Flat roof leaks in a garage are often difficult to locate because the water that causes the leak may come from any location on the roof. The most frequent cause of a leaky garage roof is a gap in between the metal flashing around the edge of the roof and the roofing material. A low spot on a flat roof may also contribute to leaks. Once you locate where the leak is, in most instances you can repair flat roof problems by repairing damaged areas in the roofing material.
Clean the area where you found the garage roof leaks. Sweep debris away from the seam edges that you are repairing using a heavy broom.
Fill a bucket with warm water. Use a sponge to wash away as much dirt as you can along the seam that is leaking. Allow the area to dry.
Separate the seam by laying the top back on itself at least 1 foot in either direction from the leak. Use a special cleaning solution for roofing rubber to remove the adhesive from the seam. Follow the directions on the container carefully. Allow the cleaning solution to dry.
Spread rubber seam adhesive with the applicator in the container over the top and bottom of the seam. Work the adhesive as far under the top piece of rubber membrane as you can.
Fold the top side of the seam back into place. Use a seam roller to press the seam together firmly so it forms a bond with the adhesive. Allow the adhesive to dry for 24 hours before proceeding.
Try to run the tip of a seam probe under the seam joint. If the adhesive dried correctly, you shouldn't be able to penetrate the seam. If you can, add more adhesive to the seam, roll it in place and allow the adhesive more time to dry.
Attach seam tape over the seam you repaired. Extend the tape at least 1 foot on either side of the repaired portion of the seam. Press it firmly in place with the seam roller.
Push gravel away from the area you are fixing on the garage roof with a heavy broom.
Make an "X" with a utility knife over the area that is leaking. Cut through the roofing material and gently pull it back and away from the centre of the "X." Allow the wood underneath to dry thoroughly once you open it up.
Spread roofing tar under the roofing material where you cut the "X." Push the tar as far under the membrane as you can with a plastic trowel. Pull the four points of the "X" back together again and push them into the tar below.
Use the trowel to cover the entire area of the repair with 1/8-inch layer of roofing tar. Feather the edges at least 6 inches beyond the repaired area.
Place an old towel or rags into the roofing tar. Smooth out any wrinkles. Cover the towel with a coating of tar.
Place the gravel you removed in step 1 back on the tar by hand. Do not use the broom to redistribute the gravel.
Use caution when climbing on ladders.