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Roof tile calculator: tiles, slates, battens, underlay, ridge and BS 5534 fixings
Measure the roof along the eaves and down the rafter, pick the tile and the pitch, and get the tiles, battens, underlay, ridge, hips, valleys, verge and fixings for the whole roof. Coverage is by named tile with the gauge set by pitch (Marley Modern is 9.9 per m² at 345mm gauge, 10.7 at 320mm), slates by size and headlap, and fixings follow BS 5534: every single-lap tile fixed, perimeter tiles twice, every plain tile and slate twice.
What you need
Tiles
- Marley Modern / Redland Regent (large interlocking)
- 872 tiles345mm gauge at 35° (maximum gauge, pitch above 22.5°), 9.9 per m², plus 10% wastagetradecalculator coverage reference; Marley, Redland data
- Roof area
- 80.0 m²2 sides on the slope
- Concrete ridge tiles
- 19 × 450mm8m of ridge plus 5%. Must be mechanically fixed under BS 5534tradecalculator ridge calculator; Marley on BS 5534
- Dry ridge kit
- 2 × 6m kitsfor ridge; or mortar bedding with clips, since mortar alone is no longer a fixing
Battens and underlay
- Battens 25 × 50mm
- 55 × 4.8m lengths15 rows at 345mm gauge, 264m with 10% wastage. 25 × 50 always for slate and at 600mm rafter centresBS 5534 graded battens; BuildWiz
- Batten nails 65 × 3.35
- 450 pcsone per rafter crossing: 15 rows × 30 rafters at 600mm
- Underlay
- 2 × Breathable membrane 1 × 50 mincludes 15% for 150mm side and 100mm head laps
- Eaves protector trays
- 11 × 1.5m16.0m of eaves, plus eaves comb or vents where the tile profile needs them
Fixings
- Tile clips or nails
- 982 pcsone per tile plus one more for each of about 109 perimeter tiles at eaves and verges. Check the fixing spec for your site exposureBS 5534:2014+A2:2018 (Marley summary)
- Dry verge units
- 61 pcsone per course: 20.0m of verge at 345mm gauge plus 5%, plus starter and ridge-end caps
- BS 5534
- every tile fixed since 2015 every single-lap tile, ridge and hip must be mechanically fixed and perimeter tiles fixed twice; mortar is not a fixing on its own
- Slope factor at 35°
- 1.22 ×plan length × this = rafter length
Round up to whole packs and bags. Figures include the wastage you chose and match TradeCalc in the app.
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Or claim your free profile link nowHow the maths works
- Roof area is the eaves length × rafter length × number of sides, measured on the slope. If you only have the plan size, multiply the horizontal run by the slope factor for your pitch. Hipped ends are counted as full rectangles, which the wastage covers.
- Tiles per m² are by named tile (tradecalculator coverage reference; Marley and Redland data): Marley Modern and Redland Regent 9.9 at the 345mm maximum gauge or 10.7 at 320mm, Double Roman 9.5 to 10.5, Redland 49 and Marley Ludlow 16.3 to 17.7, plain tiles 60 at 100mm gauge with a 65mm lap (JJ Roofing). Maximum gauge is only allowed above about 22.5°, so the pitch picks the gauge. Slates are worked out from the size and headlap: gauge = (length − lap) ÷ 2 (SIGA), then 1,000,000 ÷ (gauge × (width + 5)), about 21 per m² for 500 × 250 at 75mm lap.
- Wastage is 5% for a simple gable, 10% with hips or a dormer, 15% for a complex roof with lots of cutting (squote; materialcalculator).
- Battens run at the gauge: rows are the rafter length ÷ gauge, times the eaves length and sides, plus 10%, in 4.8m lengths. 25 × 38mm up to 450mm rafter centres, 25 × 50mm at 600mm and always for slate, BS 5534 graded, one 65 × 3.35mm nail at every rafter (BuildWiz).
- Fixings follow BS 5534:2014+A2:2018 (Marley): every single-lap interlocking tile gets a clip or nail, tiles at the eaves and verges get a second fixing, every plain tile and every slate is twice-nailed, and every ridge and hip tile must be mechanically fixed, so mortar is bedding not fixing. The old 0.3 a tile was pre-2014 practice.
- Ridge tiles are 450mm for concrete or 300mm for clay, plus 5% (tradecalculator); hips the same along each hip, with a hip iron at the foot. A dry ridge kit covers 6m of ridge or hip. Underlay is in real rolls: breathable membrane 1 × 50m (50m²) or 1.5 × 50m (75m²), 1F felt 1 × 15m, with 15% for laps. Eaves protector trays are 1.5m each along the eaves.
- Verges are dry verge units at one per course, or undercloak strips in 1.2m lengths with mortar and clips. Valleys are 3m GRP troughs, valley tiles at 3 a metre, or lead.
Worked example
A gable roof 8m along the eaves with 5m rafters at 35° is 80m² over both sides. With Marley Modern at 9.9 per m² (345mm gauge) and 10% wastage that is 872 tiles, 55 lengths of 4.8m batten in 15 rows, 2 rolls of 50m² breathable underlay, 19 concrete ridge tiles, 2 dry ridge kits and 982 clips: one per tile plus the perimeter twice.
Questions people ask
How many roof tiles per square metre?
It depends on the tile and the gauge. Big interlocking tiles like Marley Modern or Redland Regent are 9.9 per m² at their maximum 345mm gauge and 10.7 at 320mm on a low pitch; small interlocking tiles like Redland 49 are 16 to 18; plain tiles are 60 because they are double-lapped at 100mm gauge; 500 × 250 slates at a 75mm lap are about 21. Check the datasheet for your exact tile, as the gauge range and the minimum pitch vary.
How many fixings do I need?
Since BS 5534:2014 every single-lap tile has to be mechanically fixed with a clip or nail, tiles round the perimeter (eaves, verges, abutments) fixed twice, and every ridge and hip tile fixed with a dry ridge system or a clip as well as any mortar. Plain tiles and slates are all twice-nailed. The calculator counts one per tile plus one extra per perimeter tile, or two per plain tile or slate; the manufacturer's fixing spec for your postcode and exposure is the final word.
How do I measure the rafter length?
Run a tape down the slope from the ridge to the eaves. If you cannot get up there, measure the horizontal distance from the wall to the centre of the ridge on plan, then multiply by the slope factor: about 1.22 at 35°, 1.31 at 40°, 1.41 at 45°.
What batten size do I need?
25 × 50mm BS 5534 graded batten for rafters at 600mm centres and always for slate; 25 × 38mm is allowed up to 450mm centres. Standard lengths are 3.6 and 4.8m, nailed with one 65 × 3.35mm nail at every rafter, and the batten gauge is set by the tile and the pitch, not the batten.
What size are underlay rolls?
Breathable membrane comes in 1 × 50m (50m²) and 1.5 × 50m (75m²) rolls; 1F bitumen felt in 1 × 15m rolls. Laps of 150mm on the side and 100mm at the head eat about 15% of the roll, which the calculator adds. A breathable membrane replaces felt and still needs a ventilated or dry-fixed ridge.
How much does it cost to retile a roof?
It varies a lot with the tile, the access and whether the battens and underlay need replacing. Use the numbers here to sanity-check a quote, and get two or three quotes for the job including scaffolding.
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