Kai Roofing Calculator

Calculate the surface area of your roof

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Roof area calculator Shed

No pitch required
Roof calculator - Shed
* This image is to clarify which is the rise of the roof (H) and the rise of the dormer (h). The image is for general informational purpose.
3D

Your roof dimensions:

L
l
H
* L is the length of the roof
* l is the width of the roof
* H is the rise (height) of the roof (3D image)
* For decimal dimensions, use the dot notation! (.)

Total Roof area:

...

Roof Lengths

AB = CD = ...
BC = DA = ...
Roof diagram showing dimensions and measurements

Roof Surfaces

ABCD = ...

Roof Slopes

ABCD =
  • Slope Grade = ... %
  • Angle = ... deg
  • Pitch = ... x12
Roof calculator - Shed
* This image is to clarify which is the rise of the roof (H) and the rise of the dormer (h). The image is for general informational purpose.
3D

Your roof dimensions:

L
l
H
Small dormer:
Lu 0
Lu 1
h
* L is the length of the roof
* l is the width of the roof
* H is the rise (height) of the roof (3D image)
* For decimal dimensions, use the dot notation! (.)

Total Roof area:

...

Roof Lengths

AB = CD = ...
BC = DA = ...
Roof diagram showing dimensions and measurements

Roof Surfaces

ABCD = ...

Roof Slopes

ABCD =
  • Slope Grade = ... %
  • Angle = ... deg
  • Pitch = ... x12
Roof calculator - Shed
* This image is to clarify which is the rise of the roof (H) and the rise of the dormer (h). The image is for general informational purpose.
3D

Your roof dimensions:

L
l
H
Large dormer:
Lu 0
Lu 1
Lu 2
h
* L is the length of the roof
* l is the width of the roof
* H is the rise (height) of the roof (3D image)
* For decimal dimensions, use the dot notation! (.)

Total Roof area:

...

Roof Lengths

AB = CD = ...
BC = DA = ...
Roof diagram showing dimensions and measurements

Roof Surfaces

ABCD = ...

Roof Slopes

ABCD =
  • Slope Grade = ... %
  • Angle = ... deg
  • Pitch = ... x12

How to calculate the area of a shed roof

A shed roof (also called a mono-pitch or lean-to roof) has a single flat slope running from a high wall to a low wall. It is the simplest roof to measure, which makes it common on garages, extensions, garden sheds and modern houses.

What you need to measure

  • L: the length of the roof, measured along the eave (the low edge).
  • l: the horizontal width of the building that the slope covers.
  • H: the rise, the height difference between the high and the low end of the slope.

How the area is calculated

The slope is a rectangle tilted at an angle. Its length is L and its true width is the rafter length, which you get with Pythagoras from the width and the rise: rafter = √(l² + H²).

Roof area = L × √(l² + H²). The pitch is the rise over the width: H / l, or (H / l) × 12 in inches per foot.

Worked example

  1. Roof length L = 10 m, width l = 4 m, rise H = 1 m.
  2. Rafter length = √(4² + 1²) = √17 = 4.12 m.
  3. Roof area = 10 × 4.12 = 41.23 m².
  4. Pitch = 1 / 4 = 25%, or 3/12 (14 degrees).

Tips before you order materials

  • Add the overhangs: if the roof sticks out 0.3 m at the eave and the top, add 0.6 m to the width before you calculate.
  • Low slopes need roofing rated for them: most shingles need at least 2/12, below that use membranes or metal sheets.
  • Order 10% extra for cuts and waste.

Frequently asked questions

Why is the roof area bigger than the floor area?

Because the slope is tilted. The steeper the roof, the longer the rafter compared with the horizontal width, so the roof covers more surface than the floor below it.

Can I measure the rafter directly instead of the rise?

Yes. If you know the rafter length, the area is simply L × rafter. The calculator asks for the rise because it is easier to measure from inside the building.

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Roof Inspection

Why to inspect your roof

Fixing minor roof problems before they become costly is a very good way to prevent more severe damage to your roof and reduce home maintenance costs. You also need to be aware of the aging and weathering of your roof - know when it's time to replace it.

When to inspect your roof

Most experts agree that it's a good idea to check your roof regularly at least twice a year, ideally in spring and fall.
Check your roof in fall to prepare for winter - it will be more difficult to fix the roof in bad weather when it's cold or you have ice and snow on your roof.
Check your roof in spring to find if winter damaged your roof.
Another good time to do a roof inspection is after a severe storm.

What to look for during roof inspection

  • Broken or missing shingles
  • Shingles that are blistering, cracking or buckling
  • Patches of moss and lichen
  • Loose, cracked or rusted gutters
  • Look for signs of leaking - they might indicate roof damage you haven't noticed
  • Check if the roof ventilation is working - look for mold, mildew and wood rot
Roof inspection expert