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Free general builder calculator
Extension materials calculator: footings, bricks, blocks and roof
Type in the footprint of an extension or garage and get a first-pass list of everything from the footing concrete to the plasterboard: three new walls (or four if it is detached), 600mm footings as trench fill or strip and blockwork, a 150mm oversite slab on 150mm of Type 1, a cavity wall by default, and a warm flat roof. Treat it as an ordering guide, not a design; Building Control will want to see the trench before you pour.
What you need
Groundworks
- Footprint
- 12.0 m²10.0m of new wall (three sides, the house closes the fourth), 21.0m² net of openings
- Footing trench to dig
- 6.00 m³600mm wide × 1m deep. Bulks to about 7.8m³, 9.9 tonnes of muck away; see the hardcore calculator for skipsmaterialcalculator muck-away densities
- MOT Type 1 under the slab
- 4.16 tonnes150mm compacted at 2.1 t/m³ plus 10%, or 6 bulk bags (~800 kg)tradecalculator Type 1: 2.1 t/m³ compacted
- Blinding sand
- 2 bulk bags (~800 kg)50mm of sharp sand over the Type 1 to protect the DPM, 1.06 tonnes
- DPM 1200 gauge
- 1 × rolls, 4 × 25m (100m²)lapped 150mm and turned up to meet the DPC
- A142 mesh in the slab
- 2 × 4.8m × 2.4m sheets2.22kg per m², 300mm laps, on chairs for 40 to 50mm cover
- Floor insulation 100mm PIR
- 5 boards2400 × 1200 boards, includes 7% wastage
Concrete
- Trench-fill footings
- 6.60 m³full trench, GEN1 (C8/10) or ST2; ask for GEN3 in sulfate clay. Includes 10% contingencyCemex BS 8500 designated mixes
- Oversite slab
- 1.98 m³150mm, GEN3 (C16/20) or ST4; RC28/35 if Building Control wants a designed reinforced slab. Includes 10% contingencyCemex BS 8500 designated mixes
- Ready-mix wagons
- 1.43 × 6m³ loads8.58m³ in two pours. Under 1m³ is a part load with a surcharge
Walls
- Facing bricks
- 1349 bricks60 per m² plus 7% wastageImperial Bricks per-m² guide
- Concrete blocks 100mm
- 225 blocksinner skin, 10 per m² plus 7% wastageJewson block calculator
- Cavity insulation 100mm full-fill
- 8 packsat 2.88m² a pack; check the pack coverage for your product
- Wall ties
- 53 ties2.5 per m² at 900 × 450mm, 1 × box of 250, extra round openingsAncon tie spacing
- DPC 100mm
- 1 × 30m rollsboth skins, 150mm above ground
- Lintels
- 3 eachan allowance of one per 2m² of openings; sizes from the drawings
- Cavity trays
- 3 eachone over every lintel, with stop ends
- Weep vents
- 6 eachat least two per tray, 450mm centres on wide openings
- Cavity closers
- 16.2 maround the reveals, roughly two jamb heights per opening
- Wall starter kits
- 2 kitswhere each new wall meets the house, one per storey each side
Mortar
- Cement (M4, 1:5 to 6 with plasticiser)
- 15 × 25kg bags1835kg of mortar: 8 bags and 0.9 tonnes of sand per tonnematerialcalculator mortar; Marshalls BTB4 designations
- Building sand
- 3 bulk bags (~800 kg)bulk bags are about 800kgSelco; BMF
Roof
- Roof joists C24 47 × 195
- 11 × 3.3m lengths400mm centres along the 4m, spanning the 3m plus bearings. Confirm the size with the span tables or your engineer
- Firrings
- 11 × 3.3mtapered strips on every joist for a 1:80 fall
- Roof deck OSB3 18mm
- 5 sheetsfootprint plus 10% overhang and 7% cuts
- Roof insulation 150mm PIR (warm roof)
- 5 boardsPart L 2021 wants 0.15 W/m²K on a new roof, about 150mm of PIR; confirm on your drawingsApproved Document L 2021, Table 4.1
- Roof covering
- 13.8 m²EPDM or GRP, includes upstands
- Fascia, soffit and guttering
- 10.0 m
Inside
- Plasterboard 12.5mm
- 13 × 2400 × 1200 sheetswalls and ceiling, 33.0m² plus 7%
- Thistle MultiFinish skim
- 4 × 25kg bags10m² a bag at 2mm plus 10%British Gypsum product data
- Structural design
- check Lintels, steels, joist sizes, footing depth and insulation thickness must be confirmed against your drawings and Building Regulations
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
- An attached extension has three new walls: the footprint length plus twice the width. Tick detached for four. Wall area is that run × wall height × storeys, less your openings. Bricks are 60 per m² and blocks 10 per m² (Imperial Bricks; Jewson), with 7% wastage on everything that is cut.
