Rafter Length Calculator — Common, Hip & Valley Rafters

The rafter length (common rafter) equals the run divided by the cosine of the pitch angle, where the run is half the span less half the ridge board thickness. For an 8m span at 40° pitch with a 25mm ridge board: run = 4 − 0.0125 = 3.9875m, so rafter length = 3.9875 / cos(40°) = 5.21m. (Ignoring the ridge deduction gives the simplified 4 / cos(40°) = 5.22m.) Add the sloping eaves overhang for the true cutting length.

A birdsmouth is a notch cut into the rafter where it sits on the wall plate. It consists of a seat cut (horizontal) and a plumb cut (vertical). The seat cut depth should be no more than one-third of the rafter depth to maintain structural integrity — e.g., 50mm for a 150mm deep rafter.

Hip and valley rafters run diagonally at 45° in plan view, so they are longer than common rafters. The hip/valley factor multiplies the common rafter run by √(1 + 1/cos²(pitch)) — equivalently √(2 + tan²(pitch)) — which accounts for the diagonal travel across the roof plan. Their plumb (and seat) cut angles are also shallower than the common pitch, because the hip works off a longer run (run × √2). The plumb cut angle for a hip/valley is atan(rise ÷ (run × √2)), not the common pitch angle.

In the UK, 600mm centres are standard for most domestic roofs with tile or slate coverings. Use 400mm centres for heavier coverings like concrete interlocking tiles on lower pitches, or where building control specifies closer spacing for wind uplift resistance.

No — the rafters needed, timber volume and estimated cost are given per slope (one roof plane). A standard duo-pitch (gable) roof has common rafters on both slopes, so for the full roof you should double these take-off figures. The rafter count assumes the spacing runs along the eaves length you enter (it defaults to the span if you leave Wall Length blank, which only matches a square building); set Wall Length to your building length along the ridge for an accurate count.

Common domestic rafter sizes in the UK range from 47×100mm for short spans up to 47×200mm for longer spans. The required size depends on span, spacing, pitch and loading. BS 8103-3 and approved span tables (e.g., TRADA) provide the correct sizes for your specific situation.

Hip and valley rafters share the same geometry in this calculator: in plan view they run diagonally across the corner where two roof slopes meet (a hip is an external corner, a valley is the internal corner where two roof slopes meet, such as an L-shaped extension). For a standard 45° plan corner, the rafter's horizontal run in plan is the common rafter's run × √2 (Pythagoras across the square formed by the two adjoining common runs). It climbs the same rise as a common rafter, but over that longer diagonal run, so by Pythagoras its true sloping length² = (run × √2)² + rise² = 2 × run² + run² × tan²(pitch) = run² × (2 + tan²(pitch)), giving length = run × √(2 + tan²(pitch)), the hip/valley factor this calculator uses. Worked example, continuing the 8m span / 40° pitch / 25mm ridge case above (run = 3.9875m, rise = 3.346m): factor = √(2 + tan²40°) = √(2 + 0.704) = 1.644. Hip/valley rafter length = 3.9875 × 1.644 = 6.557m before overhang. Adding the default 450mm eaves overhang (0.45m × 1.644 = 0.740m) gives a true hip/valley rafter length of 7.297m, with a plumb cut angle of about 30.7°, shallower than the 40° common pitch because the hip/valley climbs the same rise over a longer run. Set Rafter Type to Hip or Valley above to reproduce this. Once cut, the roof covering along that line is finished with hip tiles (see the Hip Tile Calculator) or a valley trough (see the Valley Trough Calculator), depending on which junction it is.

Yes, with one adjustment. This calculator assumes a duo-pitch roof, where Roof Span is the full building width and a ridge sits in the middle, so it halves whatever you enter (Run = Span ÷ 2 − Ridge ÷ 2). A mono-pitch (lean-to) extension roof has no ridge: a single rafter runs the full horizontal distance from the high wall to the low wall. To model that correctly, enter Roof Span as double your actual wall-to-wall horizontal distance, set Ridge Board Thickness to 0, and keep Rafter Type as Common. For example, a lean-to with a 3m horizontal run at 15° pitch and a 300mm eaves overhang: enter Roof Span 6m, Ridge Board Thickness 0mm, Roof Pitch 15°, Eaves Overhang 300mm. That gives Run = 6 ÷ 2 − 0 = 3m (matching the real run), rafter length = 3 ÷ cos(15°) = 3.106m, rise = 3 × tan(15°) = 0.804m, and a true length of 3.106 + (0.3 ÷ cos(15°)) = 3.416m including the overhang. If you do not already know your roof's pitch, work it out first with the Roof Pitch Calculator.

No. The rafter length and true length shown by this calculator are the theoretical lengths along the top (back) edge of the rafter, from the ridge line to the tail. A birdsmouth is a notch cut into the underside of the rafter where it crosses the wall plate, and it does not shorten that top-edge line. What the birdsmouth does affect is the timber remaining above the seat cut at the bearing point (the heel), which is why UK convention keeps the seat cut to no more than one-third of the rafter's depth. This calculator gives you the notch depth and the plumb cut angle for marking out: it is a geometry and marking-out tool, not a structural check on whether the remaining depth is adequate for your span and loading. Note this applies to traditional cut-roof framing: trussed rafter roofs (see the Roof Truss Calculator) bear on the wall plate through a truss shoe rather than a birdsmouth notch.

No. This calculator works out rafter length, run, rise and cutting angles from your span, pitch and rafter type; it does not check whether that timber section is strong enough for your span, spacing, pitch and loading. Section size (for example 47×100mm versus 47×200mm) depends on span, spacing, snow and wind loading and the weight of the roof covering, and needs checking against a structural span table or a structural engineer's design. See the Rafter Span Tables reference for C16/C24 maximum spans by rafter size, spacing and pitch band, and get hip/valley layouts, long spans or unusual loading confirmed by a structural engineer or building control before cutting timber.