Roofing
Roofing calculator
This calculator works out the area of a pitched roof from the house footprint and roof pitch, and from that the sheets or tiles to cover it, the ridge cappings, and the trusses and purlins or battens underneath. It handles the three common shapes: a gable roof (two slopes and two triangular ends), a hipped roof (four slopes) and a mono-pitch or lean-to.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
IBR and corrugated sheets are quoted by their cover width, the width each sheet actually covers once it overlaps its neighbour, not by the sheet's full width. Sheets are usually cut to the length of the slope, so for gable and mono roofs the calculator gives the number of sheets and the length to order them at. Roof tiles are counted per square metre.
The steeper the roof, the more area there is to cover for the same footprint. Sheet roofs in Zimbabwe are often pitched between 5 and 17.5 degrees; concrete tiles need at least about 17.5 degrees and are usually laid at 26 degrees or more.
How it is worked out
- Plan area including the overhang = (length + 2 × overhang) × (width + 2 × overhang); a mono-pitch roof has no overhang on the high side added twice.
- Roof area = plan area ÷ cos(pitch). For a hipped roof with all slopes at the same pitch, the same formula holds.
- Rafter (slope) length = half the span including overhang ÷ cos(pitch); for a mono-pitch, the full span.
- Sheets across each slope = slope width ÷ cover width, rounded up. Tiles = roof area × tiles per m².
- Ridge = building length plus overhangs (gable); length minus width (hipped), with four hips; none for a mono-pitch.
What it assumes
- Valleys, dormers and L-shaped plans are not handled; split them into rectangles.
- Gutters, fascias, flashings and insulation are not included.
Common questions
How many IBR sheets do I need for my roof?
Divide the length of each slope (including the overhang) by the sheet cover width of 686 mm and round up; that is the number of sheets per slope, each cut to the rafter length. The calculator does this for you.
What is the minimum pitch for an IBR roof?
Manufacturers generally allow IBR down to about 5 degrees with correct end laps and fixings. Corrugated sheet needs more pitch, and concrete tiles at least about 17.5 degrees.
How far apart should roof trusses be?
Commonly about 1 m apart for tiles and up to about 1.2 m for sheeting, but truss spacing is set by the truss design. Follow the truss supplier's layout.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.