How to Apply Roll Roofing Over Old Roofing Material

Sweep any and all debris from the existing roof to clean away anything that would otherwise rip the roll roofing material or cause an improper seal. This includes downed foliage, rocks or collected roofing sediment. Once the entirety of the surface is free and clear, prep the roofing primer as directed on the product's packaging. If you've purchased multiple cans to cover a large area, only prep one can at a time to prevent the primer from drying out.

Fill a painter's tray with primer and use the roller to apply a thin coat of primer -- no more than 1/8 inch -- over the roof section. Be advised that if you're working on a flat roof you'll need to add a double layer of the material to prevent leaks and reduce pooling of rainwater. If you're working on a slanted roof, only one layer will suffice.

Brush on roofer's cement along the first 2 inches of the edging of the existing roof in order to apply a new drip edge.

Lay 9-inch-wide starter strips over the newly applied cement and hammer roofing nails every 3 inches to secure them in place.

Roll the roofing material in strips over the roof and newly installed edging, overlapping the strips about 17 inches or so to create a double layer. If you're working on a slanted roof, only overlap the material by 2 to 3 inches since the additional layer isn't needed. Hammer roofing nails every 3 to 4 inches to secure the new roofing to the existing roof.