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.

How Much Does a New Roof Cost?

A new roof is one of the largest home improvement expenses, but prices vary enormously depending on the material, roof complexity, and where you live. This section breaks down every cost component — materials, labour, scaffolding, and extras — so you can budget accurately and compare quotes with confidence. Use the tile and area figures above together with the per-m2 prices below to turn your roof size into a total project budget.

Roof Replacement Cost by Material

The table below shows total installed costs (materials + labour + scaffolding) for a full re-roof. Prices assume stripping the existing roof to rafters, new breathable membrane, new battens, new tiles, and ridge work.

Concrete interlocking tiles are the most popular choice in Northern Europe because they offer the best balance of cost, durability, and appearance. Natural slate is the premium choice, typically used on period properties or high-end new builds.

Cost Breakdown: What You Are Paying For

A full re-roof involves many cost components. Understanding each one helps you evaluate quotes and spot anything missing.

Cost by House Type

These are realistic total costs for a full re-roof using concrete interlocking tiles, which is the most common and cost-effective option.

Hipped roofs add 15-25% to the cost compared to gable roofs because of the extra hip tiles, cutting waste, and more complex labour.

Roof Repair vs Full Replacement Costs

Not every roof problem needs a full replacement. Here is what common repairs cost compared to a full re-roof.

As a rule of thumb: if more than 20-30% of tiles are damaged or the roofing felt underneath is tearing, a full re-roof is more cost-effective than repeated spot repairs.

Material Cost Comparison Per m2

This table shows material costs only (excluding labour and scaffolding) so you can compare the true cost of each roofing material.

When viewed as cost per year of lifespan, concrete tiles offer the best value. Natural slate has a high upfront cost but its exceptional longevity makes the per-year cost competitive with cheaper materials.

Labour Cost Factors

Roofing labour rates vary based on several factors. Understanding these helps you evaluate why quotes differ.

• Roof complexity — A simple gable roof with no dormers costs less per m2 to tile than a hipped roof with valleys, dormers, and multiple flashings. Complex roofs require more cutting, more lead work, and more time.
• Access — Difficult access (narrow lanes, no space for scaffold tower, overhead cables) increases scaffolding costs and slows work.
• Height — Three-storey properties or tall two-storeys with high eaves require more scaffolding and increase risk, both of which add cost.
• Season — Roofers are busiest from late spring to early autumn. Winter quotes may be 10-15% lower, but work depends on weather windows.
• Region — Urban areas and high cost-of-living regions typically have higher labour rates than rural areas.

Hidden Costs to Budget For

The most common hidden cost is rotten timber. You cannot see the condition of rafters and battens until the old tiles and felt are removed. Budget a 10% contingency for unexpected timber repairs.

7 Ways to Save Money on a New Roof

1. Get at least three quotes — Roofing prices vary significantly between contractors. Always get three written, itemised quotes and compare like for like.
2. Choose concrete interlocking tiles — They offer the lowest cost per year of lifespan and are quick to lay (reducing labour costs).
3. Schedule in the off-season — October to February is quieter for roofers. You may negotiate 10-15% off.
4. Combine work — If you need gutters, fascias, or loft insulation, do it during the re-roof while the scaffold is up. You save on a separate scaffolding hire.
5. Keep the roof shape simple — If building new, a gable roof is significantly cheaper than a hipped roof. Avoid unnecessary dormers and valleys.
6. Use a dry ridge system — Although it costs slightly more upfront than a mortar ridge, it never needs repointing and reduces long-term maintenance to zero.
7. Do not pay everything upfront — Pay a deposit (20-30%), then staged payments, with the final 10% held back for 2-4 weeks to confirm no leaks after the first rain.