If the roof is below a 2/12 pitch you can't put composition on it. If it is below a 3/12, you shouldn't.
The International Residential Code, or IRC, which used to be the Uniform Building Code, states that the lowest slope for comp is 2/12 but there are caveats. Two ply of 15 lb. felt must be used.
God forbid you use nail guns on a low slope. The nail has a compression seal when driven flush but air nailing always cuts the shingle.
I recommend using three tabs on low slopes and you can't see them anyway so what's the difference. GAF says you can use Timberline on low slopes but that's crazy. The second strip makes a hump like a second starter does on a reroof. Gravity cannot be defied.
GAF also doesn't want to reduce the exposure of the roofing but I strongly disagree. Even though blow off may be more likely because the self seal is higher up the lap, it has never happened to any of our roofs. But we hand nail anyway.
If you reduce a 12" tall shingle from a 5" exposure, which leaves 2" headlap (5" for the first course, 5" for he next, leaving 2" that tucks under the second shingle) to 4", you get three ply of roofing. (Four inches on the first course, four inches on the second, and four inches remaining instead if two.)
Besides, due to slower moving water and more ice build up, the low slope roofs should have extra waterproofing on all penetrations, valleys, and overhangs. That means using expensive StormGuard or other ice and water barriers.
Shut up and quit complaining. Your saving a fortune by not putting on a flat roof system that looks okay on a low slope. The cost of a built up roof with a color coordinated chat (gravel, marble, or lava rock) or an ugly modified bitumen, EPDM, or TPO is easily two to three times a low slope three tab with all the amenities.
I'd never shingle my home if it were below a three pitch. In fact, I'd raise the pitch.
Water needs encouragement to run and the less the pitch, the slower it becomes. At a 3/12 it is more of a walk and at a 2/12 it is no more than the three legged mosey at the retirement center field day event.
There are three way, and only three ways, to waterproof something.
One: Turn water with gravity. That's what shingles do.
Two: Seal water with adhesives. This may be caulking here and there or a monolithic flat roof system.
Three: Compression seals like corks. That what window glazing does. On glass it is a putty that compresses. Glass, being a frozen solid, cannot be caulked. The natural state of glass is liquid so the frozen glass still has a thermoplasmic migration. The caulking will be released by the movement of the molecules. It is slow but it is still there. That is why old mirrows distort. The glass is migrating. I must reference Monty Banner. He died in a sad way but he loved big words. Million dollar words. Despite his lack of higher education he strived to be of the chattering classes.
Exposed nails on the last piece of ridge or through the flashing on a galvanized stack pass through the metal and several layers of asphalt shingle. The sun never sees those shingles thus ultraviolet degradation (Monty didn't teach me that) does not destroy the compression seal. The darn home inspectors today like to show off and make us caulk those nails but it does more harm than good. Not only would they never leak but now the nail might be unrepairable if the stack needs to be replace. When NP1 caulking is used, the caulk is stronger than the metal. It will require forces stronger than the metal to separate the nail from the flashing.
These inspectors are espousing procedures that sound good but have no basis in reality. They must have learned roofing form Nietzsche.
Low slope roofs must lay completely flat if they are covered with composition. Any bulges will cause a loss in the down hill tangent. Wallah: leak city.
If the 15 lb. felt can't be laid right before the shingles are installed, then Shinglemate should be used. It has no wrinkle properties. Thirty pound felt is right out. It might wrinkle after you put on the roof. Plus 30 lb. is for amateurs who don't know how to roof. They count on it to keep the water out until their warranty has expired or they have left the premises.
Plus when water runs under comp roofing and on top of felt, a slim mold develops. It is a creepy white slim that is peculiar to asphaltic products.
Low slope roofs also need to installed very meticulously. The felt must go over the down hill side metal edge. The metal edge needs to be slightly larger so capillary action or water tension, take your choice here, does not encourage the water to run uphill underneath the starter course and on top of the metal edge.
With little gravity to push and a lot of slow moving water, the possibility that the water migh suck upwards between the starter and metal is great.
Have you ever taken two pieces of glass and held them vertical in a pot of water. As you try to pull them apart the water starts to climb upward between the glass. If you hold a string level and pour water on either end, the water will travel sideways toward the non-pour side.
Water, like electricity and people, takes the path of least resistance. And we all know the Borg said "resistance is futile." I love that stuff. A lot. Really.
