Roofing to Resist Wind and Rain Damage

Shandy Heil, Moore, David, Skinner, Patricia, Attaway, Denise

roof cut-away showing elements for a strong roof

Whether building a new roof or repairing an existing roof in South Louisiana, there are steps you can take to improve your roof's resistance to hurricane winds and driving rain. Here are the essentials for selecting and installing a roof and roofing components.

  1. Decking: Roof decking is more than a place to attach shingles, tile or metal; it strengthens the roof and helps transfer wind loads to the ground. If replacing small sections, use the same thickness as the original. If replacing large sections, 1/2-inch plywood is good; 5/8-inch plywood is better. Fasten deck to rafters with 2- to 2-1/2-inch screws or 8- to 10-penny galvanized nails (ring-shank are best). Fasten roof sheathing along the roof corners, ridges and eaves using additional nails four inches apart. (Wind forces are concentrated at these points.) Be careful not to use too many nails or screws; they can split the plywood and weaken your roof. Sheathing not located on the edges or ridges of the roof should have eight-penny nails or #8 screws every 6 inches. (See also Strengthening the roof-to-wall connection, below.)
  2. Metal drip edge at eaves: Fasten it directly to the deck with nails that are the same metal as the decking strip (steel or aluminum). If underlayment is already in place, slip the drip edge under the underlayment at the eaves and trim the underlayment so 1 inch to 1-1/2 inches of drip edge is exposed. The shingle starter strip will adhere to this drip edge, so its attachment is an important part of keeping the shingles on the roof. (See Starter strip, below)
  3. Bitumen tape: For maximum water resistance apply 4-inch bitumen tape over seams in the roof deck. Decking sheets must be installed with an 1/8-inch gap between them to allow for expansion and contraction. The tape will keep these joints from leaking if the underlayment blows away. Do not use duct tape.
  4. Underlayment. Choose a synthetic underlayment -- several brands are available. Synthetic underlayment is more wind-resistant than traditional "tar paper" felt and will give better protection for your home if the roof loses some shingles in a storm. Synthetic underlayment also holds up better during construction, so it's not such a problem if there is a delay between putting down the underlayment and getting the roofer to your job site. Underlayment is the primary rain-shed for your roof. Roofing (such as shingles or metal) protects the underlayment from wind, debris and solar degradation.
  5. Metal drip edge on rakes: Install the drip edge over the underlayment along the rakes. On the rakes, drip edge servers as an anchor for the underlayment for wind resistance.
  6. Starter strip: Use starter strip along eaves and rakes. Starter strip is a specially designed shingle that has strong adhesive patches placed where it will stick to the drip-edge material. Do not use regular shingles cut in half and mounted upside down for this purpose. Starter strip should extend beyond the drip edge about 3/4 inch.
  7. Shingles: Select shingles with a rating equal to or greater than the basic wind speed of your location (see Louisiana basic wind speed map). Laminated or "architectural" shingles provide more wind resistance than tabbed shingles. Install shingles following the manufacturer's high-wind installation guidelines. Generally, high-wind installation includes using six nails per shingle rather than the more typical four nails per shingle. Proper nailing procedure is also important; nails should be driven straight into the decking (not leaning) and should penetrate the back side of the roof deck at least 1/4 inch. Heads should be flush with the surface of the shingle, not overdriven. Improper nailing accounts for many of the wind-related roofing failures.

Strengthening the roof-to-wall connection

When you're at deck-level in repairing the roof, you may want to use this opportunity to strengthen the connection of the roof structure to the wall. Here's how:

  • Remove the bottom row of roof sheathing at the eave lines.
  • Check to see if the roof is fastened on the top of the exterior wall with metal hurricane straps or clips that provide the proper measure of strength and safety. (The common practice of toenailing the trusses or rafters often is not sufficient to hold a roof in place in high winds.) These clips or straps are usually difficult to see from the attic because of insulation.
  • If there are no clips or straps, install them where each roof rafter meets the exterior wall. Use straps that wrap over the rafter for best protection.
  • Re-fasten the roof sheathing.

Many of the recommendations here are not standard building practice in Louisiana. Consumers can be good advocates for themselves by understanding these principles and sharing them with their roofing contractors. More details are provided in the LSU AgCenter publication "Build Safer, Stronger, Smarter: Add Wind and Water Resistance when Repairing your Roof."

Video Demonstration - Installing Underlayment and  Drip Edge

9/19/2008 1:24:09 AM
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