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How to Build Roof Trusses Over a Flat Roof

Bob Haring

Flat roofs are not really flat. All have some slight slope to allow water to drain. But all flat or very-low-slope roofs are subject to water problems. Heavy rain, snow and ice will not drain off them as quickly as from a pitched or sloped roof. Some flat-roof materials also are more prone to degeneration than pitched-roof shingles. Some homeowners also want or need more attic space than a flat roof provides. It is not uncommon to have a flat roof replaced with a pitched roof, and that requires roof trusses.

Step 1

Measure the roof with a tape measure and decide where the peak of the new roof should be and which sides should slope. Generally make the pitched roof run the direction of the longest dimension, but check the drainage on the sides so the roof does not dump water into a depression or some area that might impact the house foundation.

Step 2

Decide on a pitch or slope for the roof and a style of truss. Pitch is expressed in inches of rise per foot, as in 5/12 (a common house roof pitch), which slopes up from the wall at 5 inches per foot. There are many styles of truss, depending on the width of the roof and the amount of space needed inside the truss. A basic type is called the fink, with two sloping rafters, a bottom beam or chord between them and diagonal braces in the center in a sort of W shape.

Step 3

Start building trusses with a bottom chord, to go from wall to wall. Each end of the chord should be flush with the edge of the wall. Mark rafters at a 5/12 pitch. Put a framing square at the bottom of an end of a rafter board with the point at the bottom of the board, the 5-inch mark on the thin tongue and the 12-inch mark on the wide blade at the top of the board. Mark that angle, for a top or plumb cut, on the end of the board.

Step 4

Use the "length of common rafter per foot of run" to determine the length from the plumb cut to the end of the bottom chord, where it will rest on the top of the wall. Multiply that differential by the rafter span, half the width of the roof, to calculate that bearing point. Add a foot to provide an overhang or eave and mark an end or tail cut, like the plumb cut but with the point of the square at the top of the board. Go back to the plumb cut and take off another 3/4 inch so two plumb cuts will have a gap for a ridge board.

Step 5

Cut one pair of rafters with a circular saw at those angles and lay them on a flat surface with the bottom chord between them. Set the plumb cuts together with a 2-inch (actually 1 1/2 inches thick) spacer board between them to duplicate a ridge board, with the bottom chord level between the rafters. Mark the angle the rafters form across the bottom chord and cut it to fit. Remove the spacer and secure the peak and the two side bearing points with metal gussets, which have spikes on them. Drive the spikes in with a hammer to secure the gussets to both rafters, then add nails in all the nailing holes in the gusset plates. The gussets fit across the rafters just below the ridge board slot (some gussets are made with a ridge board slot).

Step 6

Measure to the center of the bottom chord, directly under the peak, then measure half that distance on each side. Mark those quarter points and make four braces, two to fit from the peak to the quarter point on the bottom chord and two to go from that point to the rafter halfway between the bearing point and the peak. Cut those braces and fasten them with gussets. Turn the truss over and add gussets on all connections on the other side. The finished truss should have two slopes, a flat bottom and braces that make a W shape.

Step 7

Remove the covering and decking from the old flat roof. Use a pry bar or roofing scraper to peel off any roof paper/gravel or membrane covering, then pry off the plywood or oriented strand board (OSB) decking. Remove the roof joists from wall to wall; the bottom chord of the truss will replace those. (Brace the walls temporarily if removing the joists weakens them). Use a level to make certain the wall tops are level so the bearing points of the trusses will be the same on both sides. Adjust the wall caps if needed to level them.

Step 8

Build the number of trusses needed to span the roof from end to end, spaced 24 inches apart. Mark the walls for the trusses. Start at one end of the roof and mark 1 1/2 inches in for the first truss. Measure 23 1/4 inches in from the end of the wall to mark the outside edge of the second truss, then mark it 1 1/2 inches wide (use the 1 1/2-inch wide tongue of the framing square to mark that space). Mark truss points 24 inches apart to the other end of the roof. Use the blade of the square, which is 24 inches long. The last space may be slightly less than 24 inches

Step 9

Lift the trusses to the roof, one at a time. Use at least three people for this -- one on each side of the roof with a 2-by-4 board with a notch on top to hold the truss and one (or preferably two) on the roof to set the truss upright and fasten it. Lift the truss upside down and let the roof workers set it upright. Use a level to get it vertically plumb and fasten it to the walls through the truss bearing point with framing nails and a hammer.

Step 10

Raise other trusses to the opposite end of the roof. Set a ridge board in the tops of the trusses to connect them from end to end. Nail the trusses to the ridge board; use a level to keep the ridge board level the length of the roof. Remove any temporary braces. Add "hurricane clips," metal brackets that fasten the truss ends to the wall caps, on both sides of all trusses.