Roofing calculator

The formula, in plain words

1. 1Rafter length = plan run (eaves to ridge) ÷ cos(pitch). On a gable or hip roof the run is half the width.
2. 2Sheet length = rafter + about 50 mm into the gutter, made to order to the next 10 mm.
3. 3Sheets per side = eaves length ÷ cover width, rounded up. Cover width already allows for the side lap. Hip roofs: order every sheet at one length and use the hip offcuts on the opposite corner.
4. 4Batten rows = rafter ÷ batten spacing, rounded up, + 1. Screws per sheet = the end-support pattern at the two end rows plus the internal pattern at every other row.
5. 5Hips and valleys = plan run × √(2 + tan² pitch). Barges = rafter length. Sarking = roof area ÷ (roll width − 150 mm lap) × roll length.

Worked example

A 12 × 8 m gable roof at 22.5° in 0.42 mm corrugated (762 mm cover) on battens at 900 mm.

1. 1.Rafter: 4 ÷ cos 22.5° = 4.330 m. Sheet length: 4.330 + 0.05 = 4.38 m.
2. 2.Sheets: 12 ÷ 0.762 = 15.7, so 16 per side, 32 in all.
3. 3.Batten rows: 4.33 ÷ 0.9 = 4.8, so 5 spaces and 6 rows. Screws: 32 × (2 × 5 + 4 × 3) = 704.
4. 4.Roof area: 12 × 8 × 1.082 = 103.9 m². Ridge capping 12 m; barge capping 4 × 4.33 = 17.3 m.

Answer: 32 sheets at 4.38 m, 704 screws, 12 m of ridge capping and 17.3 m of barge.

How to measure it

• Take the roof's length along the eaves and its width eaves to eaves from the plan, to the outside of the fascia.
• Find the pitch on the elevations or the truss drawings.
• Pick the profile and thickness, and the batten or purlin spacing from the frame.
• Before ordering, measure the actual sheet lengths on site: manufacturers make sheets to site measurements, not plans.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

Frequently asked questions