Roofing Felt Calculator

How This Is Calculated

Each course of felt only adds its roll width minus the overlap as genuinely new coverage up the slope, since the overlap portion sits on top of the course below rather than covering new roof. Dividing the slope length by that effective coverage, rounded up, gives the number of courses needed.

Each course runs the full eaves width, so the total linear length of felt needed is the number of courses multiplied by the eaves width, with your wastage allowance added before dividing by the roll length to get whole rolls.

Nails are estimated from one row along the length of every overlap, spaced at your chosen nail spacing - a reasonable minimum, though real installations also nail along perimeters and under-courses.

Common Questions

Why does felt need an overlap between courses?

Each course of felt overlaps the one below it so water running down the slope always meets a continuous surface rather than a gap at the joint. Without a proper overlap, water can work its way underneath and cause exactly the leak the felt was meant to prevent.

Which direction should the felt run?

Courses run horizontally across the roof, parallel with the eaves, starting from the bottom of the slope and working up towards the ridge - never the other way round, or every overlap would channel water under the course above instead of over it.

How is the number of nails worked out?

As a rough guide based on one row of nails along the length of each overlap, spaced at your chosen nail spacing. Real installations also nail along the top edge under each subsequent course and around the perimeter, so treat this as a sensible minimum starting point rather than an exact count.

Can I use shed felt on a flat roof or an extension?

Standard mineral shed felt is designed for simple pitched roofs like sheds and summerhouses. A flat roof needs a proper built-up bituminous felt system (usually three layers, torched or self-adhesive) or a single-ply membrane, installed to a higher specification - it's not a straightforward like-for-like swap.