flats and low slopes

 
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Old 01-08-2012, 03:05 PM   #1
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flats and low slopes


What are you guys doing for flats / low slopes?

We are putting down either PVC or TPO unless it's visible, then we use mod bit peel and stick base/cap sheet (mule hide). I don't like those under 1:12 even though they are rated to 1/4:12. EPDM is out because of glued seams, and shrinkage, so I'm left with TPO or PVC.

I like PVC personally, TPO hasn't proven itself over time. The only advantage I've heard is that it's easier to deal with from a supply perspective when it comes to small pieces since we can go to the supply house and buy what we want rather than having to order and have shipped.

I've got some questions about that though. Are they compatible with asphalt shingles? I've heard that it's best to go three feet up the slope (where a flat porch meets a sloped roof), but why so far up if you're punching holes all in it anyway? Will the asphalt eat the membrane? I've heard of using coil stock under shingles, and over membrane ...

Biggest question, and this may be for the commercial guys - but what's the best attachment method - mechanically fastened, or fully adhered?

I saw something about mechanically fastened being better for wind uplift (my thought was REALLY???), and I've heard it's better if it's sloped so it doesn't slide down, but I use mod bit for that anyway. Personally I'd think fully adhered would be better, because wind could potentially balloon the roof up away from the surface otherwise, and make it flap to pieces - like waves on the ocean.

Other than speed of install, is there an advantage to mechanically fastened membranes?

We've been screwing down ISO board, and mechanically fastening the membrane, and going 3 feet under the shingles fyi ...

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Old 01-08-2012, 05:29 PM   #2
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Re: flats and low slopes


We are using TPO,PVC,MB & BUR. We mop or torch our modified. In my area, epdm is not used at all. Miami has plenty of roofs that pond & the major manufacturers have created "ponding water specs" for South Florida. I've put them down a couple times but they get real expensive. Obviously, if possible, we taper our roofs.

As far as single-ply, I perfer PVC. We enjoy working with it and find it welds easier.(fewer cold welds) The suppliers carry both PVC & TPO but have more TPO in stock. We bring our PVC in direct & always keep extra on hand. It has worked well for us.

When tying in to a slope, I always check the manufacturers book & do it their recommended way. Usually, membrane through & up the break, a piece of sheet metal fastened over it with a slight kick that gets caulked & then the shingles or tile underlayment on top of the metal. It's best to keep your membrane out of contact with asphalt based products. PVC more so than TPO.

The attachment methods we use are based on the substrate & specification.
Wood and metal decks we mechanically fasten. Concrete, gyp & tectum we fully adhere.

The highest uplifts I've seen are membrane fully adhered direct to the Concrete deck. When a deck is 1.5/12 or greater we have to back nail them because they will slip in our area. If the deck is combustible it's a whole different ball game as we need a class a UL. We usually add Dens Deck.

Mechanically fastening PVC/TPO, when applicable, is more cost efficient for us both labor & material wise.

If you give me specifics on the type of deck,manufacturer and attachment method, I can get you the wind uplifts that have been submitted by the manufacturer for the High Velocity Hurricane Zone.

Do you have to use wind uplift & design pressure calcs in your area?

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Last edited by Miami Roofer; 01-08-2012 at 05:40 PM.
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Old 01-08-2012, 06:03 PM   #3
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Re: flats and low slopes


We're not in a high wind zone, I was wondering because some spec (forget which one, I think it was a global insurance data sheet) basically said that to get the higher wind rating, you had to mechanically fasten the membrane - which seems absurd. We screw down iso board and go over that.

We're using Duro-Last PVC, and I don't know which TPO we're using offhand. The Duro-Last has to be ordered for the job. I'd rather just get 10' goods and weld the seams personally.

We don't sell EPDM at all. From what I've seen, it's junk.
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Old 01-08-2012, 06:37 PM   #4
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Re: flats and low slopes


I think you're talking about FM Global class 1-90. If you are, that applies to metal decks. At one time, there was a debate over how they reached their opinion and determined that fully adhered systems couldn't meet the classification even though fastener destiny was increased in the corner and perimeters. Guess that is why it's their spec.

I figured your not in the HVHZ, but all our roofing systems must meet certain wind calcs. If you wanted, I would have let you know specifically what those numbers were and you'd know the failure point of certain roof designs.

I only worked with D/L a couple times so I know where your coming from. There's a bit of a learning curve with it. However, I've seen some really good & efficient crews install it. Guess it would be a bit of a headache if there was a mistake and you were short material. But, there is a + & - to everything.

Personally, I've seen some nice EPDM jobs that lasted-(though not locally). I don't believe it's junk at all. However, it's not really for the South Florida climate & won't last as long as most other systems do down here.

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Last edited by Miami Roofer; 01-08-2012 at 06:40 PM.
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Old 01-08-2012, 07:28 PM   #5
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Re: flats and low slopes


"EPDM is out because of glued seams, and shrinkage, so I'm left with TPO or PVC."

