comment about torch-application is a good one as well. There are lots of jurisdictions now (including Houston I think) that do not allow any open flame on roofs.

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Peter - It sounds like the Owner Rep has a thing for built up roof systems, but is adding the TPO to get the reflectivity necessary for LEED credit. It seems that a PVC or TPO roof that meets the manufacturer’s extended warranty requirements would be a better solution.

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If high reflection is not an issue (low roof/high wall with windows) you could use Siplast Veral, aluminum faced cap sheet as the third ply. That should be less money than adding a 4th ply.

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You can also apply a wide variety of roof coatings to the mod bit for reflectivity. While these coatings may be require recoating from time to time, they also probably extend the underlying mod bit by protecting it from UV.

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This is a LEED project and I think the TPO is a response to the emittance requirement. However, there are beginning to be a variety of cap sheet products on the market with a highly reflective granule (Siplast’s looks more like a flake) that meet Cool Roof and LEED requirements. There are also coated sheets, but these seem to have some problems as well.

As for the slope; you know, you can make water run down hill but you can’t tell an architect that flat roofs leak.

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Manufacturers focusing on their share of the TPO market have done a good job of convincing uncritical architects that TPO’s reflectance and emittance make it THE solution for LEED projects. It is A solution among many.

There are factory- and site- coated BUR and MB systems that meet LEED SRI requirements. There are also high reflectance gravel/adhesive systems that meet LEED SRI requirements. So I’d start by providing the longest-duration, most renewable, most constructable roof system you can, and work backward until you can afford it. Don’t rule out a TPA system if you end up looking at single-plies; they have some advantages in high pollution settings. I don’t know what you do with a TPO sheet when it’s run its course after 15 years or so. Covering up an MB system with a TPO means you cannot use resaturants to renew the MB plies after 15 or 20 years.

My own preference would be for an MB 2-ply system or hybrid system for a quick dry in with the final cap in cold adhesive near closeout.

Since when do code officials and manufacturer warranty writers have the final say on what constitutes good roof system design? Sigh.

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J. Peter - I am not sure here. Are you saying a modified bitumen roof system manufacturer will give you a 20 year roof warranty if the slope is 1/8" foot? Would it be impolite to ask who?

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I have spoken with two, Firestone and Derbigum. In both cases, these would be 3-ply systems. I remain somewhat skeptical; would want the lightweight insulating concrete installation to be very flat (not level). I may require some sort of testing for birdbaths before roof installation.

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I just did some research because a client wanted a mod-bit roof on a LEED project. I thought it couldn’t be done because I could find only one mod-bit roof on the “Cool Roof” site that stated they could attain the SRI of 78.

Further investigation into LEED indicates that the point is based on 75% roof coverage and a formula is given. If the mod-bit roof covers more than 75% then a lower SRI number (72, 73, 74) will make the calculation work. So now I have convinced myself, which can be very dangerous, that a mod-bit roof with a white cap sheet is acceptable for LEED. I don’t think there is any reason to cover it with a TPO or PVC membrane, either for LEED or more importantly, the life of the roof.

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IMHO, there is only one good dead level roofing membrane system that is coal-tar built-up system. But I guess that wouldn’t fly with the LEED effort.

Peter you never mentioned the deck substrate. Since you are looking at an insulating concrete [which I haven’t used in more than 20-years because it is so problematic: drying issue that Dave mentioned and the lesser R-values available], am I correct in assuming that the existing deck is concrete. If so, have you considered a fluid-applied membrane on the existing deck, adding a rigid insulation over top with a reflective aggregate over top of the insulation.

Low-spots in the existing deck can be addressed with patching compounds to slope water to the roof drains.

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My 2 cents:

Hybrid systems which incorporate asphalt-based low slope roofing with fleece backed thermoplastic capsheets are the BOMB baby. In fact, you may not have to do a tear off at all with this type of system. Which is a huge plus in the PRO column.

TPO and PVC membranes have heat welded seams. All of the penetrations incorporate membrane flashings that are also heat welded. These systems are freaking durable. Seaman Corp Fibertite and Sarnafil have PVC membranes that have been performing for over 3 decades now! I know of no asphalt-based system that can make the same claim. That’s the problem with asphalt-based systems - your project being a perfect example - they degrade, to the point (eventually) that the Owner has to either add another asphalt-based cap sheet or do a tear off. Not so good, and not so sustainable.

I encourage these hybrid systems when Owners have a better comfort level with the redunacny asphalt-based systems provide but also see the value in a cap sheet that just refuses to fail (and will require very little maintenance, comparative to asphat-based systems). For existing asphalt-based systems requiring major repair/replacement, fleece backed single ply membranes right over the top is such an elegant solution. These puppies can be hot mopped or cold adhesively applied just like their asphalt-based brethren. True!

