Roofing question...shallow pitch underlayment question.

famous_worldMarch 9, 2009

Part of our home has a very shallow pitch. It's just steep enough for shingles though. Code is to 1/2 lap(?) with 30 lb. felt and is the absolute minimum required for a shallow pitch. One contractor said that he would heat seal rubber with a torch and another said that they would apply a peel & stick product. Which of these three methods would be the best? The peel & stick man said that the torch sealed rubber was an inferior product and had a manufacturer's warranty of only 10 years as opposed to the 30 year shingles that would be applied. Would that matter if this product is underneath he shingle?

Any experiences? thank you!

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Peel and stick would be your better bet, imo. W.R. Grace makes a great product.

Here is a link that might be useful: W.R.Grace

    Bookmark   March 9, 2009 at 7:25PM
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Minimum pitch for shingles is 2&1/2 by 12 pitch. That means the roof rises 2&1/2" for every foot of length.

The reason for that is because water will wick up under shingles normally and less pitch equals more wicking distance. Wayer under even the open edge of shingles can be a problem long term.

Which of the three you mentioned depends on how much annual rain and accompanying wind your area gets. The more rain/higher wind speeds there is, the more underlayment protection you need to reduce possible wicking problems. Freeze/thaw cycles also play a part in that as well.

The kind of shingles is a factor. Single thickness or multiple thickness, for instance.

The roof sheathing needs to be in excellent condition---the best underlayment over bad sheathing is just as bad as incorrectly applied roofing over perfect sheathing.

In my area---Kansas City---the optimum roofing seems to be architectural shingles(multiple thickness) over 30 pound felt with a membrane applied to the last 4 feet of the roof from the edge up. That membrane is referred to as an ice dam membrane as we get a lot of thaw/freeze cycles.

I do not know if the preferred membrane is self stick or not.

In your case, if you live in the area I do or farther noprth, you need membrane over the entire roof. I don't know which is better---there are going to be nail holes in any layer.

    Bookmark   March 9, 2009 at 7:33PM
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To answer the previous post: I live in central florida. And we plan to re-roof with architectural shingles. I'm not sure on the exact pitch, but was told by all 6 estimators that it was the minimum pitch allowed to use shingles (code), and that the underlayment would be different than that of the rest of the roof. A peel and stick man (for lack of a better title) mentioned that the membrane will seal around the nail? And even if the nail rusts it would close over the nailhole? True?

I understand the purpose of the underlayment, but wonder which would be the best.

    Bookmark   March 10, 2009 at 9:07AM
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In NY they are not allowed to torch down roofing membranes anymore. They have to use adhesive.
My father in law had one torched down on his garage roof. He had to make a lot of phone calls after the firemen left.
I'd go with the peel and stick myself. If you go with the other, make sure they have a hose at the standby.

    Bookmark   March 10, 2009 at 11:05AM
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You should never use a building code to design a roofing system. If you want advice about your roof you should tell us the roof pitch and the nature of the substrate and the climate.

WR Grace makes a self-adhering modified-asphalt shingle underlayment called Ice & Water Shield and it is superior to its imitators. It will seal the nails that are driven through it. It will adhere well to new bare plywood or OSB but if the substrate is more porous (like concrete) or rough, you should use their WB Primer first. It also makes a butyl based underlayment (Grace Ultra) for very hot climates (usually high altitude desert).

There are also peel & stick sheet roofing materials that overlap 50% which is why you need to be more specific about what is being proposed.

    Bookmark   March 10, 2009 at 12:28PM
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Florida Building Code is the same 2006 International Residential Code used in nearly all 50 states.

The 2006 IRC Code allowed minimum slope for asphalt shingles is 2:12.

No shingle manufacturer, however, allows their products to be installed on less than a 2 1/2:12 under any circumstance, and local wind velocity conditions may further limit shingle use.

The problem with either the peel-and-stick membrane or a 'torch-down' roof is that both only have an equal 10 year warranty.

Installing WR Grace Ice and Water shield also is NOT and adequate underlayment for shingles in any circumstances where the roof slope for use with asphalt shingles is less than allowed by Code or the manufacturer's installation instructions.

For low-sloped roofs in high velocity wind locations, you need to consult your local Code authority and local engineers for proper alternative roofing solutions.

Failing to do so in areas prone to hurricane winds may void your insurance.

Talk to your local Code and Engineering professionals fro proper solutions in your locale.

