First divide the length of the building by 2 and round up to the next whole number, add one and that is the total number of trusses you will need including the 2 gable ends.
Example 19 divided by 2 = 9.5 rounded up = 10 add one, so that is 9 trusses and 2 gable ends = 11.
The width of the building will be equal to the lineal feet of lumber needed for the bottom chord of the truss if they have the eaves tipped on the same angle as the roof. But if they are to be level soffits you will need to add 3 ft approx. to the length.
For the top chord multiply the height (at peak) of the truss by itself, multiply half of the length of the bottom chord by itself, add both totals together, find the square root and add to it 16 inches. Example 24 inch rise = 24 X 24 = 576, 12 ft (144 inches) truss divided by 2 = 72 - 72 X 72 = 5184 + 576 = 5760 square root = 76 inches approx. plus the 16 inches for the eave, so you will need a 92 inch piece of lumber for each top chords on each side. It won’t be exact to that length but will be pretty close so just add a little extra to each.
The web members will be a little harder to figure out so just I add 2 – 8 foot pieces of 2″ X 4″ for each truss for a medium size truss.
As a rule of thumb I add 1/2 sheet of plywood for each truss.
To sheath the roof with board multiply the length of the 2 top chords by the length of the building (plus 3 feet). This will be the square footage of the roof, add 10% for wastage and it will be the square feet of board you need. If using plywood divide the SF by 30.
Find the number of bundles of shingles needed by dividing the roof SF by 30 also.
1/2 pound of galvanized roofing nails is needed for each bundle of shingles you use to cover the roof.
If you will be using a pneumatic roofing nail gun to attach the shingles to the roof you will need 1 coil of roofing nails per bundle of shingles at 120 nails per coil.
Plans to build your own roof trusses.