- Footings are 600mm wide at the depth you enter. Trench fill is concrete to near ground level; a strip footing is 300mm of concrete with two skins of dense blocks up to DPC. Footing concrete is GEN1 (C8/10) or ST2, the 150mm oversite slab GEN3 (C16/20) or ST4, or RC28/35 if it is a designed reinforced slab (Cemex BS 8500 designated mixes). Both carry a 10% contingency and are shown as 6m³ wagon loads.
- Under the slab: 150mm of MOT Type 1 at 2.1 tonnes per compacted m³ plus 10% (tradecalculator), 50mm of sharp sand blinding, a 1200 gauge DPM in 4 × 25m rolls with 10% for laps, A142 mesh at 11.52m² a sheet with 10% for 300mm laps, and 100mm PIR in 2.88m² boards.
- Mortar for facing brickwork is 8 bags of cement and 0.9 tonnes of building sand per tonne of mortar (materialcalculator) at M4, 1:5 to 6 with plasticiser. A brick takes 1kg and a 100mm block 2.2kg. Sand is in bulk bags of about 800kg (Selco; BMF).
- Cavity walls get ties at 2.5 per m² (Ancon), 100mm DPC rolls for both skins, a lintel allowance of one per 2m² of openings, a cavity tray over every lintel with two weep vents, cavity closers round the reveals and a wall starter kit per storey where each new wall meets the house. Cavity insulation is in packs at the coverage you enter, because it varies by product.
- Block and render is two block skins with either two-coat sand and cement at 27kg of sand per m² and about 9 bags of cement a tonne (tradecalculator), or monocouche at 1m² a bag at 15mm.
- A warm flat roof is C24 joists at 400mm centres spanning the width plus 300mm of bearings, firrings for the fall, 18mm OSB, 150mm PIR (Part L 2021 asks for 0.15 W/m²K on a new roof) and 15% extra EPDM or GRP for upstands. Pick pitched and the roof hands off to the roof tile calculator. Inside is 12.5mm plasterboard and Thistle MultiFinish at 10m² a bag (British Gypsum).
Worked example
A 4m × 3m single-storey extension with 2.7m walls, 6m² of openings and 1m trench-fill footings has a 10m wall run and 21m² of net wall. That comes to 6.60m³ of GEN1 for the footings and 1.98m³ of GEN3 for the slab, 4.16 tonnes of Type 1, 1,349 facing bricks, 225 blocks, 15 bags of cement, 3 bulk bags of sand, 8 packs of cavity insulation and 13 sheets of plasterboard.
Questions people ask
Why only three walls?
Most extensions are built against the house, so the existing wall closes one side. Tick detached for a garage or garden building and the calculator uses the full perimeter, drops the wall starter kits and keeps everything else the same.
Trench fill or strip footings?
Trench fill, concrete poured to about 150mm below ground, is the norm on domestic jobs: more concrete, but no laying blocks in a trench. A strip footing is 300mm of concrete with blockwork up to DPC, which saves concrete on a deep dig. Either way it is GEN1 or ST2 concrete, and Building Control will want to see the trench before you pour.
How deep should the footings be?
The calculator defaults to 1m, which is common for a house extension on decent ground. Clay, nearby trees and drains can all push that deeper. Building Control inspects the trench before you pour, so ask them first rather than guessing.
What concrete do I ask for?
GEN1 (C8/10) or ST2 for trench fill and strip footings, GEN3 (C16/20) or ST4 for the oversite slab and garage floors. If the drawings show a designed reinforced slab ask for RC28/35, and in sulfate-bearing clay ask the plant for a sulfate-resisting mix. Order in two pours and allow the 10% contingency; part loads under about a cubic metre carry a surcharge.
What about a pitched roof?
Pick pitched and the deck, PIR and EPDM lines drop out, leaving the wall plate, fascia and guttering. Then use the roof tile calculator with the rafter length and pitch for the tiles, battens, underlay, ridge and fixings. Trusses or cut rafters come from the drawings.
Is this enough to build from?
It is enough to price and order the bulk materials. Lintels, steels, joist sizes, footing depth and insulation thickness all need to be confirmed against your drawings and Building Regulations, and Building Control will inspect the trench, the DPC and the roof.
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