The biggest error in roof estimating is using the plan area. A roof is sloped, so its true surface area is always bigger than its footprint, and the steeper the pitch, the bigger the difference. On a steep roof, pricing the footprint can leave you a third short on materials. Start with the right area and the rest of the takeoff follows.
The pitch factor
Pitch factor = √(1 + (rise ÷ run)²). Multiply the plan area, including the overhangs, by this factor to get the true roof area.
| Pitch | Degrees | Factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
For a hip roof where every slope has the same pitch, the same factor applies. A hip roof over a given footprint has the same area as a gable. Only the edges differ: hips instead of verges. Roofs with different pitches on different faces need each face worked out separately.
Measuring the roof
Take the building's length and width from the wall faces, then add the overhang at the eaves and verges. On complex roofs, split the plan into rectangles, one for each roof plane or wing, and work out each one. Use the drawings where you have them, and check at least one dimension on site, especially on older houses.
Tiles, sheets and battens
Most Australian roofs are either steel sheet (Colorbond or Zincalume) or concrete or terracotta tile. Sheets are counted by the cover width along the eaves and cut to length: corrugated and Trimdek cover 762 mm. Tiles per m² depend on the batten gauge and the tile's cover width: concrete tiles run about 10–11 per m² and terracotta about 11–13. Batten run = roof area ÷ gauge, so check the tile maker's gauge table for your pitch.
The edges
Measure eaves, verges, ridge, hips and valleys separately. They drive many material lines:
- Eaves fillers, and barge capping or verge tiles
- Gutter apron and eaves protection
- Ridge and hip capping, bedded and pointed on tiles, or capping on metal roofs
- Valley gutters (valley irons)
- Ventilation at eaves and ridge
- Flashings at walls, chimneys and roof windows
Hip length = (half the building width) × hip factor, where hip factor = √(2 + (rise ÷ run)²). Valleys use the same factor.
Underlayment
Sarking (reflective foil laminate) goes under tiles and often under metal sheet, in rolls of about 1.35 × 30 m. Allow about 15% for laps. Under metal roofs, a roof blanket adds insulation and cuts condensation and rain noise. In bushfire-prone areas, sarking and sealing must suit the site's BAL rating.
Fixings and sundries
Tile clips or screws as AS 2050 and the tile maker specify for your wind classification, and batten screws or nails. For metal roofs, roofing screws with sealing washers, at the spacing the sheet maker gives for your wind region.
Check the minimum pitch
Corrugated sheet needs at least 5°, Trimdek 2° and Klip-Lok 1°. Concrete and terracotta tiles usually need 15° or more, depending on the profile and the sarking. Below the minimum the roof will leak no matter how well it's laid.
Don't forget the rest of the job
- Tear-off and disposal of the old roof (price by m², and by layer)
- Batten and rafter repairs, priced as an allowance
- Scaffolding or edge protection
- Gutters, if they're being replaced
- Skip bins
The roofing calculator works all of this out for gable, hip and mono-pitch roofs, and the roof pitch calculator converts between every pitch format.
A worked example
A 10 × 8 m house with 450 mm eaves and a 22.5° pitch, in concrete tiles: plan area 10.9 × 8.9 = 97 m². True roof area 97 × 1.082 = 105 m². With 10% waste at about 10.6 tiles per m², about 1,225 tiles. Battens at 345 mm gauge: about 335 m, or 70 lengths of 4.8 m. Three rolls of sarking, and capping for 10.9 m of ridge.
Pricing checklist
- Area from the plan × pitch factor, not from the footprint
- Waste set for the roof's complexity
- Every edge measured and its materials listed
- Underlay, ventilation and flashings included
- Tear-off, disposal and repairs priced or excluded in writing
- Access, edge protection and skips included
Reading the roof on site
Before you price, walk the roof or inspect it from a ladder or drone. Note anything that adds work: soft or sagging areas, which point to rotten battens or rafters; old layers that must come off; chimneys that need new flashing or repointing; skylights and vents; and how close the roof is to power lines, trees and neighbours. Photograph everything. The photos back up your price, and they help if there's a dispute later about what was there before you started.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check the NCC, Australian Standards and your building surveyor or certifier for your project.