Roofing Calculator

Building length, m

Building width, m

Pitch, °

Overhang, m

Effective sheet area, m²

Allowance, %

Result

Pitch factor1.155
Plan area, m²99
Roof area, m²114.32
With allowance, m²125.75
Sheets70
Ridge length, m11

Pitch adds area, and quite a lot of it

Roof area allowing for pitch and overhangs, the number of sheets needed and the ridge length.

A roof is always larger than the house beneath it, and the pitch decides by how much. A 30-degree slope is 1.15 times its horizontal projection; 45 degrees is 1.41 times. Ordering roofing by floor area is the classic mistake.

Overhangs are the second multiplier. Half a metre all round on a 10 by 8 house adds nearly twenty square metres. Then comes the allowance for cutting and laps, about ten percent for profiled sheet.

What this calculator shows

The pitch factor — how much longer the slope is than its projection
Plan area, roof area and roof area with an allowance
Sheet count and ridge length

What to keep in mind

The calculation is for a simple gable roof. Hipped and mansard roofs should be split into planes and added.
Effective sheet area is less than the overall size because of laps; use the figure from the product datasheet.

FAQs

How does pitch change the area?

Area is divided by the cosine of the angle. At 15 degrees that adds 3.5 percent, at 30 it adds 15, at 45 a full 41.

What pitch should I choose?

Usually between 20 and 45 degrees. Below 20, snow sits and loads build; above 45, wind loading and material use climb. Metal tile needs at least 14.

Do I need an allowance for laps?

Yes. Ten percent for profiled sheet and metal tile, fifteen for membrane roofing on a complex roof with valleys and penetrations.

How do I estimate a hipped roof?

Split it into two trapezoids and two triangles, work out each plane and add them. There is no single factor — geometry dominates.

Worked example

A gable roof on a 10 by 8 house

Input: 10 × 8 m, 30° pitch, 0.5 m overhang, 1.8 m² sheets

Output: Factor 1.155, roof 114.1 m², 70 sheets

Note: Plan area is 99 m² but the roof is 114. Those fifteen extra square metres are eight sheets it would be easy to leave off the order.

Useful links

All tools in Building & Repair

Roofing Calculator

Result

Related tools

Insulation Calculator

Lumber Calculator

Square Footage Calculator

Useful links