Roof Pitch Calculator

What Is Roof Pitch and Why Does It Matter?

Roof pitch is the angle of your roof measured in degrees from horizontal. It determines which roofing materials you can use, how much material you need, how quickly water and snow shed from the roof, and how much usable loft space you have. Getting your pitch right — or knowing the pitch of your existing roof — is the foundation of every roofing calculation.

How to Measure Your Roof Pitch

You can measure pitch from inside the loft without climbing onto the roof.

Method 1 — Spirit level and tape measure (inside loft):

1. Hold a spirit level horizontally from a rafter, level it, and mark a point exactly 100 cm along the level.
2. From that 100 cm point, measure vertically down to the top surface of the rafter. This measurement in centimetres is the rise per metre of run.
3. Use the formula: Pitch (degrees) = arctan(rise / 100)

Method 2 — Smartphone inclinometer app:

1. Place your phone flat against the underside of a rafter in the loft.
2. Open a clinometer or spirit level app and read the angle.
3. This gives the pitch directly in degrees.

Method 3 — From outside with a photo:

1. Stand well back from the house and photograph the gable end.
2. Use a protractor on the photo (or a digital angle tool) to measure the angle between the roof slope and horizontal.
3. This is less accurate but gives a reasonable estimate.

Pitch Angles and Slope Factors

The slope factor converts the horizontal footprint area of your roof into the actual sloped surface area. This is essential for calculating tile, felt, and batten quantities.

How to use: Multiply your building footprint area by the slope factor. A 10m x 8m building (80 m2 footprint) with a 35-degree pitch has an actual roof surface area of 80 x 1.221 = 97.7 m2. You then need to buy enough tiles, battens, and underlay for 97.7 m2 (plus waste).

Minimum Pitch by Roofing Material

Every roofing material has a minimum pitch below which it cannot reliably keep water out. Using a material below its minimum pitch leads to leaks, moisture ingress, and premature failure.

If your roof pitch is between 15-25 degrees, your options are limited to concrete interlocking tiles, fibre cement, or metal. Below 15 degrees, only metal or membrane roofing will work reliably.

Standard Roof Pitches by House Type

If you cannot measure your pitch, these typical values for common house types give a reasonable starting estimate.

Pitch, Snow Load, and Wind

Roof pitch interacts with weather loads in important ways.

Snow: Shallow pitches (under 15 degrees) accumulate snow, adding significant weight. Building regulations in snowy regions require roofs to be designed for this load. Steeper pitches (above 40 degrees) shed snow quickly, which reduces roof loads but can dump snow onto paths, drives, and conservatories below.

Wind: Steeper roofs present a larger surface area to wind, creating more uplift force. In high-wind zones, tiles on steep roofs must be secured with additional fixings (every tile clipped, not just every third or fifth). Very low-pitch roofs can create suction effects at edges and ridges.

Rain: Steeper pitches shed rainwater faster, which is why plain tiles (which rely on overlap, not interlocking channels) require steep pitches to prevent water creeping uphill by capillary action. Interlocking tiles with deep water channels can handle shallower pitches because the channel drains water before it reaches the overlap.

Pitch and Loft Space

The pitch of your roof directly affects the usable space inside the loft.

Building regulations typically require a minimum head height of 2.2 m over at least 50% of the floor area for a habitable room. A 30-degree pitch on a standard-width house is usually the minimum for a practical loft conversion.

Converting Between Pitch Formats

Pitch can be expressed in several formats. Here is how to convert between them.

Degrees to ratio: Ratio = tan(degrees) x 12 : 12. For example, 30 degrees: tan(30) x 12 = 6.93, so the ratio is approximately 7:12.

Ratio to degrees: Degrees = arctan(rise / run). For a 6:12 pitch: arctan(6/12) = 26.6 degrees.

Degrees to percentage: Percentage = tan(degrees) x 100. A 30-degree pitch = 57.7%.

Percentage to degrees: Degrees = arctan(percentage / 100). A 50% slope = arctan(0.5) = 26.6 degrees.

Pitch Considerations When Re-Roofing

If you are replacing your roof with the same material, the existing pitch is already correct. But if you want to change materials, check that your current pitch meets the minimum for the new material.

Common scenarios:

• Replacing concrete interlocking with natural slate — Only possible if your pitch is 25 degrees or more. Many 1960s-1980s houses at 25-30 degrees qualify.
• Replacing plain tiles with interlocking — Almost always possible since interlocking tiles work at lower pitches (15 degrees) than plain tiles (35 degrees).
• Replacing slate with concrete tiles — Usually fine since slate requires 25 degrees and concrete interlocking works from 15 degrees. The main consideration is the weight increase.
• Adding a flat-roof extension — Must use membrane or metal roofing. Tiles are not suitable below 15 degrees.

Always check with the tile manufacturer's technical data sheet for the specific product you intend to use. Minimum pitch can vary between products in the same material category.