How to Measure Roof Pitch for VELUX Windows

Why Roof Pitch Matters for VELUX Windows

Before choosing a VELUX roof window, it’s important to know the pitch (angle) of your roof. Each window and flashing kit is designed to work within a certain pitch range to ensure correct water drainage and performance.

This guide explains simple ways to measure roof pitch and what to keep in mind when planning a VELUX installation.

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1. What Is Roof Pitch?

Roof pitch is the angle of the roof compared to horizontal. It’s usually expressed as:

• Degrees (for example, 30°), or
• Rise over run (for example, 150mm rise over 300mm run).

VELUX product information normally refers to pitch in degrees, such as “suitable for roof pitches between 15° and 90°” (always check the specific product details).

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2. Tools You Can Use to Measure Roof Pitch

You don’t need specialist equipment. You can use:

• A spirit level and tape measure.
• A roof pitch gauge or angle finder.
• A smartphone app with an angle or level tool.
• Information from architectural drawings if available.

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3. How to Measure Roof Pitch from Inside the Loft

This is often the safest and easiest method if you have access to the loft or roof space.

1. Place a spirit level horizontally against the side of a rafter.
2. Mark a point on the rafter exactly 300mm along the level (the “run”).
3. From that mark, measure vertically up to the underside of the rafter (the “rise”).
4. Use a roof pitch chart or calculator to convert the rise over 300mm into degrees.

For example, a rise of around 173mm over 300mm corresponds to roughly a 30° roof pitch.

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4. How to Measure Roof Pitch from Outside

If you can safely access the roof (for example via scaffolding and with proper safety measures):

• Place a spirit level flat on a roof tile or slate.
• Measure vertically from one end of the spirit level down to the tile surface.
• Use the level length and the measured height with a pitch chart or calculator to find the angle.

Always follow safe working at height practices. If in doubt, ask a professional roofer or installer to measure the pitch for you.

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5. Using Plans or Drawings

If your project has architectural drawings or structural calculations, the roof pitch may already be specified. Look for notes such as:

• “Roof pitch: 30°” or
• “Roof pitch: 35°”

These values can usually be relied upon for new builds and recent extensions, but older properties may differ slightly from historic drawings.

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6. Typical Pitch Ranges for VELUX Windows

Most VELUX roof windows are designed to work within a specified pitch range, for example:

• Suitable for roof pitches from around 15° up to 90° (vertical).
• Some products or flashings may have different minimum or maximum pitches.

Always check the product data for the specific window and flashing kit you are using.

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7. Why Pitch Affects Flashings & Installation

Correct roof pitch is important because:

• Flashings are designed to shed water at certain angles.
• On very low pitches, water may not drain fast enough if the wrong products are used.
• On very steep roofs, different detailing may be required.

Our VELUX Flashing & Installation Guide explains the main flashing options (EDW, EDL, EDP) and how they relate to roof coverings and pitch.

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8. Double-Checking Before You Order

Before ordering a VELUX roof window and flashings, it’s sensible to:

• Measure the roof pitch at least once (ideally twice) to confirm the angle.
• Note the roof covering type (tile, slate, plain tile, etc.).
• Confirm the window size you need (see our VELUX Size & Code Guide).
• Choose glazing and finish based on the room and comfort requirements.

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9. When to Ask a Professional for Help

If you are unsure how to measure roof pitch safely or accurately, a roofer, builder or surveyor can usually confirm it quickly using professional tools.

• They can also confirm if your chosen VELUX product is suitable.
• They will advise on flashings, insulation and roof build-up.
• They can help ensure the installation complies with building regulations.

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