Installing any type of roof, laying down the felt underlayment is always one of the first steps. This underlayment is used as a water barrier and helps protect the roof from leaking. Putting down felt on a flat roof is similar to laying it down on a sloped roof. Flat roofs can be easier to work with and less time consuming. Any homeowner with the proper instructions can lay down a flat roof.
Pull up all the existing felt from the roof with a hammer or pry bar to remove it. Remove all nails that are set in place. Pull up all three layers of existing felt, being careful not to pull up the plywood underneath.
Measure the length and width of the roof and multiply them together to get the square footage, which will determine how many rolls of No. 30 felt you will need.
Roll the felt out and cut it to the size of the length of the roof with a utility knife. Lay out the first piece and nail it onto the roof near the edge. Insert a nail every six inches across the felt along the front of the roof and nail in the other direction along the side in the same manner.
Lay out the second piece and overlap it a few inches. Nail it on the roof in the same manner as the first and continue down the roof until you reach the edge. Cut the felt to size at the end with a utility knife if necessary.
Cut the second layer in the same manner and cut the first sheet to half the size so they can overlap throughout. Apply construction adhesive with a trowel over the first few pieces of the first layer and lay the second layer on top. Press down to make sure it is firmly attached and there is no space for air to get through. Continue down the entire roof the same way until the second layer is complete.
Lay the third layer just like the second. Cut all the pieces to length and spread the adhesive all over the second layer. Overlap the felt and continue until you have reached the end of the roof.