    Bookmark   March 10, 2009 at 9:49PM
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It is helpful to actually read what shingle manufacturers have to say about this subject. CertainTeed's asphalt shingle installation instructions define a "Low Slope" roof as between 4:12 and 2:12 and recommends using an underlayment over the entire roof of (1) one layer of self-adhering "WinterGuard" (its Ice & Water Shield imitation) or its equivalent (Ice & Water Shield, etc.), or (2) two layers of 36" wide felt shingle underlayment lapped 19". The CertainTeed warranty simply requires that these instructions be followed.

It is also helpful to read what the NRCA Roofing and Waterproofing Manual has to say about this subject. The minimum slope for asphalt shinges is 2:12 and the required underlayment is 15# or 30# felt lapped 19" or a self-adhering polymer modified bitumen sheet that complies with ASTM D 1970. It should be no surprise that all shingle manufacturers use the NRCA for their installation standards.

It should also not be a surprise that the IRC minimum standards are the same as the NRCA Roofing and Waterproofing Manual since the NRCA members essentially write that section of the code. Because this manual costs $350, non-professional designers usually rely on the limited standards contained in the IRC or manufacturers' instructions.

    Bookmark   March 11, 2009 at 9:27AM
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Out here the code is 3/12 for a minimum pitch any composition shingle. Because we are in a high wind area, it's simply not a good idea to install them on even a 3/12. Most of the pitches in our valley are 4/12 and the old three tab shingles roll up and blow away. It's good to be a roofer here. I would suggest skipping having shingles in that area that you probably wont see any how with that low of a pitch and have a built up or elasomeric encapsulted roofing installed in it's place, especially if your area is prone to high winds.

    Bookmark   March 11, 2009 at 11:26AM
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I phoned both GAF and Tamko and both said that their products should be applied over a double layer of felt (30 lb. is code here). And the minimum pitch is 2:12. Neither company mentioned a peel and stick product and when I brought it up, neither said it was necessary.

I also spoke with code enforcement and their requirements are exactly what the manufacturers require.

The substrate is 1x6 decking. I wonder if peel & stick would be a moot point if 100% contact is necessary? Hmmmm...a call to the peel and stick makers tomorrow morning.

    Bookmark   March 11, 2009 at 10:49PM
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famous world, you talked to the wrong person at GAF.

GAF makes their own version of WR Grace's "peel & stick" Ice & Water Shield called "StormGuard" for use where wind blown rain is an issue. Otherwise, it recommends the use of their "DeckArmor" vapor permeable synthetic (polypropylene) underlayment rather than organic asphalt-saturated felt.

GAF/Elk has a strange, out-of-touch approach to asphalt shingle underlayment. The company insists that underlayments must be "breathable to allow moisture to escape from inside your attic to help prevent structural decay mold and mildew." The company "does not recommend the use of non-breathable roof deck protection under its shingles." Since modern roof systems are required to be ventilated by other means, this approach makes little sense but I suspect it helps them sell a lot of their Typar-like synthetic underlayment.

However, GAF/Elk makes an exception to their breathable underlayment rule when attic ventilation is provided by other means and says it's OK to cover the entire deck with their non-breathable "peel & stick" StormGuard Leak Barrier. (The granule coated version is called "WeatherWatch" and "UnderRoof"is fiberglass reinforced for use under tile.)

Their sales brochure says:
"The use of full deck coverage with Weather Watch®,
StormGuard®, and UnderRoof Leak Barriers under fiberglass asphalt shingles provides a major advantage in protecting the roof and building in severe wind and hail events. How? Since they are solidly adhered to the roof deck and self-seal around each nail, leak vulnerability is significantly reduced from causes including wind driven rain and lost or damaged shingles. Simply, these Leak Barriers provide an unsurpassed, sound, and secure secondary roof."

For a 2:12 roof slope you should use whatever brand of "peel & Stick" waterproofing membrane you prefer (nothing can match WR Grace products IMHO) rather than rely on double coverage of a loosely applied conventional underlayment. This is because for a low slope shingle roofing system exposed to cold or wind driven rain, it is essential to seal the nail holes. It's the cheapest insurance you'll ever buy.

These membranes should be 40 mils thick, aggressively tacky, and reinforced with a strong polyethylene film so there should be no problem installing them on dry board decking in suitable weather. Anywhere an extra good seal is desired the membrane edge should be rolled with a small steel hand roller.

Here is a link that might be useful: GAF WeatherWatch/StormGuard info

    Bookmark   March 12, 2009 at 7:57AM
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Good to know. I can't believe that the GAF guy I spoke with didn't mention any of the products you mentioned...even if only as a sales pitch? And I spoke with someone in their tech dept. Crazy.

