Sagging roof lines can cause undue concern for the uninformed homeowner. Unless the roof has been impacted by falling debris, or has suffered catastrophic failure of one or more of its trusses, the fix is typically simple. The procedure for reinforcing roof trusses is similar to adding a "cripple" stud to support a sagging common rafter. A piece will need to be cut to fit the affected truss and nailed in place.
Drive a screw into the bottom edge of the upper cord or rafter, on the gable truss at each end of your attic. Leave it standing out from the face 1 1/2 inches. Tie a piece of mason's twine between the two screws, 1 inch from the board's surface.
Follow this line from one end of the attic, examining each rafter or upper cord as you go. Any rafter that is closer than 1/2 inch to the string needs to be examined for sagging. Drive in screws at the top and bottom of these rafters, 1 inch above the bottom edge of the board.
Tie mason's twine between them. Check along the string to see how straight your rafter is. If the edge of the board is 1 1/4 to 1 3/4 inches below the string, the rafter does not require reinforcement. Drive a screw into the face of the rafter centred between the top and bottom ends. Hang a plumb bob from it and wait for it to stop swinging. Mark the floor or lower cord (also known as a joist), directly under the plumb bob.
Cut a 2 by 4 to fit under the roof, standing upright, overlapping the affected rafter, with its bottom on the floor or lower cord. Place the 2 by 4 centred on the mark you made below the rafter. Place a level on the edge of the 2 by 4 and adjust it until the bubble is centred. Mark the lower edge of the rafter on the face of the 2 by 4.
Measure the distance from the bottom edge of the rafter to the string running from the top to bottom. If the measurement is less than 1 inch, add that to the length of your 2 by 4 by redrawing the rafter edge line you just traced, parallel to the original with the difference between the short measurement and 1 inch. For example, if the rafter is sagging 1/4 inch, make the 2 by 4 brace 1/4 inch taller.
Cut your 2 by 4 along this new line. Stand it centred on the mark on the floor or joist, with the angled top against the bottom of the rafter or upper cord. Hammer this brace into place with a 1.36kg. hammer, checking it for plumb with the level. Drive 3-inch treated deck screws on an angle through the top and bottom ends of the brace into the floor or joist below and rafter above.