How to build a roof to withstand extreme weather
The damage caused inside a home from a storm-damaged roof is often much more expensive to fix than the roof itself. By taking extra care and paying close attention to some very important details, you can help ensure the roofs you install stand up to most extreme weather events. Here’s how to build a roof to withstand extreme weather.
To install a roof that is storm resistant with new construction or when repairing an entire roof structure that has been damaged by extreme weather, keep the following in mind:
• Moderate pitch – A hip roof is the most wind-resistant roof design (as opposed to gabled), and those with a pitch between 4/12 and 6/12 withstand strong winds much better than steeper roofs.
• Advanced framing members – Not only are pre-fabricated rafters faster and easier to install than traditional rafters, but they are almost always stronger.
• Hurricane straps – Use hurricane straps that wrap over the top of a truss or rafter and tie to a load-bearing wall or bracing. Strap each part of the wall assembly together or anchor each part from the foundation to the roof to provide a continuous load path, effectively connecting the roof to the foundation.
• Secure decking – Use roof sheathing with a minimum thickness of 19/32 inch for added strength and use 8d ring-shank nails spaced 6 inches on-center (never use staples). With trusses spaced 24 inches on-center, make sure you use “H” clips to gap the sheathing and support it at mid-span
How to build a roof using a systems approach
Most roofing warranties today require use of an entire system, meaning you have to use underlayment, starter strips, flashing and ventilation systems from one manufacturer or in partnership with a third-party manufacturer. Most roofing system components are manufactured specifically to work with each other and are clearly labeled with which particular system they should be used with.
Using a mix-and-match approach to installing roofing components will not only void the warranty, it could cause premature failure.
How to build a roof
- Close the seams – Tape sheathing seams with a self-adhering asphalt/rubber or modified bitumen tape at least 4 inches wide. This allows the roof decking to act as a water and wind barrier in case the coverings blow off in high winds. You can also apply self-adhering underlayment over the entire roof deck, just make sure it complies with the roof system you are using and provide attic ventilation or install rigid insulation over the decking to avoid moisture problems during winter months.
- Properly install underlayment – Again, use a product that is recommended or approved with whatever system you are using, and install it according to manufacturer specifications using the approved fasteners.
- Use starter strip and flashing – Don’t use an upside-down shingle as a starter strip. Apply drip edge to all roof edges – including the rake – and overlap asphalt and fiberglass shingles on the drip edge by ½ to ¾ inch. Install flashing (either step flashing or continuous, depending on the roof) whenever there is a change in surface plane or cladding material.
- Adhere to proper fastener schedules – Mandated fastener schedules tend to change with different products and vary between wind zones, so pay close attention to these details and consult manufacturer specifications. Often, increasing the frequency of fasteners from four to six per shingle will also increase a shingle’s wind-resistance rating.
Installing proper roof and attic ventilation will not only decrease utility costs (cooling the attic and decreasing air-conditioning loads), it will also provide maximum service life for the framing and roofing materials and help the roof system perform at an optimum level.
Baffled vents provide good airflow while preventing air and wind-driven rain and snow from entering the attic. Extend fascia boards below soffit to create a drip edge so buffeting winds don’t drive the rain across the soffit surface and into the eaves of the building. Install plastic baffles between rafters or trusses at the top of the wall and extend up the rafter at least 18 inches.
This will keep attic insulation away from the vented soffit and roof decking, improving airflow and providing a barrier and drainage plane for rain.
—By Rob Fanjoy