And ventilation is a non-issue as the shallow pitched roof in question is a carport w/ decking exposed underneath.

I'm going to cal Grace today. It seems like the best situation.

    Bookmark   March 12, 2009 at 8:14AM
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Grace will only tell you what you already know. I deal with them on a regular basis and they know how to waterproof a building. They believe all shingled roofs should have full coverage of Ice & Water Shield regardless of the roof slope and that is rapidly becoming the conventional wisdom for design professionals and builders in the Northeast. Many custom builders include it whether it is specified or not. One recently insisted on using it on a 12:12 pitch roof in Maine.

    Bookmark   March 12, 2009 at 10:48AM
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If you have room for the extra height, you might consider "skinning" over the 1x6's with an exterior grade plywood. I would go with 5/8", 7-ply ccx. CCX has a relatively finished on one side surface that will give you a good grip with the W.R. In this case you would put the finished side up.

    Bookmark   March 12, 2009 at 11:11AM
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You're over-thinking this detail.

If the boards are level enough for modern fiberglass reinforced asphalt shingles they are certainly level enough for Ice & Water Shield underlayment. The major beneficial feature of this material is that it seals the roofing nail holes and seals to itself at the laps. How well it adheres elsewhere is a side benefit that will only matter if you are ever in a hurricane. How much wind uplift resistance do you think stapled double coverage felt underlayment provides?

I have a sample of this stuff from the 70's and the two lapped pieces are still stuck together. If you ever put the two sticky sides together you will not be able to separate them without tearing the polyethylene sheets away from the asphalt layers and that is quite difficult to do.

You can run this stuff vertically, lapped the wrong way, or forget to remove the backing paper and it will still perform better than any other underlayment.

Use the real Ice & Water Shield or make sure any substitute is 40 mils thick with a smooth polyethylene top sheet. This is the same material as Vycor 40 but with a non-slip surface. You still want to be careful walking on it when wet.

    Bookmark   March 12, 2009 at 3:26PM
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Good point, as usual, M.A. We used protecto wrap for flashing out our windows, sill pans and door openings and it's pretty much a one shot deal. If you allow it to fold back under itself, you pretty much aren't going to get it apart. I stand corrected on the "skinning" part of the show!

    Bookmark   March 12, 2009 at 7:34PM
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"Because this manual costs $350, non-professional designers usually rely on the limited standards contained in the IRC or manufacturers' instructions."

Doesn't matter if you have a $350 NRCA roofing manual or a $49 used IRC Code Book if one cannot read or one has no practical experience in knowing that the shingle manufacturer's won't permit you to install their products on slopes lower than permitted by the manual or the Code.

Shingles on a 2:12 or lower slope are prohibited by manufacturers and by building codes....and designing roofs with shingles on roofs of slopes of 2:12 or less goes against "accepted engineering practice".

And why so many roofs designed by 'architects' exactly the way mightanvil suggests simply get refused for building permits. Period.

Just refused one.

The 'architect' still does not understand 'why'?


    Bookmark   March 12, 2009 at 9:47PM
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You originally said "No shingle manufacturer, however, allows their products to be installed on less than a 2 1/2:12 under any circumstance..."

Now you're claiming the manufacturer limit is 2:12? Better late than never.

    Bookmark   March 13, 2009 at 9:19AM
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Why do you keep maligning architects? I would think that someone who reviews the designs of professionals for code compliance would have more respect for the services that architects provide - like a critic might respect an author. Living to prove others incompetent can't be much of a life.

    Bookmark   March 13, 2009 at 12:19PM
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please comments on it

    Bookmark   September 26, 2013 at 7:33AM
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please comments on what?

    Bookmark   September 26, 2013 at 10:10AM
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I am planning a year round sun room addition to the back of my house (colonial), in NJ. The roof proposed is a slope of 4/12, and the limit on the room width is 16' because the peak of the roof is at the bottom of an upstairs window. I would like to go to a very slightly lower pitch to accomodate an 18-foot wide room without encroaching on the aforementioned window.
Does this require any special underlayment for the shingles? Do I need double felt or self stick membranes I have read about on this blog?

    Bookmark   November 2, 2014 at 3:49PM
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Keep the 4/12 pitch. Frame around the window and install EPDM roofing there and tie it into the shingles.

    Bookmark   November 2, 2014 at 5:33PM
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