Roof pitch and what it means for a roof window

Pitch is the angle between the sloping roof surface and the horizontal. A dead-flat roof has a pitch of zero. A steep Victorian gable might climb past 45 degrees. Everything you might want to put glass into sits somewhere on that scale, and where it sits changes the job.

The confusion starts because the same angle gets expressed three different ways, and the trade uses all of them without warning you which one it means.

• Degrees. The straightforward one. A 30 degree roof rises at 30 degrees from level. This is the figure window manufacturers print in their fitting ranges, so it is the one you want in the end.
• Ratio, as rise over run. A roof that climbs 100mm for every 300mm it travels horizontally is written 1:3, and works out at about 18 degrees. Builders and older drawings lean on this because it comes straight off a measurement, no protractor needed.
• Percentage. Rise divided by run, times a hundred. That same 1:3 slope is a 33 per cent fall. You see this more on commercial and flat-roof drawings than on houses.

None of them is wrong. They just describe the same triangle from different corners. If a builder tells you the extension roof is “a fall of 1 in 40”, that is a shade over 1 degree, which is a flat roof with just enough tilt to drain, and it will not take an ordinary pitched roof window at all. If they say “it’s a 40 degree roof”, you have a steep slope with the pick of the market. Get the number, then get it in degrees before you go shopping.

Roof windows and flat rooflights are two different products built for two different worlds, and the line between them is drawn by pitch.

A pitched roof window, the sort that sits flush in a tiled or slated slope, is designed to shed water down its own face. It relies on the roof being steep enough that rain runs off before it can pool against the frame or work back under the flashing. Most units in this category are rated for roofs from about 15 degrees up to around 90 degrees. Below 15 degrees the water slows down, sits longer, and the standard flashing kits are no longer trusted to keep it out.

Manufacturers have answers for the shallow middle ground. Many pitched roof windows can be taken down to around 10 degrees, sometimes even lower, but only when they are fitted with a specific low-pitch flashing kit or lifted on an upstand, a raised timber kerb that tilts the unit steeper than the roof around it. The upstand is a common fix on shallow extension roofs: the roof might fall at 8 degrees, but the window sits on a kerb that presents the glass at 15 or more, so the water behaves.

Go flatter still, below about 5 degrees, and you are into flat rooflight territory. These are different units entirely: fixed or opening flat glass, or a domed or pyramid rooflight, usually mounted on a builder’s kerb to lift them clear of any standing water. That is the right product for a modern flat-roofed extension, and it is a job in its own right. If your rear extension has a roof you could stand a mug on without it sliding off, the honest answer is that you want a flat-roof skylight rather than a pitched roof window, and forcing the wrong product onto the wrong roof is where leaks are born.

The reason pitch sets these limits comes down to how water moves on glass and tile. On a steep slope, gravity pulls the runoff away quickly and the wind struggles to drive it back uphill under the flashing. As the angle shallows, three things happen at once. The water travels more slowly, so more of it is on the roof at any moment. Surface tension lets a film cling to the glass instead of sheeting off. And wind-driven rain can push water sideways and even upwards across a nearly level surface, past defences that a steeper roof would never have tested.

This is why flashing kits are pitch-rated and why fitting outside that rating is not a detail to fudge. The flashing is the skirt of metal and weatherproof material that marries the window frame to the surrounding roof covering. On a slate roof it laps over the courses below and tucks under the courses above, so every drop is handed downhill from one layer to the next. That handover only works when the slope keeps the water moving. On south Essex roofs that face the estuary, there is the added factor of exposure: an open aspect towards the water means wind-driven rain arrives with more force and from a wider range of angles than it would on a sheltered inland street, so the weathering detail earns its keep. A window fitted at its correct pitch, with the matching flashing and a proper upstand where the slope is shallow, is what keeps a roof dry for decades.

The housing across this patch was built in waves, and each wave came with its own roof geometry. Knowing roughly what you are standing under tells you most of what you need before anyone gets a ladder out.

The pattern that matters most is the split between the original house and its extension. A 1930s semi in Westcliff or Thundersley usually has a main roof at a comfortable 35 to 45 degrees, which is the sweet spot for a pitched roof window and the reason lofts across the belt convert so readily. But the rear extension bolted onto the back of that same house, over a knocked-through kitchen, is often a shallow 10 to 20 degree slope, and that is a different specification entirely. One house, two roofs, two answers. The steep original slope will take an ordinary roof window without a second thought. The shallow extension roof needs the number checked and quite possibly a low-pitch flashing kit or a kerb.

Loft conversions in the older terraces along the cliff at Leigh and Westcliff sit on steep 40-plus degree slopes, which weather beautifully and give a lot of freedom on window size and position. The catch there is rarely the pitch and more often the conservation constraints, which is a separate matter to check with the local authority before ordering anything.

Pitch does not just decide whether a window fits and stays dry. On a south-facing roof it decides how the sun strikes the glass, and that is where this business does its real thinking.

The angle the sun hits the glass at governs how much of its energy gets through. Sunlight arriving square-on to a pane delivers close to its full strength. Sunlight arriving at a glancing angle spreads across more glass and glances off, so far less energy per square metre enters the room. Pitch controls that geometry on a south slope through the whole year.