I've only seen shrinkage issues with the cheap.045. I only bid ans use .060 and thicker reinforced.


"I've got some questions about that though. Are they compatible with asphalt shingles? I've heard that it's best to go three feet up the slope (where a flat porch meets a sloped roof), but why so far up if you're punching holes all in it anyway? "

I go up 3' and leave 1'-2' exposed. I also add a sacrificial sheet to the break.
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Old 01-08-2012, 08:04 PM   #6
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Re: flats and low slopes


We will install what ever the customer wants. Mostly it boils down to EPDM TPO and one form or another of BUR.
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Old 01-08-2012, 08:27 PM   #7
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Re: flats and low slopes


I like the sacraficial sheet idea. Somebody I think in CT? said to run it up the slope, and put metal over it to prevent contact with asphalt.
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Old 01-08-2012, 08:45 PM   #8
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Re: flats and low slopes


They say to use metal. It only seems to matter where Grace and some other I&W material is in contact. As a rule, I'm using slate and just run it to the slate and slate over it. No I&W or felt.
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Old 01-09-2012, 09:14 AM   #9
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Re: flats and low slopes


I spec modbit for 99% of the roofs I replace. Over a steel or wood deck the deck overlay (gypsum or fiberboard) is mechanically fastened according to FM 1-90 and then everything else is fully adhered (mech fastened gypsum, mopped vapour barrier, mopped insulation, mopped fiberboard, mopped base sheet, torched cap). Sometimes we use adhesives. Over a concrete deck we do the same minus the deck overlay.

Low slope prefab buildings get a fleecebacked EPDM fully adhered. I've done a couple ballasted roofs with EPDM and they do stretch after 10 years, but that is the nature of the beast.

In terms of attachment method... it depends. Fully adhered is more expensive, but gets a better bond between surfaces (unless there is delamination). You dont have to worry about bubbling or fastener pullout. Mechanical fasteners can pull out and will conduct heat if they are put through insulation, but are cheaper/easier. You also don't have to worry about smell from adhesives or asphalt.
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Old 01-09-2012, 01:17 PM   #10
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Re: flats and low slopes


Great Thread guys!!!!

I am a flat roof guy and hate to hear you say that EPDM is out. What a great, long lasting product that has proven itself over the years. Definitely not Junk, in fact I have seen sheets from improper formulations that have lasted longer than “Good TPO’s. Most residential flat roofs are relatively small. I have used Non reinforced EPDM (60 mil) many times for this application. Shrinkage isn’t really the problem it used to be 10-15 years ago but we will solve that anyway by fully adhering it. Normally I would say Reinforced EPDM is the way to go for any application, but if you don’t have skilled laborers to install tape, you can order non-reinforced EPDM in wider rolls and avoid seams all together. In theory this roof will never leak. I would be confident in any climate the EPDM will hold up 30+ years and 50 wouldn’t surprise me. So I would actually recommend this system over a Dens Deck Prime cover board.

Personally I avoid TPO’s but there are some good ones out there I would choose before Durolast. Durolast seems to be extremely susceptible to hail so consider that in your area and ALWAYS use a HARD cover board below it, not just insulation.

When I designed roofs, I personally always use a metal separator between products in this detail. When designing for snow regions, I would specify 12” butyl tape below the metal as a self-sealing flange to nail through. Its expensive but avoids the inevitable headaches of winter leaks in these areas.

Without knowing the exact makeup of the system you are installing, I will open myself up for comment by saying a properly installed fully adhered roof is a better system for wind uplift. I say properly installed because I have seen many partial blow-offs of fully adhered systems and none of them were installed correctly. I have been involved previously with full scale wind uplift testing (12’ x 24’) and have come to the conclusion that fully adhered is slightly better for wind, mostly for the reason you suspected, Mechanical fasteners can allow the membrane to inflate, flutter, tear… But for the most part, in most regions within the united states, a properly installed Mechanically fastened roof system, is more than adequate to resist uplift pressures. Believe it or not the most common means of failure on a fully adhered roof is the fastener plate pulling through the insulation. Pretty neat, one more argument for using a dens deck cover board to help resist that.

I was involved mostly in commercial design so I am unaware of your residential code requirements but I designed to the specifications called out in ASCE-07 (American Society of Civil Engineers – Components and Cladding) as required by IBC. These requirements are slightly less stringent than FM’s requirements. (FM is Factory Mutual, nothing more than an insurance company who over-engineers to prevent loss) ASCE -07 also include enhancements at the corners and perimeters but keep in mind, recent studies of real world wind events are indicating these past requirements are not adequate. Keep your eyes open for more testing on the way.

All this stated, I worry more about Nails. Most of the wind damage I see as far as full and partial blow-offs go, are a result of blocking or metal fascia installed with nails which pull out or improperly nailed structural members. Threaded fasteners do wonders in wood blocking.

Also, Mod Bit is my Top Choice but rarely recommend it unless I know the people installing it are confident, and consider cold adhesive. It is the Future of built up roofing.

Let me know if I need to clarify anything...



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