Some tips for flat roof construction include calculating the joists' maximum span, ensuring that the joists do not twist and finishing the roof using rubber sheets, tar, gravel or rolled asphalt. Apply the finishing material in such a manner that the roof is made waterproof.
To calculate the joists' maximum span, measure the distance between the two walls using joists. Also, consider the weight the roof may bear.
When constructing a flat roof, use plywood to build a cover around the joists. Keep a gap of about 1/8 inch between joints in the plywood to account for wood contraction and expansion. To ensure that the joists do not twist, push in noggins between two joists. Ensure that the plywood is clean before finishing the roof.
If using rubber as the finishing material, affix a fiberglass-backed foam board first, dry fit the rubber sheet with an overlay on all sides, fold the overlay, and glue it to the roof. To waterproof the roof, apply a dry roller over it to remove air bubbles. To apply rubber on any pipes on the roof, make a hole that is slightly larger than the pipe in the rubber, and slide it over the pipe.
When using rolled asphalt, overlay the material, and affix it using nails. Use roof tar that is cold processed on the nails to create a tight seal. If using gravel and tar, lay roofing cement and tar roofing felt on the roof, and then alternate gravel and hot tar layers.