Show the formula and working
This free roofing felt calculator works out how many rolls of shed felt you need for a pent or apex roof, with the number and length of strips, the overlaps and an estimate of clout nails, for homeowners re-felting a shed, summer house, log store or garden building in the UK, Ireland and Europe. It works with 10 m and 8 m rolls or any custom roll size.
Quick answer: Cut strips the length of the roof plus overhangs, lay them from the eaves up with a 75 mm overlap, and count how many fit on each roll. An apex roof 2.4 m long with 1.2 m slopes needs four 2.5 m strips, so this roofing felt calculator shows one 10 m by 1 m roll.
What Is a Roofing Felt Calculator?
A roofing felt calculator is a tool that plans how bitumen shed felt is cut from rolls to cover a roof. Strips run along the roof, parallel to the eaves, starting at the bottom, and each strip overlaps the one below. In plain words: felt per slope = slope length + eaves overhang; strips per slope = 1 + (felt per slope – roll width) / (roll width – overlap), rounded up; strip length = roof length + an overhang at each end; rolls = strips / strips cut from one roll, rounded up.
The defaults follow the IKO Super Shed Felt technical data sheet, which lists 8 m and 10 m rolls, 1 m wide, a 75 mm overlap with the previous sheet and galvanised clout nails at 50 mm intervals on overlaps and overhangs, with 500 mm centres along the top edge. Always follow the instructions for the felt you buy.
How Do You Use the Roofing Felt Calculator?
- Choose the Roof type: pent (one slope) or apex (two equal slopes).
- Measure the Roof length along the eaves and the Slope length, eaves to ridge, up the roof surface.
- Set the Overhang at the eaves and ends, the felt you will turn down over the edges.
- Pick the Roll size, or Custom with the Custom roll length and Custom roll width, and check the Overlap between strips.
- Read the rolls, strips and nails, then open “Show the formula and working” to see the cutting plan.
How Much Roofing Felt Do I Need for a Shed?
Most small and medium sheds need one or two 10 m rolls: a 2.4 m long apex roof with 1.2 m slopes takes one roll, while a 3 m roof with 1.5 m slopes takes two. The number of rolls depends on how many strips fit in each roll without joins, not just on the area. The table uses 10 m by 1 m rolls, a 75 mm overlap and a 50 mm overhang.
| Roof | Roof area | Strips | 10 m rolls | Clout nails (approx.) |
|---|---|---|---|---|
| Apex, 1.8 m long, 0.9 m slopes | 3.24 m² | 2 x 1.9 m | 1 | 198 |
| Apex, 2.4 m long, 1.2 m slopes | 5.76 m² | 4 x 2.5 m | 1 | 370 |
| Apex, 3 m long, 1.5 m slopes | 9 m² | 4 x 3.1 m | 2 | 459 |
| Apex, 3.6 m long, 1.8 m slopes | 12.96 m² | 4 x 3.7 m | 2 | 548 |
| Pent, 1.8 m long, 1.5 m slope | 2.7 m² | 2 x 1.9 m | 1 | 146 |
| Pent, 2.4 m long, 1.8 m slope | 4.32 m² | 2 x 2.5 m | 1 | 184 |
| Pent, 3 m long, 2.4 m slope | 7.2 m² | 3 x 3.1 m | 1 | 303 |
What Is the Minimum Pitch for Shed Felt?
IKO’s data sheet says its Super Shed Felt is for roofs at or above a pitch of 20 degrees, so check the minimum pitch of the product you buy before using it on a shallow roof. Very low pitch or flat roofs need a flat roof system, such as torch on felt or a rubber membrane, laid to that maker’s instructions. The roof pitch calculator finds your pitch from the rise and run.
How Do You Lay Shed Felt on an Apex Roof?
Start at the eaves on one side, lay the first strip with an overhang and nail it, then lay the next strip above it so it overlaps the first by 75 mm, and repeat up to the ridge. Do the other side the same way, then lap the top strips over the ridge, or fit a separate ridge strip, as the instructions say.
Fold the overhangs neatly over the eaves and ends and nail them at 50 mm intervals, or bond them with felt adhesive. Work on a dry day, ideally above the 10 °C IKO recommends, and unroll the felt at least 30 minutes beforehand so it lies flat and supple, and use a safe ladder or platform rather than standing on a weak roof.
Worked Example: Re-felting an Apex Shed Roof
Example: an apex shed roof 2.4 m long with slopes of 1.2 m from eaves to ridge, using 10 m by 1 m rolls, a 50 mm overhang and a 75 mm overlap. Each slope needs 1.2 m + 50 mm = 1.25 m of felt, which is more than one 1 m strip, so it takes 2 strips. Each strip is 2.4 + 2 x 0.05 = 2.5 m long, so the roof needs 4 strips, 10 m in total, and a 10 m roll gives exactly 4 strips. That is 1 roll.
The nailing estimate is about 350 clout nails at 50 mm centres along the eaves, ends, laps and ridge, plus about 20 at 500 mm along the top edges. Buying a second roll gives a margin for mistakes, and it is worth checking the roof boards for rot before felting. If the shed needs a new concrete base, the concrete calculator sizes it, and more tools are on the building and DIY calculators page.
Roofing Felt Calculator FAQs
How many rolls of felt do I need for an 8 x 6 shed?
Measure the actual roof, because overhangs and pitch vary between sheds. An apex roof about 2.4 m long with 1.2 m slopes needs one 10 m roll for the main covering, and many people buy a second for safety.
How much should roofing felt overlap?
IKO’s shed felt instructions give a 75 mm overlap between strips, with the upper strip lapping over the lower one so water runs off. Other felts may differ, so check the roll’s own instructions before cutting.
Which way should shed felt run?
Lay strips horizontally along the length of the roof, parallel to the eaves, starting at the bottom and working up to the ridge. Each higher strip overlaps the one below, like roof tiles, so rain cannot run under the laps.
How many clout nails do I need for shed felt?
At 50 mm intervals along laps, eaves, ends and the ridge, a 2.4 m apex roof needs about 350 nails, plus about 20 at 500 mm along the top edges. Use galvanised clout nails and buy a box with spares.
Can I use felt adhesive instead of nails?
IKO’s data sheet describes applying roofing felt adhesive to the overlapping areas, and many people use adhesive on laps with nails along the edges. Follow the felt maker’s fixing instructions, and use adhesive only in dry, mild conditions.
How long does shed felt last?
Lifespan depends on the felt grade, the roof pitch, exposure and how well it was fitted. Heavier mineral surfaced felts usually last longer than thin basic felts. Check the maker’s stated life or guarantee and inspect the roof each year.
Checked October 2026. Sources: IKO Super Shed Felt technical data sheet, Asphalt roll roofing (Wikipedia).