Think of yourself as a water molecule traveling in a large herd of molecules. As you travel the trail of least resistance you find yourself being pushed around and squeezed out. If up is easier then so be it. How about sideways? You'll go where ever the least pressure is.
Low slope roofs have little of the stuff that make comp roofs work:gravity. Therefore "i" before "c" except after "c."
That's a weird rule.
This is not grammar folks. We're discussing physics and fluid dynamics. The test is tomorrow so pay attention.
You can break the "i" before "c" rule with the word "weird" but as long as this world exists and gravity is in play, we will need to determine your roofing system needs based on several needs, slope being one of them. The menu is more limited and Mother Nature will not give you a pass. It is what it is.
When gravity is gone, my warranties are void and it is judgment day but until that day passes, you must select the right type of system for your roof.
On a low slope roof a 50 year or lifetime roof becomes a no functional roof. Too thick. You need flat.
A roof is basically a skin. It follows the contours of the substrate. That's decking for the ones of you without the imagination to keep up. Thin roofing works better on a smooth surface with a low slope because it lays flat. If you pay for a better shingle, one that is thick and lumpy, it will interfere with the pull of gravity by causing bulges in the surface of the roof.
In my world slope is everything. In your world it is too but you just don't know it. That roof over your head is protecting everything you own and the health of your family. Don't blow it by wasting money on a better shingle on a low slope. If you want to spend, then buy a flat roof system.
Another concern on low slope roofs is the type of ceiling you have. Most really flat roofs have true cathedral ceilings. No ventilation occurs here. Reflective roofing, insulation for flat systems, and radiant barrier energy efficient decking systems play a much greater roll here.
If you have a Tectum deck, a system that uses the deck as the ceiling, the deck, and the insulation, there are real needs to concern yourself with. It is not considered a nailable deck but morons nail roofing right to it anyway.
If a flat roof system is not used a deck needs to be installed.
Near Plymouth Park in southwest Irving there are a lot of homes with Tectum. The centers on the beams are either three foot seven inches or random.
These homes need to be lathed and a radiant barrier deck needs to be installed vertically as the decking thinks the lathe is the rafter. Decking, neither OSB or plywood, is strong against the grain and it will sag roller coaster style like a patio deck of plywood installed parallel to the rafters.
I know this is hard to follow but I'll be glad to come out and use hand displays to make my point. Two years of Italian and six months of schooling in Roma, Italia, make me pretty good at gesticulating.
I'm looking at a roof in north Dallas near Richardson, off Hillcrest and Spring Valley, that has a vegetable bord deck. It is like Tectum. Non-nailable.
These decks have pretty good "R" value and if you put lathe and radiant barrier decking, the air space will pay for the cost in a short time. Have you heard of energy savings? If you haven't then you will soon be poor.
With the cost of utilities about to soar, excuse me here, through the roof, the money you don't spend here and now will be devoured relentlessly for many years to come. Plus you're killing bunnies. Get up and go green for yourself, your family, your posterity, your pocketbook, and forget about keeping the HVAC guy in work. The economy can be stimulated right here through me with all my green stuff.
If you let me I'll make your home comfortable and energy efficient. Why spending money here is the cheap way to go, long term.
Sorry about the rant. I believe this stuff.
A dollar spent on efficiency is a dollar invested in future efficiency.
Flat roofs are notorious for being wasteful of energy. They were quick to build and your are paying for that if you have one. Let me turn you into the envy of the barrio.
We can turn your home around and make it quieter, more comfortable, more energy efficient, and more waterproof.
We can make it even more beautiful with a low slope standing seam roof with photovoltaic additions to the system.
Imagine a post modern Frank Lloyd Wright low slope with lots of single pane windows. Put a radiant barrier deck with a giant solar collect roof on it and tell Oncor, TXU, Reliant, and all the others to leave you alone.
Iran would be jealous. They wouldn't need their peaceful nuclear bomb program if they had photovoltaic roofs that Israeli bombs wouldn't dare blow too smithereens.
I've always wanted to type that word. Mr. Spellcheck even left me alone even though he has poked me no less than fifty times to day for words that have married parents.
Ground control to Major Tom: We have met the future and it is Jon Wright Roofing.
I promise my next essay will be more down to earth, just where Major Tom will never go.
