All 43 tools
Roofing · rafters · sheets · flashing
What do I need for a lean-to roof?
A single-slope roof off a house or garden wall - over a patio, a log store or a lean-to shed: the rafters and their length, the wall plate and front beam, the posts, and the covering in clear sheets or boards and felt, with the flashing and gutter. It counts materials; timber sizes and spans are for the span tables or whoever designs it.
Measure it
Every box has its own unit next to it.
Change the unit beside any box and the number converts. The (i) beside each question says what the box wants and where the figure usually comes from. Anything marked per cent is a percentage of the quantity above it.
Check the rates
Rates and assumptions used
These are the figures the calculator works to. Change any of them and the order list updates — your merchant’s pack sizes and your own waste allowances beat the defaults every time.
Order list
The orange figure is the one you order to. Highlighted rows are the other numbers you order to. The small print under each row is the sum that produced it, so you can check it against your own take-off. Amber rows are warnings, not quantities.
How this one works
- Rafter length =
√(projection² + fall²) + overhang; rafters =length along the wall ÷ centres + 1 - Roof area =
length along the wall × rafter length - Clear sheets =
length along the wall ÷ a sheet’s cover width, rounded up, each a rafter long - Boards =
roof area × (1 + spare) ÷ one board; felt =roof area × (1 + laps) ÷ one roll
The figures the calculator works to. Every one can be changed in step 2, and your merchant’s pack sizes and your own waste allowances beat these defaults every time.
| Rate | Default | Why |
|---|---|---|
| Rafter centres | 600 mm | Check the span tables for the timber size at this spacing. |
| Posts no more than | 2.4 m | apart along the front. |
| Board size | 2.88 m² | 2400 × 1200. |
| Felt roll | 10 m² | 10 m × 1 m. |
| Felt laps | 15 % |
A worked example
With the defaults the page opens on:
- How long along the wall? 4 metres
- How far does it come out? 2.5 metres
- How much does it fall? 0.4 metres
- How high is the front? 2.2 metres
- What goes on it? Clear roof sheets
- How wide is a clear sheet, once lapped? 1 metres
- Overhang at the front? 0.15 metres
- How much spare? 10 per cent
- What does one clear sheet cost? £45.00
- What does one board cost? £30.00
Timbers
- Rafter length: 2.68 m — √(2.5² + 0.4²) + 0.15 m overhang
- Rafters: 8 no. — at 600 mm centres
- Rafter timber: 23.6 m — +10%
- Wall plate: 4 m — fixed to the wall, the rafters’ top ends
- Front beam: 4 m — on the posts
- Posts: 3 no. — no more than 2.4 m apart, 2.2 m each
- Flashing at the wall: 4 m
- Gutter: 4 m — along the front
Clear sheets
- Roof area: 10.73 m² — along the wall × rafter length
- Sheets: 4 no. — 4 m ÷ 1 m cover, each 2.68 m long
Budget
- Sheets cost: £180 — excludes the timbers, fixings, flashing, gutter, delivery and VAT
Change any figure above and every line recalculates; the small print under each row is the sum that produced it.
Questions people ask
How long are the rafters on a lean-to?
The slope from the wall to the front, plus the overhang: √(projection² + fall²) + overhang. Coming out 2.5 m with a 0.4 m fall and a 150 mm overhang, each rafter is 2.68 m.
How many rafters do I need?
Length along the wall ÷ the centres, rounded up, plus one: 4 m at 600 mm centres is 8 rafters - 23.6 m of timber with 10% spare.
How many roof sheets?
Length along the wall ÷ one sheet’s cover width (its width once lapped), rounded up, each a rafter long: 4 sheets at 1 m cover - £180 at £45 a sheet. For boards and felt, the same roof (10.7 m²) is 5 boards and 2 felt rolls.
What size timber, and how far can it span?
That is for the span tables or whoever designs the roof - it depends on the load, the timber grade and the spacing. This counts the pieces and their lengths.
Calculators for the same job
Doing this for a whole job rather than one wall? SiteLedger keeps the valuations, supplier statements, invoices and variations against the project they belong to.
Read before you order. These work out quantities, not structural design — spans, lintels, rebar and foundation depths need an engineer and Building Control sign-off. Waste allowances are defaults, not promises: awkward cuts, long runs and bad weather all eat more than 5%. Where a merchant sells whole packs, the figure is rounded up to whole packs.