Advanced options
Measure the width over the apex (both slopes), not the shed's footprint. Lay horizontal runs starting at the eaves, upper sheets overlapping lower by 75mm+ so water always crosses joints downhill, capped with a ridge strip. Galvanised 13mm clout nails at 50mm spacing on laps; adhesive under laps in exposed spots. Do it on a warm day: cold felt cracks.
What The Shed Roof Felt Calculator Works Out
- Roof area: length x width measured over the apex (both slopes), plus 15% for eaves and gable overhangs and waste. The standard failure is measuring the footprint instead and finding the shortfall at the ridge with the old felt already stripped.
- Rolls to buy: the area divided by what a roll actually covers once the laps are taken out: about 8.5m2 from a 10m x 1m standard roll, 6.8m2 from an 8m heavy-duty roll. A standard 8ft x 6ft shed wants one roll.
- Fixings: about 200 galvanised 13mm clout nails, felt adhesive for laps and edges, and a sharp knife with spare blades. Corners fold like hospital bedsheets rather than being cut and left flapping.
- Roof shape (advanced): apex roofs lay runs on two slopes and need a ridge strip; pent roofs are a single slope with the top run turned over the high edge. The cutting plan changes accordingly.
- Allowance, lap and overhang (advanced): set your own waste percentage, the lap between runs (75mm minimum, 100mm in exposed spots) and the overhang at eaves and gables. Water must always cross every joint downhill.
- Cutting plan (advanced): works out the number of 1m-wide horizontal runs each slope needs at the effective cover (1m minus the lap), the run length including overhangs, the ridge strip, and how many rolls that actually cuts from. Where the run-by-run count is higher than the area count, buy the higher one.
- Clout nails (advanced): the total length of laps and edges at 50mm spacing plus 10%, and the 500g boxes to buy, rather than the flat 200 guess.
- Felt cost (advanced): rolls x your price. A 10m roll of green mineral felt is typically GBP 20 to GBP 30 and lasts 5-10 years; polyester-reinforced grades GBP 35 to GBP 50 for 10-15 years.
- EPDM alternative (advanced): the one-piece rubber sheet size (roof plus 150mm each edge) and its area, costed if you enter a price per square metre (typically GBP 10 to GBP 18 plus adhesive). It reprices the job but deletes every joint a shed roof can leak through and lasts 25 years or more.
- Cutting list table (advanced): each piece, its length and width, laying notes and the roll left over for patches. Do the job on a warm day: bitumen felt is flexible in sunshine and cracks in the cold, which is why the January flap gets a tarpaulin, not a repair.