How to Calculate Roof Tiles, Felt and Battens

How Many Roof Tiles Do You Actually Need?

Getting your roof tile quantities wrong is expensive. Under-order and your supplier's next delivery might be a different batch — meaning a visible colour difference across your roof. Over-order by too much and you're stuck with pallets of tiles you can't return. Here's how to calculate it properly.

Step 1: Measure the Roof Area

You need the rafter length (slope length from eaves to ridge) and the horizontal run of the roof. If you can only measure the plan area from the ground, you'll need to apply a pitch factor.

For a simple gable roof:

Roof area = Plan area × Pitch factor

Pitch Factor Table

| Roof Pitch | Pitch Factor | |-----------|-------------| | 15° | 1.035 | | 20° | 1.064 | | 25° | 1.103 | | 30° | 1.155 | | 35° | 1.221 | | 40° | 1.305 | | 45° | 1.414 | | 50° | 1.556 |

For example, a semi-detached house with a plan area of 45m² and a 35° pitch:

45 × 1.221 = 54.95m² of actual roof area

Remember, most roofs have two slopes — measure both sides. They're not always identical, particularly on older properties.

Step 2: Work Out Tile Coverage

Different tile types cover different areas per tile. This depends on the tile size, headlap, and gauge (the exposed portion of each tile).

Common Tile Coverage Rates

| Tile Type | Typical Size (mm) | Coverage per Tile (m²) | Tiles per m² | |----------|-------------------|----------------------|-------------| | Concrete interlocking (large) | 420 × 330 | 0.092 | ~10.8 | | Concrete interlocking (standard) | 381 × 227 | 0.057 | ~17.5 | | Plain clay/concrete tile | 265 × 165 | 0.015 | ~60 | | Natural slate (500 × 250) | 500 × 250 | 0.037 | ~27 | | Natural slate (600 × 300) | 600 × 300 | 0.054 | ~21 | | Fibre cement slate | 600 × 300 | 0.054 | ~21 |

Important: These figures assume standard headlaps. Your tile manufacturer's data sheet will give you the exact coverage rate for the specific tile and headlap. Major manufacturers such as Marley and Russell Roof Tiles publish comprehensive technical data sheets online. Always check — getting this from a generic table rather than the manufacturer's spec is the number one cause of ordering errors.

Example Calculation

Using our 54.95m² roof with large concrete interlocking tiles at 10.8 tiles per m²:

54.95 × 10.8 = 593 tiles

Step 3: Add Wastage

Wastage depends on the complexity of the roof:

• Simple gable roof — add 5%
• Roof with hips — add 7–10%
• Complex roof with valleys, dormers, or multiple hips — add 10–15%

Cut tiles at hips and valleys are the main source of waste. For our example (simple gable):

593 × 1.05 = 623 tiles

Round up to the nearest full bundle or pallet.

Step 4: Don't Forget the Extras

Ridge Tiles

Measure the ridge length and divide by the tile length (typically 450mm). Add 2–3 spares.

Ridge tiles = Ridge length (m) ÷ 0.45

A 7m ridge:

7 ÷ 0.45 = 15.6 → 16 ridge tiles + 2 spares = 18

Verge Tiles

If you have verges (the sloping edge of a gable), you'll need verge clips or dry verge units. Count the number of tile courses from eaves to ridge on each verge.

Hip Tiles

Similar to ridge — measure the hip length and divide by 0.45. Each hip also needs a hip iron at the base.

Valley Tiles or Troughs

For valleys, you can use purpose-made valley troughs (typically 1.2m long) or cut tiles into a laced valley. Measure the valley length and order accordingly.

Felt and Membrane

Roofing underlay is sold in rolls. Standard breathable membrane rolls are typically 1m or 1.5m wide × 50m long.

To work out how many rolls:

1. Measure the rafter length (slope length)
2. Multiply by the roof width to get the area
3. Add 150mm horizontal overlaps between rows and 100mm end laps
4. In practice, add 10–15% to the roof area to account for overlaps and trimming

Rolls needed = (Roof area × 1.15) ÷ Roll area

For a 50m² roll covering our 54.95m² roof:

(54.95 × 1.15) ÷ 50 = 1.26 → 2 rolls

Breathable vs Non-Breathable

Modern building regulations (Approved Document C) generally require breathable membrane for new roofs. This eliminates the need for a ventilated air gap between the underlay and insulation. If you're using non-breathable felt (1F), you must provide a 50mm ventilated gap at eaves and ridge.

Battens

Roofing battens (typically 25mm × 50mm or 25mm × 38mm) are fixed horizontally at centres matching the tile gauge.

Batten quantity = Roof area ÷ Batten spacing × 1.1

Or more precisely:

1. Divide the rafter length by the tile gauge to get the number of batten rows
2. Multiply by the roof length to get total linear metres
3. Add 10% for waste and cuts

Common Batten Gauges

| Tile Type | Typical Gauge (mm) | |----------|-------------------| | Large interlocking concrete | 340–345 | | Standard interlocking concrete | 293–310 | | Plain tile | 95–100 | | Natural slate 500 × 250 | 190–210 |

Plain tiles need far more battens because of the tight gauge — keep this in mind for both cost and labour.

Counter-Battens

If you're creating a ventilated gap above the membrane (cold roof or non-breathable underlay), you'll need counter-battens running vertically down the rafters, typically 25mm × 38mm. Calculate based on the number of rafters multiplied by the rafter length.

Pre-Calculated Example: Typical 3-Bed Semi

For a standard 3-bed semi-detached with a plan footprint of 8m × 5m, 30° pitch, gable ends:

| Item | Calculation | Quantity | |-----|-----------|---------| | Roof area | 2 × (8 × 5) × 1.155 | 92.4m² | | Large concrete interlocking tiles | 92.4 × 10.8 × 1.05 | ~1,048 tiles | | Ridge tiles | 8 ÷ 0.45 + 2 | ~20 | | Breathable membrane | (92.4 × 1.15) ÷ 50 | 3 rolls | | Battens (25 × 50) | ~270 linear m + 10% | ~300 lin. m |

Rather than working through all of this by hand, you can plug your roof dimensions into the Roofing Calculator to get a full breakdown of tiles, felt, battens, and fixings in seconds.

Tips From the Roof

• Always order 5% extra on top of your wastage allowance for breakages on site and future repairs
• Check the minimum pitch — every tile type has a minimum recommended pitch. The National Federation of Roofing Contractors provides technical guidance on minimum pitch requirements. Using tiles below their minimum pitch voids the guarantee and risks leaks
• Store tiles properly — keep them on a flat surface, stacked no more than 6 courses high, and covered
• Distribute tiles evenly across the roof before you start — don't stack them all on one side or you'll overload the structure
• Keep 2–3 spares in the loft for future storm damage repairs — matching discontinued tiles years later is a nightmare