Strengthening an Existing Roof

When you're building a new home, or putting on a new roof according to the building code, the roof will be built to resist the winds prevalent in the area. However, an existing roof can be strengthened even when you're not reroofing and often without disturbing any portion of the home that would be visible to the occupants of the home.

Here are some roof-strengthening techniques that don't require altering the home's interior.

Strengthen roof sheathing attachment to rafters using adhesive

Improve the uplift resistance of the roof deck from the attic without removing the roof covering.

  • Using a caulking gun, apply a 1/4-inch bead of wood adhesive along the intersection of the roof deck and the roof support element (rafter or truss chord) on both sides. Make sure the adhesive is in contact with both the deck and roof support elements.
  • At places where you have limited access to each side of the roof support, such as the gable end of the house, use quarter round pieces of wood that run the full length of the roof support. Apply the adhesive along the two adjacent sides of the wood block. Press the wood pieces onto the intersection, making sure the adhesive is in contact with the deck and roof support elements. The wood pieces can be tack-nailed or clamped in place to ensure good contact between the surfaces.

Caution: Attics are typically tight, enclosed areas with poor ventilation. When applying the adhesive, be sure to follow the directions for proper application and ventilation.

According to static pressure tests, using the wood adhesive can increase the wind uplift resistance of the plywood roof sheathing by as much as three times the conventional method of securing the sheathing with nails. Various wood adhesives are available at local hardware and building supply stores. In particular, look for products which are certified as AFG-01. Ask your local hardware expert if other products are available that could provide the same strength and properties as a wood adhesive.

Brace gable-end walls

If your home has gable-end walls, those end walls take a tremendous beating during a hurricane. If not properly braced, they can collapse, causing significant damage. However, gable-end walls are easy to strengthen and deserve to be a high priority on your retrofit list.

Typically, gable-end trusses are directly attached to the top of gable-end walls. The bottom of the truss must be securely nailed to the top of the wall and braced to adjacent trusses. This prevents wind from pushing or pulling the gable end at its critical point, where the gable truss is connected along the gable wall. Without adequate bracing, the end wall may be destroyed during hurricane winds.

To secure your gable-end wall, fasten 8-foot-long braces to the bottom chord of the gable truss and the adjacent trusses with 16-penny (16d) nails. The braces should be perpendicular to the truss, spaced at a maximum of 4 feet on center. In addition, be sure to tie back the gable truss with at least one 8-foot-long brace along the ridge of the roof to several of the interior trusses as shown in the detail.

Strengthen the roof to top of wall connection - decorative brace

A Clemson University civil engineering graduate student and his adviser have developed a structural bracket with decorative molding that strengthens a home, making it more likely to survive wind storms. The system is unique because existing structures can be easily and economically upgraded. Unlike conventional retrofitting techniques, this system does not require removal of existing finish materials. Once installed, the system is easy to inspect and actually improves the appearance of the structure.

The developers, Dr. Ed Sutt and Dr. Tim Reinhold, have received a patent on the technology. The High Wind Bracket and other hazard-resistance features were demonstrated at 113 Calhoun Street Center for Sustainable Living, a South Carolina Sea Grant project.

Strap the roof to top of wall connection (requires cosmetic repairs)

Areas where the roof framing meets the top of stud walls are normally covered by drywall on the inside and by wall cladding and soffit board on the outside. To install straps or hurricane clips, remove the roof sheathing around the perimeter of the roof to reveal the top of the wall. You may also need to remove the soffit and exterior cladding to reveal the top 12 inches to 18 inches of the wall. In addition, if the exterior cladding is brick veneer, you may need to remove small sections of brick.

If your roof has trusses, make sure you tie them to the wall by either anchoring to the top plate and then the top plate to the wall stud, or strapping the truss directly to the wall stud. The figures shown illustrate ways you can anchor the roof to the top of the wall of wood or masonry homes with straps or connectors.

For more information on strengthening walls when repairing, see the LSU AgCenter Publication 3072: Build Safer, Stronger, Smarter: Add Strength and Water Resistance When Repairing Your Walls

Strengthen the roof during reroofing (requires structural repairs)

Reroofing is a great opportunity to secure your roof inexpensively. Here's what you do:

  1. Remove the bottom row of roof sheathing at the eave lines.
  2. 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.
  3. 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.
  4. Refasten the roof sheathing removed in step #1. Fasten roof sheathing along the roof corners, ridges and eaves using additional eight-penny ring-shank nails or #8 screws 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 at every 6 inches.

These and other improvements that can be made when repairing the roof are described in the related article: Roofing to Resist Wind and Rain Damage.

For more information on strengthening the roof and making it more rain resistant when repairing the roof, see the LSU AgCenter Publication 3073: Build Safer, Stronger, Smarter: Add Strength and Water Resistance When Repairing Your Roof

4/7/2005 2:16:48 AM
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