Buying a rooflight before checking your roof properly is one of the most common mistakes homeowners make. The wrong pitch can mean water pooling on the glass, failed seals, or a warranty that never applies in the first place.
Determining pitch of roof correctly is the step most people skip, usually because they assume any rooflight will fit any roof. Our team at Skylights Roof Lanterns sees this mistake regularly, which is why getting the measurement right matters just as much as choosing the style.
This guide walks through what roof pitch actually means, how to measure it safely, and how it affects which rooflight suits your project. We will also cover the difference between flat and pitched installations.
Why Determining Pitch of Roof Matters Before You Buy
Every rooflight on the market is designed around a specific pitch range. Fit the wrong product outside that range, and water either pools on the glass or drains too aggressively, stressing the seals over time. Even within the flat rooflights available, pitch requirements can vary between products that otherwise look very similar.
Determining pitch of roof accurately also protects your warranty. Most manufacturers state a clear pitch range in their product specification, and installing outside it can void cover entirely. Getting this step right first saves money and hassle later, especially on extensions where the roof shape changes across a single build.
What Is Roof Pitch?
Roof pitch describes how steep a roof surface is, measured as the angle it makes against the horizontal. In the UK, this is almost always expressed in degrees rather than a ratio, unlike some international guidance you might find online.
A completely flat roof has a pitch close to zero degrees, while a steep traditional roof can sit anywhere from 40 to 50 degrees or more. The skylights for pitched roofs guide explains how pitch angle affects glazing choice on sloped roofs, including the 15 to 75 degree range most models are built around.
Most UK homes fall somewhere in between, with many pitched roofs sitting between 30 and 45 degrees depending on the build era and region.
How to Measure Roof Pitch at Home
You do not need specialist tools to get a reasonably accurate reading. A tape measure and a spirit level are usually enough for most domestic roofs. Here is a simple, safe method:
- Access the loft space or use a ladder to view the roof slope safely from the ground or an upstairs window.
- Hold a spirit level flat against the roof slope, extending it horizontally by a fixed distance, such as 300mm.
- From the end of the level, measure straight down to the roof surface to find the vertical rise.
- Use these rise and run figures to calculate the angle, either by hand or using a roof pitch calculator online.
- Repeat the measurement in at least two spots to account for any unevenness in the roof surface.
If working at height feels risky, it is always safer to ask a roofer or builder to take the reading for you. Once you have a reading, it helps to compare it against the full range of roof light styles to see which shape suits your specific angle.
Determining Pitch of Roof: Degrees vs Ratios
Some online guidance expresses pitch as a ratio, similar to how it is done in the US construction industry. In the UK, though, degrees remain the standard used by manufacturers, planners and building control.
If you have already measured your rise and run, converting to a pitch ratio and then degrees is straightforward with a calculator, though most UK product specifications simply ask for the angle directly. Determining pitch of roof in degrees avoids confusion when comparing products across different suppliers.
Roof Pitch and Rooflight Suitability
Once you know your roof's angle, matching it to the right product becomes far simpler. The table below shows how Skylights Roof Lanterns categorises pitch suitability across its range.
| Rooflight Type | Roof Suitability | Typical Pitch | Best Suited To |
| Flat rooflight | Flat or low-pitch roofs | 2 to 15 degrees | Extensions, kitchens, minimal finishes |
| Pitched rooflight | Sloped roofs | Above 15 degrees | Lofts, bedrooms, hallways |
| Roof lantern (Wendland) | Flat roofs, extensions | Fixed 20 degree pitch | Architectural feature ceilings |
| Pyramid rooflight | Flat or low-pitch roofs | Similar to flat rooflight range | Even light spread across a room |
Determining pitch of roof before ordering means you can go straight to the correct category rather than guessing between styles.
Determining Pitch of Roof for Flat Roof Installations
Flat roofs are rarely completely flat in practice, and that small hidden pitch matters more than people expect. Most flat rooflights are actually built with a slight built-in slope, often starting around 5 degrees, purely to help water drain away from the glass.
According to the Planning Permission, roof alterations including rooflights generally count as permitted development provided the unit does not project more than 150 millimetres above the existing roofline.
Pitch and projection both feed into this, so it is worth checking both figures before ordering, particularly on listed or conservation area properties. The 5-degree rule for flat skylights explains the drainage logic behind the rule in more depth.
Choosing the Right Rooflight Once You Know Your Pitch
With your roof pitch confirmed, choosing between product ranges becomes far more straightforward. Steeper roofs above 15 degrees generally call for a pitched rooflight, while flatter roofs need a unit built specifically for low-pitch drainage.
If your roof falls close to that 15-degree threshold, it is worth checking the exact specification before ordering, since some pitched rooflights can handle the transition better than others. The collection covers fixed and custom options built specifically for sloped roofs above this angle.
Getting the Right Fit for Your Specific Roof
Every roof is slightly different, even within the same street or development. Standard sizes do not always match older properties, extensions or converted lofts with non-standard openings.
For projects needing a size that falls outside standard measurements, custom flat glass rooflights allow exact dimensions to be specified once your pitch and opening size are confirmed.
Final Thoughts
Determining pitch of roof properly before ordering is the single step that prevents most rooflight problems down the line. Whether your roof is flat, gently sloped or steeply pitched, matching the product to the correct angle protects both performance and warranty.
If you are unsure how to measure your roof pitch, or which product suits your reading, call Skylights Roof Lanterns on 020 4538 3079 or visit the contact page for tailored advice.
