How we calculate: roofing
Every formula and default figure our roofing calculators use. Each figure can be changed in its calculator to match your product. See how we calculate for the method behind all of them.
Roof slate calculator
Open the roof slate calculator
- Gauge = (slate length − headlap) ÷ 2
- Slates per m² = 1 ÷ (gauge × (slate width + side gap)), all in metres (10⁶ ÷ (gauge × (width + gap)) in mm)
- Batten per m² = 1 ÷ gauge (1000 ÷ gauge in mm)
- Slope area = plan area ÷ cos(pitch) when measured on plan; slates = slope area × slates per m² × (1 + waste), rounded up
- Nails = slates × nails per slate; slate-and-a-half = verges × courses, where courses = rafter length ÷ gauge, rounded up
Headlap depends on the slate, the pitch and your exposure: take it from the slate maker or BS 5534, never a rule of thumb.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Side gap between slates | 5 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2 (test RS1 uses a 5 mm side gap). Researcher to confirm against BS 5534 and slate makers. |
| Waste allowance | Enter from the bag, pack or tin | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2 gives no waste figure. Researcher to set from slate makers' guidance; blank adds none. |
| Nails per slate | 2 nails | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R2: 'nails (2 per slate)'. Researcher to confirm against the maker's fixing specification. |
Roof tile calculator
- Pitch from a rise: pitch = atan(rise ÷ run), where run = half the plan width (or the full width for a lean-to)
- Slope area = plan area ÷ cos(pitch) (the same for gable, hipped and lean-to roofs at one pitch)
- Tiles = slope area × tiles per m² × (1 + waste), rounded up
- From the tile size: gauge = tile length − headlap (single-lap) or (tile length − headlap) ÷ 2 (plain tiles); tiles per m² = 1 ÷ (gauge × cover width)
- Rafter length = run ÷ cos(pitch); hip length = √(2 × run² + rise²); ridge = plan length (gable) or plan length − plan width (hipped)
- Battens (m) = slope area ÷ gauge; verge units = verge length ÷ gauge, rounded up
Each tile has a minimum pitch and a headlap set by its maker for your exposure: check the maker's data and BS 5534.
Hipped roofs here have all four slopes at the same pitch and the ridge along the longer side.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Waste allowance | 5 % | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R1 (test RT1 uses 5%). Researcher to confirm against tile makers' guidance. |
Flat roof calculator
- Sheet length = deck length + 2 × (upstand + drip); sheet width = deck width + 2 × (upstand + drip)
- Membrane (m²) = sheet length × sheet width (× layers for torch-on)
- Adhesive tubs = deck area ÷ coverage per tub, rounded up
- Edge trim (m) = deck perimeter − edges against a wall
- Rolls = membrane area ÷ (roll length × (roll width − side lap)), rounded up
Upstand and drip allowances are illustrative: use the membrane maker's details for your edges and upstands.
Roll counts are by area; plan the strips across the roof and allow for end laps as the maker shows.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Upstand or turn-down at each edge | 150 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R5 (test FR1, illustrative: 150 mm upstand all round). Researcher to source from membrane makers. |
| Extra at each edge for the drip or trim | 50 mm | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R5 (test FR1, illustrative: 50 mm drip). Researcher to source from membrane makers. |
Guttering and downpipe calculator
Open the guttering and downpipe calculator
- Effective area (m²) = plan area × pitch factor (Approved Document H)
- Flow (L/s) = effective area × rainfall intensity (L/s/m²)
- Flow to each outlet = flow ÷ outlets; with the outlet in the middle, each side of the gutter carries half of that
- Fall (mm) = run ÷ n for a fall of 1 in n; brackets = run ÷ bracket spacing, rounded up, + 1
Gutter capacity depends on the gutter profile, the outlet and where it sits: use the gutter maker's flow figures.
Downpipes discharge to a drain or soakaway; check with building control where new drainage is needed.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Rainfall intensity | 0.021 L/s/m2 | Regulation or HSE figure Approved Document H (Researcher verifies) Not yet checked against the source | Source to be added by the Researcher brief v2 R6: default 0.021 L/s/m² (about 75 mm/h) from Approved Document H. Researcher verifies the clause and value. |
| Table | Clause | Nations | Status |
|---|---|---|---|
| Effective roof area: design factor by roof pitch | Approved Document H, Section H3 (Researcher verifies the table and row pitches) | England | 0/5 values entered Not yet checked against the source |
| Gutter and outlet sizes by flow capacity | Approved Document H, Section H3 (Researcher verifies) | England | 0/0 values entered |
Roof batten calculator
Open the roof batten calculator
- Courses per slope = (rafter length − gap at the ridge) ÷ gauge, rounded up
- Battens per slope = courses + extra eaves battens
- Batten (m) = battens per slope × slopes × roof width
- Lengths to buy: each batten run cut from your stock lengths, packed longest first (first-fit decreasing)
- Rafters per slope = roof width ÷ rafter spacing, rounded up, + 1; nails = rafters × battens × slopes × nails per crossing
- Counter-batten (m) = rafters × rafter length × slopes
Batten size, grade and fixings depend on the rafter spacing and the tile or slate: follow the maker's specification and BS 5534.
| Figure | Our default (adjustable) | Kind | Source |
|---|---|---|---|
| Extra battens at the eaves per slope | 1 battens per slope | Our assumption Not yet checked against the source | Our choice: change it to suit your job brief v2 R3 (test RB1: '16 courses (+1 eaves) per slope'). |
| Allowance per piece | 0.1 m | Our assumption Not yet checked against the source | Our choice: change it to suit your job Allowance per piece for squaring ends and mitres (brief v2 FL2). |
Roof pitch and rafter length calculator
Open the roof pitch and rafter length calculator
- Run = half the building width less half the ridge board (two slopes), or the full width (one slope)
- Pitch = atan(rise ÷ run); or rise = run × tan(pitch)
- Rafter length = √(run² + rise²) + overhang ÷ cos(pitch)
- Plumb cut = the pitch; seat cut = 90° − pitch
- Slope area = rafter length × roof length × slopes; rafters = (roof length ÷ spacing, rounded up, + 1) × slopes
This gives geometry only. Rafter sizes, grades and how far they can reach are a structural design: use a structural engineer, timber span tables or building control.
Rafter length is measured along the top edge from the ridge cut to the tail, before any trimming on site.
This calculator has no built-in defaults: every figure comes from you or your product's data.