Southend sits at roughly 51.5 degrees north. In the depth of winter the midday sun barely clears 15 degrees above the horizon. In midsummer it climbs to around 62 degrees. A south-facing roof window presents its glass to that moving sun differently depending on how steeply it is laid. A shallow, nearly flat pitch faces close to straight up, so it takes the high summer sun almost square-on and collects the most heat exactly when you least want it. A steeper south-facing pitch, up around 40 degrees, tilts the glass more towards the low winter sun and turns its shoulder to the high summer one, which is a friendlier balance for a room you live in year round.

This is why a shallow south-facing extension rooflight can turn a kitchen into a greenhouse between two and six on a July afternoon, while a steep south-facing loft window in the same street feels far more even. The pitch has quietly set the terms. It does not remove the need to think about the glass, but it tells you how hard the glass will have to work. On any south or west-facing slope, the pitch reading is the first input into choosing the solar-control glazing that keeps the brightness and takes out the heat.

Beyond weather and heat, pitch changes what the window does for the room underneath, and it does so in ways people rarely picture until the unit is in.

A steeply pitched roof window behaves a little more like a wall window. You can stand near it and see out at something close to eye level, catching a slice of sky, rooftops or the garden. On the villa and terrace lofts around the cliff, a steep slope lets a tall roof window frame a genuine view, sometimes across to the estuary. A shallow-pitched or near-flat rooflight, by contrast, gives you sky and little else, but it pours light straight down into the middle of the room, which is exactly what a deep-plan kitchen extension is short of.

Pitch also affects how much daylight a given pane of glass delivers and how evenly it spreads. A window low on a shallow slope throws light deep into the floor plan. A window on a steep slope casts light against the opposite wall and down, which can be lovely in a loft bedroom but leaves the far corners darker. There is a useful rule of thumb the trade leans on: the shallower the pitch and the higher the window sits in the roof, the deeper into the room the daylight reaches. It is one of the reasons flat and shallow rooflights suit big open kitchens, while steep roof windows suit lofts where the light is meant to be atmospheric rather than functional.

None of this is a reason to fight your roof’s pitch. It is a reason to place and size the window for the angle you have, rather than picking a unit off a shelf and hoping. The pitch is fixed. The response to it is where the skill lives.

You can get a workable pitch figure yourself in a few minutes, and it will make every later conversation sharper. You are not producing a survey, just getting into the right ballpark so you know whether you are shopping for a pitched roof window or a flat rooflight.

1. From inside the loft. Hold a spirit level out horizontally against a rafter so one end touches the timber. Measure the horizontal distance along the level to the rafter, and the vertical drop from the level’s far end up to the rafter. Rise over run gives you the ratio, and a phone angle app or a quick search converts it to degrees.
2. With a phone. Most phones have a level or measure app with an angle readout. Rest the phone flat against a rafter in the loft, or against the underside of the roof, and read the angle. It will not be laboratory-accurate, but it is close enough to tell a 12 degree roof from a 35 degree one, which is the decision that matters.
3. From the ground outside. Stand back, hold a protractor or a phone level up so it lines up with the slope of the roof against the sky, and read off the angle. Rough, but a useful sanity check against your loft measurement.

What you are really trying to establish is which side of the important thresholds you fall on. Below about 5 degrees, think flat rooflight. Between 5 and 15, expect a low-pitch flashing kit or an upstand and get the exact number confirmed. Above 15 degrees, you are in ordinary pitched roof window territory with the full range open to you. A proper survey pins the figure down and works out the flashing, the structural opening and the orientation, but knowing your rough pitch first means you arrive at that conversation already understanding why the answers come out the way they do.

Once the pitch is known, the specification falls into place in a sensible order. Pitch first, because it rules products in or out. Orientation next, because it decides what the glass has to cope with. Then the unit itself. Here is what to have on your list.

• Confirm the pitch in degrees, for the exact slope the window is going into. Not the main roof if the window is going in the extension. The two are often different, and the extension is usually the shallow one.
• Ask whether the pitch needs a low-pitch flashing kit or an upstand. Anything under 15 degrees should prompt this question. If it does not come up unprompted, ask it.
• Match the flashing to your roof covering. Plain tile, interlocking tile, slate and metal each take a different flashing profile. The pitch and the covering together decide the kit.
• On any south or west slope, ask for the G-value of the glass. The pitch tells you how directly the summer sun will hit it, and the glazing is what manages the result. A number should come back.
• Size and place the window for the pitch. A steep slope can carry a taller window with a view. A shallow slope wants a unit positioned to throw light where the room needs it.
• Ask who handles Building Control. A new or enlarged roof window is notifiable under the Building Regulations, with Part L covering thermal performance and Part K covering the safety glazing overhead. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the installation.

Pitch is the fact you cannot change without rebuilding the roof, so it is the fact worth starting from. It sets the product, the weathering detail, the daylight and, on a south slope over the estuary, how much the sun has to be managed. Every job we take on begins with reading the roof, and the pitch is the first line of that reading. If you want to know exactly what your slope will take, our pitched roof skylight installation starts with a survey that measures the angle, checks the orientation and specifies the glass to match, and you can ask us for that assessment before you commit to anything.