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Work out how many rolls of shed felt you need for an apex or pent roof. Choose the roof type and roll size, enter the roof length and slope width, and press Calculate to see the rolls, the lengths to cut, the ridge strip and the felt nails.
| Roll | Lengths Per Roll | Rolls | Left Over |
|---|
Each strip is cut whole from a roll, so the roll length that wastes least depends on the roof length.
You get the rolls to buy, the number of strips on each slope, the lengths to cut, the ridge strip, the felt left over and the number of felt nails. A drawing shows the strips on the roof, a table compares the two roll sizes, and you can copy the result or save it as a PDF.
Shed felt is laid in strips that run along the roof, from one gable to the other, starting at the eaves and lapping up the slope. Work it out in three steps:
Strips per slope = 1 + (slope + 0.05 − 1) ÷ 0.925, rounded up
Working in whole strips rather than square metres matters, because offcuts from the end of a roll are usually too short for another strip.
If you only know the floor size and the pitch, the slope of an apex roof is about half the width divided by the cosine of the pitch, plus the eaves overhang. Our roof pitch calculator gives the slope length from the rise and span.
| Felt | Roll Size | Notes |
|---|---|---|
| Standard green mineral shed felt | 10 m × 1 m or 8 m × 1 m | Glass fibre base, the cheapest option for garden sheds |
| Polyester (super) shed felt | 10 m × 1 m or 8 m × 1 m | Polyester base, tougher and longer lasting |
| Felt shingles | Packs of strips | Laid like tiles over an underlay; a different calculation |
A 10 m roll covers just under 10 m² once the laps are taken off. Makers such as IKO recommend 75 mm laps and 50 mm overhangs at the gables, which is what the calculator uses.
Shed felt is made for pitched roofs on sheds and garden buildings, usually 20° or steeper. For a flatter roof, or a garage, workshop or garden office you want to last, look at torch-on felt, EPDM rubber or GRP fibreglass. Felt shingles, onduline sheets and metal sheets are other choices for pitched outbuilding roofs. For the timber roof itself, our rafter length calculator and plywood sheet calculator work out the rafters and roof boards.
8 × 6 ft apex shed, roof 2.6 m long with slopes of 1.1 m:
8 × 6 ft pent shed, roof 2.6 m long with a 1.9 m slope:
The calculator adds 50 mm at each gable to the roof length to get the length of each strip. It adds 50 mm at the eaves to the slope width, and on a pent roof 50 mm more to fold over the back. The number of strips is one for the first metre and one more for every 925 mm after that, because each 1 m strip laps 75 mm. An apex roof has two slopes and one extra length for the 300 mm ridge strip. Each roll gives as many whole strips as fit along it; if the roof is longer than a roll, strips are joined with a 150 mm lap. Nails are counted at 50 mm centres along the eaves, gables, laps and ridge, and 500 mm along the top edge of each strip.
About 2 rolls of 10 m × 1 m felt for an apex roof, and 1 roll for a pent roof, depending on the overhangs.
A 10 m × 1 m roll covers up to 10 m², a little less once the 75 mm laps are allowed for.
Felt makers recommend 75 mm laps between strips and 50 mm overhangs at the gables and eaves.
Along the length of the roof, starting at the eaves and working up, so each strip laps over the one below and water runs off.
13 mm galvanised large-head felt nails, at about 50 mm along laps and edges.
Felt adhesive in the laps and under the ridge strip stops wind and rain getting under the felt.
It is made for pitched roofs. A flat roof needs torch-on felt, EPDM or GRP.
Standard felt typically lasts several years, and polyester felt longer, depending on exposure and how well it was fitted.
Roll sizes, laps, overhangs and nailing from shed felt makers' instructions. Reviewed November 2026.