Roof squares vs square metres: converting a Canadian takeoff both ways

One roofing square is 100 square feet, which is 9.29 square metres, so a Canadian takeoff converts between the two with one division: square metres ÷ 9.29 = squares, and squares × 9.29 = square metres. The trouble on a Canadian job is never the factor. It is that the measurement arrives in one system, the shingle data sheet speaks the other, and the membrane roll is sold in metres. Here are the factors once, a takeoff worked both ways, and the rounding steps that quietly add or drop a bundle.

The factors, once#

A "square" is the trade's unit for 100 ft² of roof surface, and a bundle is sold as a fraction of a square. Everything else is arithmetic.

The bundle row only holds for a product that covers a square with three bundles; shake-look and heavier laminates take four or five, and the data sheet wins. The pitch factor has no unit at all: a 6/12 pitch multiplies the footprint by 1.118 whether the footprint is in ft² or m². Metric drawings write the same pitch as 500 mm of rise per metre of run.

Which direction, and when#

Convert as late as you can, and convert once. A takeoff that flips units at every step collects a rounding error at each flip, and on a 15-square roof those errors are the size of a bundle.

• Measurement in feet, supplier in metric. Finish the takeoff in ft² and squares, waste included, then convert the final figures for the items sold in metres: membrane, underlayment, trims on a 3 m price list.
• Measurement in metres, supplier in squares. Most shingle data sheets in Canada still state bundles per square, so take the metric roof area, add waste, and divide by 9.29 at the end.
• A quote in m² for a homeowner or a tender. Price in the unit the document asks for, but keep the material count in the supplier's unit. Our guide to pricing a roof per square metre covers the rate conversion.

Lengths follow the same rule: convert the eave, ridge, hip, valley and rake totals once, then divide by the piece length on the price list.

A Canadian takeoff worked both ways#

Take a two-storey gable in Ontario, footprint 40 ft × 30 ft, pitch 6/12, laminated shingle at three bundles per square, 10% waste.

Both columns land on 45 bundles because the conversion happened once, at the end. Now the lengths for the same roof:

• Eave: 2 × 40 ft = 80 ft, which is 24.4 m, for starter strip, drip edge and eavestrough.
• Ridge: 40 ft, which is 12.2 m, for cap shingles and ridge vent.
• Rake: 15 ft of run × 1.118 = 16.8 ft each; four rakes make 67.1 ft, which is 20.4 m of drip edge.

Membrane forces the reverse direction. The provincial code sets a minimum run of eave protection up the slope, covered in our eave protection article. Whatever width you settle on, measure the strip in ft², divide by 10.76 to get m², and divide by the roll's coverage in m² from the data sheet. If the roll is listed in ft², skip the conversion. Never convert the roll and the roof separately and then compare them.

Drawing the roof in RoofStruct's measurement tool removes most of this: the builder's unit setting switches the whole drawing between imperial and metric (m and m²), so the area and every edge length change together and both columns above come from one drawing.

Rounding rules that keep the bundle count honest#

• Keep two decimals on the area until the final step: 1,341.6 ft² is 13.42 squares, not 13 and not 14.
• Round to bundles, not to whole squares. 14.76 squares at three bundles per square is 44.3 bundles, so 45. Rounding to 15 squares first gives 45 here too, but on a 14.2-square roof it gives 45 instead of 43.
• Round lengths to the piece, in the piece's own unit. A 20.4 m rake total on a 3 m trim is 6.8 pieces, so seven.
• Use 9.29, not 9, and 10.76, not 10. The short cuts move the roof by 3% one way and 7% the other, which on 15 squares is up to a full square.

Where you trip#

• The pitch factor is applied twice. Once in feet, once in metres. If your metric column is 12% larger than the converted imperial one, this is why.
• Waste goes on in one unit and the bundle count comes from the other. Add the waste to the area, convert once, then count bundles.
• Lineal metres and square metres are both "metres" on the supplier's sheet. Drip edge, starter and ridge cap are lineal; membrane and underlayment are area. A ridge of 12.2 m is not 12.2 m² of anything.
• 500 mm per metre is read as 5/12. It is 6/12: a 1:2 slope either way. A 5/12 pitch is about 417 mm/m.
• The Québec plan is in metres but the shingle quote came in squares. Normal. Take the metric roof area, add waste, divide by 9.29, and write both figures on the order so the yard and the crew see the same roof. Our bundles per square in Canada article has the coverage by product type.

Convert once, at the end, in the direction the price list demands, and write both numbers down. The roof does not change size when the unit does.

FAQ#

How many square metres are in a roofing square?#

One square is 100 ft², which is 9.29 m². To go the other way, 1 m² is 0.108 square, so 100 m² of roof is 10.76 squares.

Is the pitch factor different in metric?#

No. The factor is the ratio of slope length to run, which has no unit. A 6/12 pitch, or 500 mm per metre in metric drawings, multiplies the footprint by 1.118 in either system.

Should I convert before or after adding waste?#

After. Finish the takeoff in one unit system, add the waste percentage, then convert the final area once. Converting at every step collects rounding errors that can add or drop a bundle.

Can a roof measurement report give both units?#

In RoofStruct the builder's unit setting switches the drawing between imperial and metric, so the same roof reads in ft² and squares or in m² without re-measuring. Our metric measurement guide for Canadian roofers walks a full roof in m².