Roofing Calculator

Written byOleh Soloninupdated 27 September 2026

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
Minimum slope in the code
at least 20% (12°) for corrugated metal profiles and metal tiles — SP 17.13330.2017, table 4.1
Your slope30° is at or above the minimum slope for this material
How it was calculated
(10 + 2 × 0.5) × (8 + 2 × 0.5) = 99 m² in plan 99 ÷ cos 30° = 99 × 1.155 = 114.32 m² along the slopes 114.32 × (1 + 10%) = 125.75 m²; ÷ 1.8 m² = 70 sheets

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 slope is longer than its own plan: at 30° the roof area is 1.155 times the footprint. A 10 × 8 m house with 0.5 m overhangs is 99 m² in plan and 114.32 m² along the slopes; with a 10% allowance, 125.75 m², or 70 sheets of 1.8 m².

The other thing the pitch decides is which covering is allowed at all. Table 4.1 of Russian code SP 17.13330.2017 sets minimums: 20% (12°) for metal tiles and corrugated profiles, 40% (22°) for clay and cement-sand tiles, 10% (6°) for sheet metal roofs. The calculator compares your angle with that minimum.

How to use

Enter the footprint of the building and the overhang.
Set the pitch in degrees; the code comparison is for corrugated metal profiles and metal tiles.
Add the usable area of one sheet and the allowance; read the sloped area, the number of sheets and how the pitch compares with the code.

How the pitch changes the area

Every row is a separate run of this calculator for a 10 × 8 m house with 0.5 m overhangs, 1.8 m² sheets and a 10% allowance.

Every value was produced by the calculator on this page.
Pitch	Slope, %	Factor	Plan area	Along slopes	With allowance	Sheets
8°	14.1%	1.01	99 m²	99.97 m²	109.97 m²	62
15°	26.8%	1.035	99 m²	102.49 m²	112.74 m²	63
30°	57.7%	1.155	99 m²	114.32 m²	125.75 m²	70
45°	100%	1.414	99 m²	140.01 m²	154.01 m²	86

Minimum slopes in SP 17.13330.2017

Table 4.1 ties the covering to the pitch: at least 10% (6°) for sheet metal roofs — galvanised, coated, stainless, copper, zinc-titanium and aluminium; at least 20% (12°) for bitumen shingles, corrugated chrysotile-cement and bitumen sheets and for corrugated metal profiles including metal tiles; at least 36% (20°) for corrugated cement-fibre sheets; at least 40% (22°) for cement-sand, clay and polymer-cement tiles and for slates; at least 47% (25°) for metal interlocking shingles.

Percent and degrees are linked by tan α = 0.01 × x. A 15° pitch under clay tiles is flagged as below the minimum: from there you either change the covering or add the waterproofing measures clause 4.3 calls for. Outside Russia the numbers belong to your own roofing standard, but the physics of a shallow pitch is the same everywhere.

What the calculation leaves out

Valleys, abutments, dormers and complicated shapes: this calculator assumes a rectangular plan with equal slopes. A hipped or multi-gable roof is measured slope by slope and added up.

Trims are out too — ridge, barge boards, snow guards. The ridge length is printed as a guide: for a 10 m house with 0.5 m overhangs it is 11 m.

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 do I get the roof area from the floor plan?

Divide the plan area by the cosine of the pitch. For 99 m² at 30° that is 99 × 1.155 = 114.32 m²; at 45°, 140.01 m².

What is the minimum pitch for metal tiles?

At least 20% (12°) under table 4.1 of SP 17.13330.2017 — the same as bitumen shingles and corrugated sheets. Metal interlocking shingles need more: 47% (25°).

How many sheets does a roof take?

The sloped area with allowance divided by the usable area of a sheet. For 125.75 m² and 1.8 m² sheets that is 70; usable area is always less than the overall size because of the overlap.

How do percent and degrees relate?

Percent is rise over horizontal run: tan α = 0.01 × x. So 30° is 57.7%, 45° is exactly 100%, and 8° is 14.1%.

Why does the overhang matter?

It throws water clear of the walls and counts as roof: 0.5 m overhangs turn a 10 × 8 m house into 11 × 9 m, which is 99 m² in plan instead of 80 m².

What if my pitch is below the minimum?

Clause 4.3 of SP 17.13330.2017 then requires extra measures to keep the roof watertight. The calculator does not design them — it only shows that the minimum is not met.

Worked example

A 10 × 8 m house at 30°

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

Output: 114.32 m² along the slopes, 125.75 m² with allowance, 70 sheets

Note: A 57.7% slope is well above the 20% minimum for metal tiles.

Useful links

All tools in Building & Repair

Roofing Calculator

Result

Related tools

Insulation Calculator

Lumber Calculator

Square Footage Calculator

Useful links