How to ensure success with arch shingles on sub 4/12 roofs

Ironically there are a ton of sub 3/12 roofs here in the Pacific Northwest and customers want arch shingles and not torchdown. I’m actually preparing a bid now for a roof that has about 5 systematic leaks and is only about 15 years old. There was also some pressure washing done that probably didn’t help. What are some steps to successfully roofing these roofs in the 2/12 and 3/12 range? I was thinking a full layer of water and ice and the additional layer of tiger paw synthetic underlayment (mostly to meet code for the building inspector). I imagine overdriving nails would be especially bad. One of the guys thinks increasing the offset from 5-5/8” to 10” but I don’t like going against the manufacturer specs. We also offer a reduced warranty of 10 years and hope the buyers move and don’t transfer. Lol

Absolutely use ice/water shield and any kind of buffer layer above, tiger paw is overkill and overpriced for that application. You are just separating comp from ice/water shield to allow for any potential roof repairs and tear off down the road. Even a rosin slip sheet would probably work. If you don’t do this the future tear off will involve spudding bars (for you young uns these were used to remove tar and gravel).And yes, for sure increase your side lap distances. 2-12 is doable but only in certain circumstances; 12-15 ft. vertical spans without a lot of water flowing down from upper sections. Anything under trees adding debris to the roof would also be bad. I can provide proof that we have made many of these situations work and also show you pictures where it was done improperly and there was a water line on the plywood where every key way landed. I always love when the homeowner questions our extreme underlayment portion of the bid with “it hasn’t ever leaked the other way”. We then tear off their low slope roof and have to replace the whole deck, which never leaked into the house but rotted around all the nails and sidelaps.

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And by the way, we are also in the Pacific Northwest.

Sorry for the addendums but another thing about our region is Malarkey is available and has a uniquely tapered nailing area that is very well suited for low slope applications.


Use Grace Ice & water shield, use hot dipped galvanized or stainless steel nails, increase the offset to 8" or larger, lower the exposure by an inch or more per course.

In addition be extremely mindful to keep all nails at least 2" from any seam.

If you do all this the likelihood of the roof lasting as long as it should is greatly increased, it would be ideal to hand nail it also.