Tiles and roofing are both bought by the box or square metre to cover an area — but the area you order is never the area you measured. Wastage, cuts, and roof pitch all mean you need more. Here's how to get the numbers right for an Australian job.
Tiling starts with the area to be covered, in square metres — length × width for a floor, or height × width for each wall.
A bathroom floor of 3m × 2.5m is 7.5 m². A wall 2.4m high and 3m wide is 7.2 m². Add every surface you're tiling, then subtract large openings such as a window or a shower recess if it isn't being tiled. Our tile calculator and area calculator handle the arithmetic.
You never order exactly the measured area, because tiles are cut to fit edges, around fixtures, and to run patterns. Those offcuts are waste, and a sensible allowance protects you from running out of a batch you may not be able to match later.
Our 7.5 m² floor, with a 10% allowance, means ordering about 8.25 m² of tile.
Tiles are sold by the box, and each box covers a stated number of square metres — so the final step is dividing your total area (including wastage) by the box coverage, then rounding up to whole boxes.
If a box of 300mm × 600mm tiles covers, say, 1.44 m², then 8.25 m² needs 8.25 ÷ 1.44 ≈ 5.7, rounded up to 6 boxes. Always round up — you cannot buy part of a box, and the spare tiles are your repair stock.
Roofing has a trap that tiling doesn't: the roof area is larger than the building's footprint, because the roof slopes.
Measuring the building from above (its plan area) understates the actual sloped surface. A pitched roof over a 12m × 8m footprint has a plan area of 96 m², but at a 22.5° pitch the true sloped area is closer to 104 m² — and it grows as the roof gets steeper.
Our roofing calculator takes the footprint and pitch and returns the true area, then adds an allowance for overlaps and ridges.
Once you have the true roof area, materials are estimated by coverage, much like tiles by the box. Sheet roofing (such as Colorbond) is ordered by length to suit the roof, while roof tiles are ordered by the number per square metre, which depends on the tile profile.
Both need extra beyond the bare area: overlaps between sheets or courses, ridge and hip capping, valley flashing, and cuts. A wastage and accessories allowance on top of the sloped area is standard, and the exact figure depends on the roof's complexity — a simple gable wastes little, a roof full of valleys and dormers wastes far more.
In an Australian bathroom, laundry, or any wet area, waterproofing is not optional and is not a place to save money. It is a regulated requirement, and in most cases it must be carried out and certified by a licensed waterproofer before tiling begins.
Waterproofing sits underneath the tiles, so it is invisible in the finished room — which is exactly why it is tempting to skimp on and disastrous to get wrong. A failed membrane behind a shower can rot framing and damage rooms below, and rectifying it means removing all the tiling you just paid for.
For both tiling and roofing, the material is only part of the bill, and often not the largest part.
Tiling cost is driven by the tile price per square metre, plus adhesive, grout, waterproofing (essential and regulated in wet areas), and labour — and labour rises sharply for large-format tiles, intricate patterns, and rooms with many cuts. Roofing cost includes the roofing material, battens or purlins, insulation, flashings, guttering, and the significant labour of working at height.
Because material prices and labour rates vary widely, use the calculators to fix your quantities, then get itemised quotes. A quote should separate materials from labour so you can compare fairly.
How many tiles do I need for a bathroom?
Calculate the area of each surface being tiled in square metres, add them up, subtract large untiled openings, then add around 10 per cent for wastage. Divide the total by the coverage stated on the tile box and round up to whole boxes.
How much wastage should I allow for tiling?
Around 10 per cent for a standard layout with regular tiles. Allow more for large-format tiles, diagonal or herringbone patterns, and small rooms with many cuts. Large tiles waste more, because even a small cut can ruin a whole tile.
Why is my roof area bigger than my house footprint?
Because the roof slopes. The plan area measured from above understates the true sloped surface. A pitched roof over a 12m by 8m footprint (96 square metres) can be around 104 square metres of actual roof at a 22.5 degree pitch, and more as the pitch steepens.
Should I buy all my tiles at once?
Yes. Tiles are produced in batches and colour or size can vary slightly between them. Include your full wastage allowance in the first order and keep a few spares, so you don't risk a visible mismatch buying more later.
How do I convert roof pitch to roof area?
Divide the plan (footprint) area by the cosine of the pitch angle, or use a roofing calculator that applies a pitch factor. Steeper roofs have a larger true area for the same footprint, so always order to the sloped area, not the footprint.
Is the tile or roofing material the main cost?
Often not. Tiling also needs adhesive, grout, waterproofing and labour, which rises with tile size and pattern complexity. Roofing includes battens, insulation, flashings, guttering and the labour of working at height. Get quotes that separate materials from labour.