Shed Roof Felt Calculator
How many rolls of shed felt do you need? The usual area rule can leave you a roll short, because felt goes on in overlapping strips longer than the roof. Enter your roof and get a real layout: strips along the eaves with the overhangs and laps from the manufacturer's instructions, a ridge strip if it is an apex, and everything packed onto the roll. You get the rolls to buy, and the nails and the length of lap to glue.
Last reviewed: 30 September 2026
Roof
Measure the roof deck itself, not the shed walls. For an apex roof, enter the length of one slope, from the eaves up to the ridge. The calculator adds the 50mm overhangs. The roof deck is bigger than the shed base, because it overhangs the walls. The default is the roof deck of a real 8' × 6' apex: Forest's Overlap Apex Shed assembly instructions list two roof boards of 2365 × 1070mm.
Leave this at 0%. The overhangs, laps and ridge strip are already in the layout, and a spare roll is shown on its own line in the results.
How Many Rolls of Shed Felt Do I Need?
The usual answer is the roof area plus 10%, divided by the roll area, and it can leave you a roll short. For an apex roof 3.7m long with 1.7m slopes it gives 2 rolls of 10m × 1m, but the roof needs 3. For a pent roof 3.6m long with a 2.4m slope it gives 1, and the roof needs 2. In the table below the rule is short in 2 of 7 roofs.
The rule counts area, but felt goes on in strips. Each strip is the length of the roof plus 50mm at each end, each overlaps the one below by 75mm, an apex needs a 300mm ridge strip, and no strip can be split between two rolls. So this calculator lays the roof out the way IKO's instructions say, then packs every strip onto whole rolls.
Cutting the roll lengthwise. The last strip on each slope is often narrow, and the plan cuts narrow strips side by side from one length of roll where that saves felt. The results also show the count with every strip cut from the full roll width. For an apex roof 2.4m long with 1.4m slopes that is 1 roll against 2.
No slivers. No strip is narrower than 150mm (our rule). On an apex roof a sliver is left out and the ridge strip widened to cover the gap with at least 50mm over each side strip. A ridge strip over IKO's 300mm is our adjustment. On a pent roof the last strip is cut to 150mm and simply laps further over the one below.
No percentage is added on top, because the overhangs and laps are already in the layout. If the plan leaves less than 500mm spare on the roll, there is no room for a mis-cut, and the calculator tells you to buy the spare roll.
How the Layout Works
- 50mm overhang. IKO: cut each strip so it overhangs the edges of the roof by 50mm, with the first strip overhanging the eaves by 50mm. The overhangs are folded and nailed at 50mm.
- 75mm lap. IKO: each subsequent sheet overlaps the previous one by 75mm, with adhesive between the sheets.
- Ridge strip. IKO: a capping strip 300mm wide, dressed down 150mm each side. The side strips may stop up to 100mm short of the ridge line, which is our choice.
- No strip under 150mm. Our rule. On an apex the ridge strip is widened instead, which is our adjustment to IKO's 300mm; on a pent the last strip is cut to 150mm and laps further.
- Strips packed onto the roll. The narrow last strips and the ridge strip share the 1m width where they fit, then the rows are packed into whole rolls.
- Nails. IKO: top edges at 500mm centres, overhangs, laps and the ridge at 50mm intervals, using 13mm galvanised large head felt nails.
What it does not do: it does not lay hip roofs, dormers or skylights, count the timber edge strips, or work out how much adhesive a lap needs, because IKO gives no coverage figure.
Roll Sizes, With Sources
Every roll in the dropdown has a product page that states its size, and no product page means no dropdown entry. Check the roll you are buying.
| Roll (length × width) | Product page |
|---|---|
| 10m × 1m | IKO Green Shed Felt, 10 x 1m, 10m2 a roll |
| 8m × 1m | IKO Shed Felt 8m x 1m, The Roofing Merchant |
Where the Rules Come From
The overhang, lap, ridge strip and nail spacings are from IKO's Shed Felt application and installation instructions, hosted by Wickes. IKO says its shed felt is for the roofs of garden sheds, kennels, hutches and other non-inhabited buildings, and gives rough counts of 1 roll for a pent or apex shed up to 7' × 5', 1 roll for an 8' × 6' pent and 2 rolls for an 8' × 6' apex. Those are rules of thumb. The default roof here is a real 8' × 6' apex: Forest's Overlap Apex Shed assembly instructions list two roof boards of 2365 × 1070mm, and the kit ships one roll of felt cut into three lengths. This calculator gives 1 roll of 10m for it, which agrees with Forest rather than IKO's rough count of 2. The minimum strip width and the wider ridge strip are our own rules. The packing of strips onto the roll and the 100mm gap at the ridge are our own working.
