How do you cut roof underlayment?
Short answer. Cut synthetic or felt roof underlayment with a sharp utility knife, ideally with a hook blade, pulling it along a straightedge or over a scrap board so you do not score the deck or the sheet below. Cut self-adhered membrane with the release film still on, and peel split-back film one half at a time.
Tools that work
- Hook blade: cuts from underneath the sheet, so you slice only the top layer and not the course below.
- Straight utility blade: fine for cuts made on a scrap board or on the ground.
- Heavy scissors or snips: handy for detail cuts around pipes and vents.
Change blades often. Synthetic underlayment dulls blades fast, and a dull blade drags and stretches the sheet.
Cutting by material
- Synthetic: roll out a length, mark it, and cut on a board. Leave a few extra inches at rakes and trim later along the drip edge.
- Felt: cuts easily but tears when wet, so cut and install it dry.
- Self-adhered membrane: cut to manageable lengths, usually 10 to 15 feet, with the film on. In Georgia summer heat the adhesive gets very aggressive, so position it before peeling.
Around pipes and vents
Cut an X or a hole a little smaller than the pipe, slide the sheet over it so the material hugs the pipe, and let the pipe boot or vent flange lap over the underlayment above and under it below as the manufacturer directs. Do not cut big open holes that rely on caulk to seal.
At walls and valleys
Turn underlayment up walls a few inches rather than trimming it flush. In valleys, center the leak barrier and lap field underlayment over it rather than cutting it short.
Keep blades closed on the roof. See synthetic underlayment vs felt and ice and water shield in Georgia.