Roof area calculator with pitch, and roof tiles per m²

The area of a roof whose slopes share one pitch is the plan area, overhangs included, divided by the cosine of the pitch: at 35°, 100 m² on plan becomes 122.1 m² of roof, for a gable or a hipped roof alike.

Plan area × pitch factor

A pitched roof covers more than the ground it shelters. For a slope with a constant pitch, the true area is the plan area divided by the cosine of the pitch. That ratio is the pitch factor: 1.035 at 15°, 1.155 at 30°, 1.221 at 35°, 1.414 at 45°. Measure the plan area with the overhangs: the house width plus both eaves overhangs, the length plus both verge overhangs.

Example: a 10 × 8 m house with 0.50 m eaves and 0.30 m verges covers 10.6 × 9 = 95.4 m² on plan. At a 35° pitch, the gable roof measures 95.4 × 1.221 = 116.46 m², two slopes of 58.23 m². The rafter length, from eaves to ridge, is 4.5 × 1.221 = 5.49 m, and the ridge stands 3.15 m above the eaves.

Pitch factor: from plan area to roof area

Multiply the plan area, overhangs included, by the factor for the roof pitch. Example: at 35°, the factor is 1.221, and 100 m² on plan gives 122.1 m² of roof, whether it is a mono-pitch, gable or hipped roof.

Gable or hipped: the same rule

When every slope shares the same pitch, the rule holds whatever the number of slopes. A hipped roof on the same house, with a 0.50 m overhang all round, covers 11 × 9 = 99 m² on plan, which gives 120.86 m² of roof at 35°. The two main slopes are trapezia of 35.71 m² and the two hip ends triangles of 24.72 m². The ridge shrinks to 11 − 9 = 2 m, and the four hips add up to 28.41 m.

If you know the rafter length rather than the pitch, measure it from eaves to ridge, overhang included: the area of a gable roof is 2 × rafter length × ridge length. For a 5.40 m rafter and an 11 m length, 2 × 5.40 × 11 = 118.8 m².

How many roof tiles per m²

A tile only covers its exposed part: the cover width times the gauge, which is the tile length minus the headlap. Tiles per m² is therefore 1 ÷ (cover width × gauge). Marley gives 9.9 tiles per m² for its Modern concrete tile at a 75 mm headlap and 10.7 at 100 mm, with a 292 mm cover width and a 420 mm tile: 1 ÷ (0.292 × 0.345) = 9.93 and 1 ÷ (0.292 × 0.320) = 10.70. Its Plain concrete tile needs 60 tiles per m² at a 100 mm gauge on a roof.

The count is rounded up and leaves out breakage and cuts: add your own allowance. The headlap depends on the pitch: Marley lists a minimum pitch of 22.5° for the smooth Modern at a 75 mm headlap and 17.5° at 100 mm, and 35° for the Plain tile. For another tile, choose “Other tile” and enter the tiles per m² from its data sheet.

Special cases

• Mono-pitch roof: the eaves overhang counts at the bottom only; add any top overhang to the width.
• Slopes with different pitches: work out each slope on its own, with its own pitch and plan area.
• Dormers, chimneys and roof windows: the area is not reduced; the tiles around them are cut.
• Ridge tiles, verge and eaves components are counted per linear metre, using the lengths from the calculator and the accessory data sheet.

Mistakes that skew the area

• Using the floor area or the plan area without overhangs: 0.50 m eaves on an 8 m wide house add 12.5% to the area.
• Multiplying by the pitch as a percentage instead of the factor: 35° is a 70% slope, but only 22% more area.
• Counting tiles on the plan area: 95.4 m² instead of 116.46 m², over 200 Modern tiles short.
• Taking the tiles per m² of another tile or headlap: 9.9 and 10.7 for the same Marley Modern, 60 for the Plain tile.

To carry on, the slope calculator converts percentages and degrees, the angle calculator gives the rafter cuts, the insulation calculator starts from the same roof area, and the room area calculator helps with the plan size.