How to Fix a House Roof That Is Sagging in the Center

There are a few different problem situations that may account for a sagging rooftop.

Inspect the Rooftop

A sagging rooftop is often the result of damaged rafters.A sagging rooftop is often the result of damaged rafters.
The most common cause is a broken rafter. Localized sagging could be caused by a damaged section of roof decking. The first step is to determine the cause of the sagging by carefully inspecting the underside of the rooftop. The course of action depends upon the cause of the sagging. Repair split rafters by fastening angle iron over the split; replace a damaged gusset plate with plywood; and insert a piece of foam insulation and plywood to temporarily fix sagging decking.

Enter the attic of the house. Inspect the individual rafters holding the roof decking in place. Look for cracked or rotted rafters.

Take a careful look at the metal gusset plates that hold the roof's rafters to the frame of the house. Look for signs of these metal plates being twisted, warped or broken.

Examine the roof decking installed over the rafters for signs of rot or other damage.

Repair a Split Rafter

Fabricate a piece of angle iron at least 1 ½-by-1 1/2 inches thick and 2 feet in length. Cut the iron to length with an angle grinder fitted with a cutting wheel.

Drill four 3/8-inch holes along each side of the length of the angle iron--eight holes total. Space the holes equidistant along the length of the angle iron.

Clean away any jagged shards or splinters from the split roof rafter. Use a reciprocating saw to remove this loose material.

Fasten the angle iron to one side of the split. Place the angle iron on the edge of the roofing rafter and drill 3/8-inch holes through the four holes that lie on the first side of the split rafter. Drive 3/8-by-2 inch carriage bolts through the portion of the angle iron against the underside of the rafter. Run 3/8-by-2 ½ inch carriage bolts through the width of the rafters and fasten them with washers and nuts. At this point you will have the angle iron fastened securely to one side of your split rafter.

Select a 3/8th inch carriage bolt that measures about 1 ½ to 2 inches longer than the gap between the rafter and the loose end of your angle iron.

Drill 3/8-inch holes through the holes in the angle iron and into the underside of the rafter, then insert the two carriage bolts into them. Work the bolts tight by alternating from one to the other until the bolts are tight. The action of tightening the carriage bolts into the angle iron and the underside of the rafter will bring the broken rafter into alignment.

Drill 3/8-inch holes through the side holes in your angle iron and through the wooden rafter. Insert 2 ½-inch carriage bolts and fasten them with wasters and nuts.

Brace the damaged rafter with 2-by-4 inch boards cut to length and angle. Use a miter box saw to cut the boards to length and insert one board on each side of the split in the rafter, running from the ceiling joist to the rafter. Use deck screws to fasten these bracing boards in place.

Repair a Damaged Gusset Plate

Remove a damaged gusset plate, which is a spiked metal plate that holds the pieces of your roofing trusses together. Use a pry bar to lift and remove the metal plate.

Fabricate a plywood gusset out of ¾-inch plywood. Cut the plate to the proper angle to fit in the place of the removed metal gusset.

Move the truss pieces back into place. If necessary, have a partner work with you, or use a 2-by-4 inch board to lift and hold the truss pieces in place. Set your plywood gusset in position and nail it to the truss pieces with a pneumatic nail gun. Drive the nails 1 inch apart along the lengths of the plywood gusset and into the underlying truss pieces.

Repair Damaged Roof Decking

Cut plywood to fit in between the rafters and over the length of the sagging roof section; use a circular saw. Use a utility knife to cut 1-inch-thick rigid foam insulation to the same dimensions as the piece of cut plywood. The foam insulation will allow you to position the plywood board over the exposed roofing nails that extend through the decking.

Glue the foam insulation to the plywood with a construction adhesive, such as liquid nails. Let the glue dry completely.

Position the foam and insulation between the rafters, under the sagging section of rooftop. Place the foam side up so that the exposed nails of the roof sink into the insulation.

Cut two pieces of 1-by-2 inch wood strapping to the same length as the plywood and foam piece. Position these strapping boards along the rafters under the plywood and foam. Fasten the strapping boards in place to hold the foam and plywood board tight against the roof decking. Drive nails every few inches through the strapping boards into the rafters with a pneumatic nail gun.

Things You Will Need

  • 1 1/2-by-1 1/2 inch angle iron
  • Drill
  • 3/8th-inch carbide drill bit
  • 3/8th-inch carriage bolts of assorted lengths
  • Ratcheting wrench and appropriate sockets
  • 2-by-4 inch boards
  • Miter box saw
  • Decking screws
  • Screwdriver
  • Pry bar
  • Circular saw
  • Pneumatic nail gun
  • 3/4-inch-thick plywood
  • 1-inch foam insulation
  • Construction adhesive
  • 1-by-2 inch strapping boards

Warnings

  • Using a traditional hammer when working with rafters can cause reverberations that can damage the Sheetrock in your house. If you don't have a pneumatic nail gun, rent one from your local rental shop.
  • The repair of the localized section of sagging roof (Section 4) is only a temporary fix. The only way to truly solve this problem is to tear apart the roof and replace the rotted or broken roof decking material, then re-shingle the roof.

About the Author

Robert Howard has been writing professionally since 2004 and writes a weekly column for the "Synthesis," a Chico, Calif.-based newspaper. He maintains a blog and has published articles and works of fiction in a variety of different print and online magazines. Howard holds a Bachelor of Arts in visual arts from the University of California, San Diego.