Common Roofs: Rolls Needed
Generated from the calculator above, with nothing added. Each roof is measured on the deck. The "area + 10%" column is the usual rule of thumb on 10m rolls, for comparison.
| Roof | m² | Strips | 10m Rolls | Area + 10% (10m rolls) | 10m Rolls, Full-Width Strips Only | 8m Rolls |
|---|---|---|---|---|---|---|
| Apex, 1.8m long, 1.0m slopes | 3.6 | 3 | 1 | 1 | 1 | 1 |
| Apex, 2.4m long, 1.4m slopes | 6.7 | 5 | 1 | 1 | 2 | 2 |
| Apex, 3.0m long, 1.6m slopes | 9.6 | 5 | 2 | 2 | 2 | 2 |
| Apex, 3.7m long, 1.7m slopes | 12.6 | 5 | 3 | 2 (short) | 3 | 3 |
| Pent, 2.4m long, 1.8m slope | 4.3 | 2 | 1 | 1 | 1 | 1 |
| Pent, 3.0m long, 2.4m slope | 7.2 | 3 | 1 | 1 | 1 | 2 |
| Pent, 3.6m long, 2.4m slope | 8.6 | 3 | 2 | 1 (short) | 2 | 2 |
Use the calculator for your own roof. The roll length can change the count more than you would expect.
Frequently Asked Questions
How many rolls of shed felt do I need?
The usual sum is the roof area plus 10%, divided by the roll area, and it can leave you a roll short. For an apex roof 3.7m long with 1.7m slopes it gives 2 rolls of 10m × 1m, but the roof needs 3. For a pent roof 3.6m long with a 2.4m slope it gives 1, and the roof needs 2. The rule counts area, but felt goes on in strips as long as the roof plus 50mm at each end, each strip overlaps the one below by 75mm, and an apex needs a ridge strip, and a strip cannot be split between two rolls. This calculator lays every strip out and packs them onto whole rolls.
How much should the felt overhang the edges?
IKO's installation instructions say to cut each strip so it overhangs the edges of the roof by 50mm, and to place the first strip at the lowest part of the roof so it overhangs the eaves by 50mm. The overhangs are folded over the edges and nailed at 50mm intervals. So every strip is 100mm longer than the roof.
How much should the strips overlap?
IKO says each subsequent sheet overlaps the previous one by 75mm, with adhesive applied between the overlapping sheets and the joint finished by nailing at 50mm intervals. So each extra strip on a slope only adds 925mm of a 1m wide roll.
What about the ridge?
On an apex roof, IKO says to finish the ridge by cutting a capping strip 300mm wide, fixing it with adhesive and dressing it down 150mm each side, nailing the lower edge at 50mm intervals. Because the strip covers 150mm each side, the side strips can stop a little short of the ridge line. The calculator lets them stop up to 100mm short, which is our own choice and leaves at least 50mm of cap over the strip.
Why does the narrow last strip matter?
The last strip on each slope is cut narrower than the roll, and the ridge strip is only 300mm wide, so several pieces can come from the same 1m of roll. That means cutting the roll lengthwise. The calculator does it where it saves felt, and also shows the count if every strip is cut from the full roll width. For an apex roof 2.4m long with 1.4m slopes the plan is 1 roll of 10m, or 2 if every strip is cut full width. No strip is narrower than 150mm, which is our own rule: a thinner last strip adds almost no cover past the 75mm lap, and nobody would fit it.
Why is the ridge strip sometimes wider than 300mm?
IKO says to cut a 300mm capping strip. Where the last strip on each slope of an apex roof would be under 150mm, the calculator leaves it out and widens the ridge strip instead, so it still covers the gap at the ridge with at least 50mm over each side strip. A ridge strip wider than 300mm is our adjustment, not IKO's instruction. The results show the width to cut. On the default Forest 8' x 6' roof it is 340mm.
IKO says an 8' x 6' apex needs 2 rolls. Why does this say 1?
The default roof is a real 8' x 6' apex. Forest's assembly instructions for its 8' x 6' Overlap Apex Shed list two roof boards of 2365 × 1070mm, fixed flush with the roof battens, so the roof deck is 2365mm long with two 1070mm slopes, overhang included. Laid out, each slope takes one full-width strip, and the ridge strip is widened to 340mm to cover the rest. That is 3 strips of 2.465m, 7.39m in all, which fits one 10m roll whether or not the roll is cut lengthwise (1 and 1). Forest itself ships one roll of felt with that shed and says to cut it into three equal lengths. IKO's figure of 2 rolls is a rough count for any 8' × 6' apex, so it is on the safe side for this one. A spare roll is shown in the results if you want the margin.
How many nails do I need?
IKO says to nail the top edge of each strip with galvanised clout nails at 500mm centres, to nail the folded overhangs, the laps and the ridge strip at 50mm intervals, and to use 13mm galvanised large head felt nails. For the default roof that comes to about 290. The results give the number, which you can round up to a box.
How much adhesive do I need?
IKO says to use its roofing felt adhesive on the laps and ridge strips, but it gives no coverage figure, so the calculator gives the length of lap and ridge to glue instead: 4.9m for the default roof. Check the tin for its coverage.
What roll sizes are there?
IKO Shed Felt comes in 8m × 1m and 10m × 1m rolls. Wickes sells the 10m × 1m roll (10 m²), and The Roofing Merchant sells the 8m × 1m roll. Each size has a product page, listed below. The 8m roll suits a short roof, since a strip cannot be split across two rolls, and the calculator packs the rows into whole rolls.
What if a strip is longer than the roll?
A roof more than about 9.9m long on a 10m roll would need a join along the strip, which is not laid here. The calculator warns you if a strip is longer than the roll. Sheds rarely need it.
Does it cover hip roofs, dormers or the timber edge strips?
No. It covers pent and apex roofs only. IKO says timber strips can be fixed to the edges of the roof for a professional finish, and we do not count them. For the shed base or cladding, see our shed base calculator and cladding calculator.