How do I calculate rafter length for a 30 degree roof?

For a 30-degree roof, you take the horizontal run (half the span) and divide it by the cosine of 30 degrees. For example, a 4m run / cos(30°) = 4.618m rafter length (plus any overhang).

What is a 'Birdsmouth' cut in roofing?

A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the wall plate. It consists of a vertical 'plumb cut' and a horizontal 'seat cut' to ensure the rafter is stable and properly supported.

Why do I need to deduct the ridge board thickness?

Calculations usually measure from the centre of the roof. However, rafters meet at a ridge board (usually 25mm to 50mm thick). You must subtract half of this thickness from the top of each rafter to ensure the roof span remains accurate.

What is the standard spacing for roof rafters in the UK?

In the UK, the standard 'centre-to-centre' spacing for rafters is 400mm, 450mm, or 600mm. The choice depends on the timber size and the weight of the roofing material (e.g., heavy concrete tiles vs light slates).

How do I calculate rafter length for a 30 degree roof?

For a 30-degree roof, you take the horizontal run (half the span) and divide it by the cosine of 30 degrees. For example, a 4m run / cos(30°) = 4.618m rafter length (plus any overhang).

What is a 'Birdsmouth' cut in roofing?

A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the wall plate. It consists of a vertical 'plumb cut' and a horizontal 'seat cut' to ensure the rafter is stable and properly supported.

Why do I need to deduct the ridge board thickness?

Calculations usually measure from the centre of the roof. However, rafters meet at a ridge board (usually 25mm to 50mm thick). You must subtract half of this thickness from the top of each rafter to ensure the roof span remains accurate.

What is the standard spacing for roof rafters in the UK?

In the UK, the standard 'centre-to-centre' spacing for rafters is 400mm, 450mm, or 600mm. The choice depends on the timber size and the weight of the roofing material (e.g., heavy concrete tiles vs light slates).

Roof Rafter Calculator

Calculate cutting lengths and angles for a professional UK timber roof.

Total Span (Width in m)

Rise (Vertical Height in m)

Eave Overhang (Horizontal in m)

Cutting List

Fitting Note: These measurements are the 'line length'. When cutting, you must deduct half the thickness of the ridge board from the top cut.

Roof Framing: Rafters, Cuts & Geometry

Precision Cutting

Building a 'cut roof' is one of the most technical skills in carpentry. Getting the rafter length and birdsmouth cut exactly right ensures the weight of the roof is distributed correctly down to the load-bearing walls.

Our rafter calculator provides the 'true' line length, including the diagonal length of your eave overhang for fitting fascias and soffits.

Common Rafter Specs

Standard Centres400mm / 600mm

Timber GradeC16 / C24

Min Birdsmouth Depth1/3 Timber Depth

Plumb Cuts & Seat Cuts

In roofing geometry, the Plumb Cut is the vertical cut at the top (where the rafter meets the ridge) and at the bottom of the overhang. The Seat Cut is the horizontal cut that rests on the wall plate. Together, they form the 90-degree geometry required for a stable structure.

Eave Overhang

The overhang protects your walls from rainfall. In the UK, a standard overhang is between 300mm and 450mm. Remember that our calculator asks for the horizontal overhang distance.

Birdsmouth Notch

Never notch a rafter deeper than 1/3 of its depth. Doing so significantly weakens the timber and can lead to structural failure under heavy snow loads.

Pro Tip: The 'Master' Rafter

Always cut one 'master' rafter first and test it against the ridge and wall plate before cutting the rest. This ensures any slight variations in the wall plate levels are caught early.

Roof Rafter FAQ

Structural Safety Disclaimer

Rafter sizes (e.g. 47x150mm vs 47x200mm) and timber grades (C16/C24) must be specified by a structural engineer based on the span and the weight of your chosen tiles. This tool provides geometric lengths only and does not substitute for structural design.