First and foremost is to make sure you have good, firm decking for the nails to grab; no soft spots or water soaked areas. Hopefully, when you have the roof re-shingled the roofer made sure this was the case by removing all of the old shingles as well as the old underlayment. How can you make sure the roof deck is in good condition if it cannot be inspected?
It should be noted that good practice is to have asphalt shingles installed over plywood or solid sheathing vs. solid boards. This is because the wood boards typically have dried an shrunk since installation and created gaps of half an inch or more. If a roofing nail were to be installed in the gap, there would be no fastening power (air is does not hold nails well) and more than likely the nail will be over-driven and tear the shingle, and possibly the underlayment, creating an area for water to enter. So while the roof deck in exposed, check to see what type of material it is. To correct this issue, the old deck could be removed and replaced with new, solid sheathing. Another option is to install sheathing directly over the existing boards. However, this last option can add quite a bit of weight (roughly 50-55 pounds per 4x8 sheet of 3/4" plywood), so verification the roof can hold that additional weight is essential.
Also, it is not good practice to have a new asphalt roof placed over an existing asphalt roof. The installers may tell you it will be fine, but in my experience homeowners are disappointed (except for the lower initial cost) with an overlay. Do the installers use longer nails when they add another layer? How can a proper inspection of the decking be performed to make sure it is not rotten or soft from an un-noticed roof leak? Are properly sized new flashing and drip edges being installed as if the existing shingles were underlayment? Additionally, will the new asphalt shingle manufacturer still warranty the shingles if they are installed over existing shingles?
Once you determine the roof sheathing is in good condition, install new underlayment. Properly installed underlayment is a good secondary water barrier. For roofs with a slope greater than 4:12, a single layer of 15# asphalt saturated felt (tar paper), with proper overlap and fastened using capped nails is recommended. In some areas of the country with a high probability of hurricanes 30# felt is recommended. The cost difference is minimal when compared to the overall project cost. An added precaution when fastening the underlayment to the deck is the use of plastic capped nails to reduce the likelihood of pullout. Yes, not as fast or cheap as staples, but if shingles happen to be blown off the use of this type of fastener reduces the likelihood of the underlayment being blown off.
Another option for an underlayment is adhesive backed bitumen (ice and water shield). This material is more expensive than felt, but once it is stuck to the wood, and itself on the overlap, it provides more protection if a shingle is blown off than felt. Other than the cost, a down-side to this material is when the roof needs to be re-shingled again, removing the sticky mess is more difficult than felt.
With good sheathing, properly installed underlayment with the eave drip edge under the underlayment and the drip edge at the rake on top of the underlayment, it is time for the shingles. First and foremost, in high wind areas, my recommendation is to not allow a roofer to install the shingles using staples. Either hand nail or use a pneumatic nailer set to the proper depth. Yes, there could be some complaining from the contractor that staples are fine and he’s been doing roofing for 20 years with staples and has had no problems. Just say “no” to staples in high wind applications. If, and that is a big IF, staples are installed at the right depth and both sides are flush with the shingle, they can perform as well as nails. However, during installation the roofer staples in a slight arc which reduces proper staple placement and one side of the staple is either set too far or not far enough.
Nails used for shingles should be galvanized (or stainless steel) and have a minimum head diameter of 3/8” and a minimum shank diameter of 12 gauge. They must be long enough to penetrate into the sheathing at least 3/4”. If the sheathing is less than 3/4” thick, the nails should be long enough to penetrate fully and extend at least 1/8” through the roof deck.
The next question is what type of shingle are you installing? Traditional 3-tab or architectural? Architectural cost more, but have an advertised life longer than that of 3-tab. It just depends upon the look you are trying to achieve. In either case, six nails should be used. Some installers only use four nails per shingle because they do not deal with high wind areas or they want to save nails and/or time. Nail placement is very important. On both 3-tab and architectural shingles they should be installed just below the tar line. For 3-tab shingles the nails should be installed on either side of the gap and away from the middle of each individual shingle so that the gap in the shingle above doesn’t expose the nail head. With architectural shingles you don’t have to worry about nails poking through the gaps because, unlike 3-tab shingles, there are no gaps. As a result, the idea is simply to space out the nails evenly across the width of the shingle, being sure to keep around 1-inch in from the sides and ensure the nails don’t interfere with the tar line or appear below the level of the shingle which will rest on top of the nails.
Best practice is to install a starter strip. This can either be a dedicated starter strip or the type of shingle you are using with the tabs cut off just below the adhesive line. Asphalt roof cement should be used to connect the starter strip to the first course. As the roofer installs each course, three spots of adhesive should be installed under each tab. Yes, this will add cost and time, but this step significantly reduces the likelihood of the shingles being blown off during a high wind event.
In conclusion, keeping shingles on a roof in high wind areas requires:
- Sheathing in good condition (no soft or rotted spots).
- New 15# underlayment (minimum) installed with capped nails.
- Eave drip edge under the underlayment.
- Rake drip edge over the underlayment.
- The use of 3/8” head (minimum), 12 gauge (minimum), galvanized or stainless steel roofing nails.
- Use of six nails per shingle located in accordance with manufacturer.
- Ensure the nails are set properly.
- Use of a starter strip.
- Use of asphalt roof cement under all tabs.