Peter, I think you have yourself a particularly well informed client, and you should support him/her in pushing for this system. You get lots of benies in doing so (LEED among them). The best being a roofing system that will be there for the next 30 years, without having to go through this all over again in 10-15 using the same old tired asphalt-based membrane system(s).

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There are several mineral-surface mod bit cap sheet products out on the market that do meet the LEED requirements for a “cool roof”. Siplast was the first one I saw several years ago. I just saw Firestone’s last week. I understand there are others as well. There are also smooth-surface cap sheet products on the market as well. These have a factory-applied coating on them, but they simply do not seem like a good idea to me.

Existing building was originally built as a rice warehouse. Existing deck is concrete if I remember correctly. Lightweight insulating concrete (LIC) is being proposed to achieve slopes as well as add insulation values. Since nothing every dries out in Houston… LIC is used here some, but is more expensive and weighs more than rigid foam. Can’t remember why they did not want to use tapered insulation.

My client (the architect) did tell me last night that they might be revisiting the 1/8-inch per foot issue.

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Peter,

Soprema also has membrane cap ply with a reflective white reinforced film. I have used Soprema at MicroSoft campus on Redmond that met LEED NC; SS 7.2 Heat Island Effect Roof

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Interesting thread.
Ron, I agree with you regarding the hot-rubberized asphalt direct-to-deck membrane in PRMA application. Talk about a long life plus you can use it as a virtually dead-flat waterproofing application by Code. As to the single-ply salesman, please give it a break. Too many of us have seen too many single-ply failures to drink that kool-aid. Yes, there are some good single-ply membranes but youre still talking about a sheet of plastic about the thickness of a quarter protecting your building. Anything other than a good PVC, KEE or TPA, and I say keep walking. They have their place, but I still prefer a good 2 ply mod bit, and yes, there are many good reflective systems that are LEED compliant. My preference is Soprema, but there are other good membranes out there as well.

Method of application is critical in any BUR. If you’re using Type III asphalt you want to minimize the number of plies and the amount of asphalt. Traditional BUR were passable as 3-ply systems and preferred at 4-plies but failed at 5-plies because there was just too much asphalt. The plies would separate. Keep in mind that the asphalt was the waterproofing and the plies held the system together. For mod-bit systems, the intent when using standard asphalt is to heat-weld the mod bit sheets together, not glue them. You want a homogeneous membrane when the application is done. That’s one reason why torch applications are preferred; there are no ‘pollutants’ added to your modified bituminous sheets. Maintaining a fire-watch is no big deal if you use a reputable roofer. Using an SEBS mopping bitumen maintains consistency of the modified bitumen throughout the system and you can make an argument that you are adding to the waterproof envelope. Frankly, detailing using a torch is much easier and you’re not adding unnecessary weight. As long as you torch down the ‘cap’ sheet to the assembly, there are probably no negatives with using either method of installation as long as all 3 plies are mod bit (SBS preferred). Please do not apply a mod bit cap sheet to a standard BUR. Again, Ive seen too many failures, blisters, separations, etc. The two systems are not the same.

According to NRCA, applying anything over LIC requires a bond breaker. If you’re using mod bit, plan on using a vented base sheet mechanically fastened to the LIC and then adhere your roofing system. When using a single-ply membrane, NRCA requires that you use a slip sheet or separator sheet. NRCA suggests using a fully-adhered system so your separator sheet or board has to accommodate adhesion of your membrane. Please read the NRCA manual carefully if you want to use board insulation or overlay board over the LIC; think moisture plus gypsum, or wood, or polyiso. Also check out the venting requirements when using LIC over concrete.

There are manufacturers who will warrant single-ply membranes adhered over BUR’s. Then again, there are those who will tell you that TPO in the USA isn’t such a bad idea. I don’t generally let those people into my office, but that’s just my opinion.

Similar materials prefer being with each other. I’ve seen too many dissimilar materials with good warranties fail. Oddly, the warranties always seem to have disclaimers to let the manufacturers off. Bottom line, I’ve never seen a piece of paper keep a building dry.
Good luck. Hope you get that inch per foot.

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Peter:
its ultimately the Owner’s decision, so I would (with their permission) write the specs to price out the system that they want; and also price out the effective system (mod bit, or TPO) that seems sufficient for the project so that the Owner knows where their money may be going. or at the very least give them preliminary cost information that will help the decision. I’ve “converted” several owners in this manner to a more appropriate roof system.