Roof Timber Quantity Calculator

Estimating roof timber for an extension comes down to two questions: how many rafters, and how much timber in total. This calculator answers both. Enter the width and length of the extension, pick a lean-to or pitched roof structure, set the pitch and the rafter spacing, and it returns the rafter count, the wall-plate and ridge lengths, the number of restraint straps, and a materials-only supply cost.

The result is a budgeting figure, not a specification. Your structural engineer sets the actual rafter sizes against span tables, and those sizes move with the span, the pitch, and the load. Use the numbers here to sanity-check a merchant list or a builder's timber allowance, then confirm the sections on the engineer's drawings.

How the rafter count works

Rafters run across the span, bridging the house wall and the outer wall of the extension. The calculator places one rafter at each end and fills in the gap at the spacing you pick, either 400mm centres for a stiffer deck or 600mm centres as standard. A lean-to roof has a single slope, so the count is straightforward. A pitched roof has two slopes meeting at a ridge, so the rafter count doubles and a ridge board is added along the top.

Two more members complete the frame. The wall plate is the treated timber bedded on top of the blockwork that the rafters sit on, and it runs along both side walls. Restraint straps then tie each rafter down to the plate so wind cannot lift the roof. The calculator counts one strap per rafter foot.

What the roof timber costs

The supply cost is built from two rates that come straight from current merchant pricing. Rafter timber, indicatively 47x150mm C16 treated timber, is costed at £3.20 – £5.00 per metre. Wall-plate timber, at a 75 × 100 mm section, is costed at £2.50 – £3.50 per metre. The ridge board and the straps are small supplementary items on top.

Everything here is materials only and excludes VAT. It does not include the roof covering, insulation, or any steel ridge beam, which are separate lines in a roof budget. It also assumes a simple rectangular roof with a modest eaves overhang, so a complex hip or valley design will need more.

A worked example

Take a 4m by 5m single-storey rear extension with a lean-to roof at a shallow pitch and rafters at 400mm centres. The calculator finds the rafter length across the 4m span, then counts one rafter every 400mm along the 5m wall plus one extra at the end. It multiplies that out to a total length of 47x150mm timber and prices it at the £3.20 – £5.00 per-metre rate. It adds the two wall plates along the side walls at the £2.50 – £3.50 rate, plus the straps, and shows a materials-only supply cost above.

Switch the same roof to pitched and the rafter count doubles, because both slopes need covering, and a ridge board length appears in the breakdown. Before you order, remember that building control will want to see the frame before it is covered, and the engineer's specified sections take priority over these defaults. See building control guidance on the roof structure inspection for when to book it.