Your Roof Sags? Buck Up By Dwight Barnett
Scripps Howard News Service
Q: I have a 2-inch sag in the center of a 20-foot span of my roof. The pitch of the roof (two-by-six rafters) is relatively shallow, probably a 1 2/2 or 1 2/3. --F.D., Vernon, Mich.
A: With winter here and snow building up on roofs, it's good to answer this question again. Roofs need to be designed to support the weight of the roofing materials, such as decking and shingles, plus any additional weights that might be placed on the roof due to snow, water or wind. Without proper bracing, the combined weights cause the roof's rafters to sag.
The span of the rafter is the horizontal distance the rafter covers and is not the actual length of the rafter. This horizontal distance is usually the middle of the house or the mid-span. The pitch of the roof is represented by the number of inches the rafter will "rise," in your case 3 inches, over the horizontal "run," or 12 inches.
Your 3/12 roof design is at the extremes of a low-sloped roof. A steep pitched roof is more like a 9/12 or 12/12 pitch and would be much easier to brace. Why? Because the bracing should be from about the midspan of the rafter (in your case 10 feet) down to a bearing wall. The loads are then transferred from the bearing wall to the foundation.
You'll have to have working room in the attic to properly install rafter bracing. Your attic will be cramped at 54 inches high at the center and only 24 inches high midway down the rafter, but it can be done. I have seen wannabe carpenters install a trough perpendicular to the ceiling joists, where they then brace the rafters off the trough. When the roof starts to sag, it pushes down on the trough, which in turn pushes down the ceiling joists. This is not a good idea or a proper way to brace a rafter.
Many older homes have a distinct ceiling crack caused by improper bracing such as this. A trough is usually a two-by-six nailed flat side down to the top of the ceiling joists and then another is nailed vertically to the side of the first two-by-six forming an L-shaped brace. If you have to brace from a trough, the trough needs to be designed by a structural engineer. If you have the working space, you need to locate the interior bearing walls from which to brace the roof. A two-by-four is then used to brace from this wall to a point about midway up the rafter on each side of the roof.
The point on the rafter where the brace is to be attached will vary from city to city, depending on the amount of snow or wind the area receives, the type of wood used, the pitch of the roof and the size of the rafter.
You can brace every other rafter and stop a lot of the sagging; however, it is better to brace each rafter.
Contact a qualified building contractor or engineer for more specific information on spans and loading for your area.
(C. Dwight Barnett is a master inspector certified by the American Society of Home Inspectors. Questions may be addressed to him at PO Box 14091, Evansville, IN 47728, or e-mail dbarnett@evansville.net .)
