Roofing Calculator

How Many Roof Tiles Do I Need?

Whether you are re-roofing your entire house or replacing damaged tiles after a storm, knowing the exact tile count prevents costly over-ordering or panic mid-project trips to the supplier. While the roof is open, it is the ideal time to upgrade your loft insulation — use our insulation calculator to estimate rolls and thickness. This guide walks through the full calculation — from measuring your roof footprint to applying the slope factor, choosing your tile type, and working out battens, felt, and ridge tiles.

The Roof Area Formula

True Roof Area = Footprint Area x Slope Factor

The footprint area is the plan view of your roof — essentially the floor area of the rooms directly below the roof. For a simple gable roof, this equals the building length multiplied by half the building width (for each slope), times two.

The slope factor adjusts for the angle of the roof. A flat roof has a factor of 1.0. A steep roof has a factor well above 1.0 because the sloped surface is longer than the horizontal distance it covers.

Example: A detached house measuring 10m x 8m with a 35-degree gable roof:

• Footprint area = 10 x 8 = 80 m2
• Slope factor for 35 degrees = 1.221
• True roof area = 80 x 1.221 = 97.7 m2
• With 7% waste = 104.5 m2

Roof Pitch Slope Factors

Use this table to convert your roof footprint to actual roof area. The slope factor is calculated as 1 / cos(pitch angle).

Most houses in Northern Europe have pitches between 25 and 40 degrees. If you do not know your pitch, measure the rise and run in the loft and use the formula: pitch = arctan(rise / run).

Tiles Needed by Roof Size

This table shows approximate tile counts for a standard concrete interlocking tile (about 10 tiles per m2) including 7% waste, at common roof pitches.

For clay plain tiles (60 per m2), multiply the above figures by 6. For natural slate (20 per m2), multiply by 2.

Tile Coverage Rates by Type

Different tile types have very different coverage rates. This affects the total quantity, weight, and cost of your roof.

Plain tiles and slate require a much steeper pitch because they rely on overlap rather than interlocking channels to keep water out.

Battens, Felt, and Fixings

Tiles are only one part of the roofing materials list. You also need battens, underlay, ridge tiles, and fixings.

Battens: Standard 25 x 50 mm treated softwood battens are spaced at the tile gauge (typically 32-34 cm for concrete interlocking tiles). Calculate total batten length as: (rafter length / batten gauge) x roof width. Battens come in 3.6m or 4.8m lengths.

Roofing underlay/felt: Modern breathable membrane has replaced traditional bitumen felt on most reroofing projects. One roll typically covers 45-50 m2 with overlaps. Allow 15% extra for overlaps and waste.

Ridge tiles: Count the number of ridge metres (along the apex of the roof). Standard ridge tiles are 450 mm long, so you need about 2.3 tiles per linear metre including overlap.

Fixings: Current building regulations require all tiles to be mechanically fixed (nailed or clipped) in most wind zones. Budget for 10-12 nails or clips per m2 for interlocking tiles, or 60+ nails per m2 for plain tiles.

Step-by-Step: Calculating Your Full Roof Material List

1. Measure the footprint — Length x width of the building (or the portion under the roof). For L-shaped buildings, break it into rectangles.
2. Determine the pitch — Measure rise and run in the loft, or check original building plans. Use the slope factor table above.
3. Calculate true roof area — Footprint x slope factor.
4. Add waste — 5% for simple gable, 10% for hipped or complex roofs.
5. Choose your tile — Select the tile type and look up the coverage rate (tiles per m2).
6. Multiply — True roof area (with waste) x tiles per m2 = total tile count.
7. Add ridge tiles — Measure ridge length, multiply by 2.3.
8. Calculate battens — (Rafter length / batten gauge) x roof width = total batten metres.
9. Calculate underlay — True roof area x 1.15 (for overlaps) / roll coverage.
10. Budget fixings — Nails/clips per m2 x true roof area.

Common Mistakes When Ordering Roof Tiles

• Using footprint area instead of true roof area — This underestimates by 10-40% depending on pitch. Always apply the slope factor.
• Forgetting hip and valley waste — Hipped roofs require tiles to be cut at angles along every hip and valley. This generates significantly more waste than a simple gable.
• Not ordering spare tiles — Order 3-5% extra beyond the waste allowance as spares. Matching tiles years later can be difficult or impossible if the colour batch changes.
• Mixing tile types without checking compatibility — Concrete and clay tiles have different batten gauges, headlaps, and minimum pitches. Never assume interchangeability.
• Ignoring structural load limits — Switching from lightweight metal to heavy concrete or slate tiles may require strengthening the roof structure. Always check with a structural engineer.

Once the roof is complete, you may want to finish the ceiling below with plasterboard — our drywall calculator estimates sheets and fixings for the job. If the exterior walls also need freshening up, our exterior paint calculator covers masonry and cladding paint quantities.