Roofing
The roof is the most critical element of a building's weather protection. Learn about roof types, minimum pitch per material, battens, counter battens and underlay — and avoid the most common mistakes.
Common Roof Types
| Roof type | Pitch | Advantages | Disadvantages |
|---|---|---|---|
| Gable / duo-pitch (two slopes) | 18–45° | Classic, good run-off, well-understood construction | Cold loft / requires ventilation |
| Hipped roof (four slopes) | 18–45° | Good wind resistance, small gable | More complex to build |
| Mono-pitch / lean-to (one slope) | 5–20° | Simple construction, contemporary appearance | Greater demands on underlay and membrane |
| Flat roof | 1–5° | Maximises usable area, roof terrace possible | Demanding waterproofing, high maintenance |
| Mansard roof | 30–70° + 15–30° | More habitable space in upper storey | Complex construction |
Minimum Pitch by Roofing Material
Every roofing material has a minimum pitch. Below this, capillary action, wind pressure and water back-splash can drive water into the construction. The figures below are typical/indicative examples — the binding minimum for any given product is the value in its data sheet, tied to the underlay specification and local rules, so always check the specific manufacturer's figure.
| Roofing material | Min. pitch | Recommended | Notes |
|---|---|---|---|
| Concrete roof tiles | 15° (27%) | 25–45° | Some profiles 18°. Below 22° (40%) a type-approved underlay must act as the primary barrier. |
| Clay / terracotta tiles | 22° (40%) | 30–45° | 18° permitted in low-exposure zones. Heavy (~45 kg/m²). |
| Corrugated / profiled steel sheets | 7° (12%) | 14–25° | Some trapezoidal profiles from 5°. Fast to install. |
| Standing seam (steel / aluminium) | 3° (5%) | 5–90° | Continuous decking required. Popular on contemporary buildings. |
| Asphalt shingles | 14° (25%) | 19–45° | 9° possible with double underlay (product-dependent). |
| Green roof / sedum | low-slope (often ~1.5–10°) | up to ~30° with slip protection | Usually built on low slopes over a fully waterproof, root-resistant membrane with a drainage layer — it is not a minimum-pitch material. Saturated weight 150–300 kg/m²; needs a structure designed for it. |
| EPDM / PVC / TPO membrane | 1.4° (1:40) | 3–5° | Flat roofs — internal downpipe drainage required below 2°. |
| Timber shingles / shakes | 30° (58%) | 40–60° | Traditional. Birch bark under-layer for historic buildings. |
Roof Build-up — Cross Section
Typical insulated pitched roof
Batten Centres and Tile Coverage
| Material | Batten c/c | Batten section | Coverage / lap |
|---|---|---|---|
| Concrete tiles 22–25° | 340–355 mm | 36×48 mm | 9–10 tiles/m² |
| Concrete tiles 30–35° | 365–375 mm | 36×48 mm | ~8.5–9 tiles/m² |
| Concrete tiles 40–45° | 375–385 mm | 36×48 mm | ~8–8.5 tiles/m² |
| Clay tiles 22–25° | 340–355 mm | 36×48 mm | 10–14 tiles/m² |
| Clay tiles 30–40° | 370–385 mm | 36×48 mm | ~9–11 tiles/m² |
| Trapezoidal steel sheet | Rafter / purlin c/c | purlins sized by design (e.g. 50×150 mm) | Purlin size depends on span, snow/wind load and the sheet profile. Side lap: typically 1 corrugation (profile-dependent) |
| Asphalt shingles | — | 19 mm plywood deck | 7–10 strips/m² |
Always check the manufacturer's fixing instructions — centres vary between profiles. The eaves batten is typically raised ~20 mm to give the correct angle to the first tile course.
Counter Batten (Ventilation Gap) Height
Minimum ventilation gap between underlay and battens, by pitch and rafter length:
| Pitch | Rafter length ≤ 5 m | 5–10 m | 10–15 m |
|---|---|---|---|
| 10–15° | 36 mm | 48 mm | ≥ 48 mm |
| 15–25° | 23 mm | 30 mm | 36 mm |
| 25–35° | 23 mm | 23 mm | 30 mm |
| > 35° | 23 mm | 23 mm | 23 mm |
Based on SINTEF Byggforsk 525.102 guidance. Standard counter batten thicknesses: 23, 30, 36 mm.
Building Code — Key Requirements
Calculating Roof Area
The actual roof area is always larger than the building footprint because of the pitch. The formula is:
| Pitch (α) | Equivalent % | cos(α) | Area uplift |
|---|---|---|---|
| 15° | 27% | 0.966 | +3.5% |
| 22° | 40% | 0.927 | +8% |
| 30° | 58% | 0.866 | +15% |
| 40° | 84% | 0.766 | +31% |
| 45° | 100% | 0.707 | +41% |
Step by Step: Laying Roof Tiles
Check that rafters and purlins are straight, dry and free from rot. Verify that any changes to the roof build-up do not significantly alter the snow load.
Start from the eaves. Roll out the underlay membrane horizontally with 100–150 mm laps on horizontal joints and 200 mm on vertical joints. Tape all laps with the manufacturer's approved tape.
Counter battens (vertical, along each rafter) are fixed over the underlay — 25×48 mm or 36×48 mm — to secure the membrane and create the ventilation gap.
Calculate the correct centres based on tile type and pitch. Start from the eaves with a tilting fillet (eaves batten raised in thickness to set the correct angle for the first tile course). Use the correct batten section — see the table above.
Start from the lower left corner. Keep tiles square to the eaves line. Fix tiles in exposed zones (ridge, eaves and verge) in accordance with the manufacturer's requirements for clips and screws.
Material estimate: Number of tiles = (roof area / area per tile) × 1.10 (10% waste allowance).
Ridge tiles are laid with laps in accordance with the manufacturer's instructions (typically ~50 mm) and fixed with screws. Verge details seal the sides. Gutters and downpipes are sized to the drained roof area, the local rainfall intensity and the number and size of downpipes — not by gutter length. A long gutter with one small outlet can still overflow, so size the gutter and outlets from a rainwater-drainage calculation or the manufacturer's capacity tables for your rainfall region.
Gutters and Downpipes
| Gutter size | Max roof area | Downpipe size |
|---|---|---|
| 100 mm half-round | ~20 m² | Ø 75 mm |
| 125 mm half-round | ~35 m² | Ø 87 mm |
| 150 mm half-round | ~60 m² | Ø 100 mm |
| 150 mm square / box | ~70 m² | 76×52 mm |
Gutter fall: minimum 1:200 (approx. 5 mm per metre).
Common Mistakes
Ready to calculate your roof?
Use the roof calculator to work out the number of tiles, battens, underlay and total material cost.
Open roof calculator →Frequently asked questions
What is a "square" in roofing?
A roofing square equals 100 sq ft of roof surface. Most shingle bundles cover 33 sq ft, so you need 3 bundles per square.
What is the minimum roof pitch for asphalt shingles?
Asphalt shingles require a minimum 2:12 pitch with special low-slope installation (double underlayment). Standard installation starts at 4:12. Below 2:12 requires a flat roofing system.
What is roofing underlayment and is it required?
Underlayment (felt paper or synthetic) is installed under shingles as a secondary moisture barrier. It is required by most building codes and provides protection if shingles are damaged.