What is a roof window?

The parts a roof window is built from

Four components make up the unit. The outer frame is fixed to the rafters or to timbers trimmed between them, and it never moves again. The sash carries the glass and swings on the hinges. The sealed glazing unit is normally two panes with a laminated inner leaf, argon filled, with a low emissivity coating on one internal face. Around the outside sits the flashing, a set of pressed metal sections that lap over the covering below the unit and under the covering above it.

Two smaller items matter more than their size suggests. There is a drainage channel formed between sash and frame that carries away the water which inevitably gets past the outer gasket, and there is usually a ventilation flap in the head of the sash that lets air through with the window shut and locked.

Why it sits in the plane of the slope

A roof window is designed to be recessed into the covering rather than raised above it. The tiles or slates run up to the flashing and stop, the flashing laps over them, and water travelling down the roof passes across the unit and continues on its way without ever meeting an upstand. That is why the fitting depth matters and why a unit sunk too deep or standing too proud will collect water at the head no matter how good the sealant is.

It also explains why the same window looks different on different roofs. A deep interlocking concrete tile needs a taller flashing profile than a slate does, and the two kits are not interchangeable.

The flashing kit is half the product

The flashing is chosen by covering type and by how many units sit together. Slate and plain tile take a low profile kit. Interlocking pantiles take a deep profile with a lead or lead alternative apron that is dressed over the contour of the tile. Standing seam and single ply take something different again. If two units sit side by side or one above the other, a combination kit with the correct spacing bars is ordered, because the gap between them is fixed by the metalwork and cannot be adjusted on site.

Get the kit wrong and the unit is not weathertight, regardless of how well the carpentry was done. This is the most common reason a self installed roof window lets water in.

It opens, and that is part of the definition

The word window is doing real work. A unit that does not open is a fixed rooflight, and while the two are often lumped together in conversation, they are different products with different frames. A roof window opens for three reasons: to ventilate, to give access to the outer face of the glass for cleaning, and in a converted loft, sometimes to provide an escape opening.

Two hinge arrangements dominate. Centre-pivot units rotate about a horizontal axis roughly halfway up the sash. Top-hung units swing outward from a hinge at the head. We have set out the full comparison on centre-pivot against top-hung, because the choice changes how the room below is used.

Pitch range, and where the product stops working

Most roof windows are made for pitches between about 15 and 90 degrees. Below 15 degrees the water does not shed quickly enough across the flashing and the unit is outside its design range, which is why a shallow roof usually calls for a different product on a kerb rather than a roof window laid flat. Above about 55 degrees a centre-pivot sash becomes heavy to control on the way back and a top-hung unit is generally easier to live with.

If your roof is genuinely flat, a roof window is not the answer. A flat roof takes a kerb mounted rooflight, and the detailing is a different discipline altogether.

Sizes, and why they are written as codes

Roof windows come in a fixed range of stock sizes rather than being made to order, and manufacturers write those sizes as short codes rather than dimensions. A code of two letters and two numbers is normal: the letters indicate the width band, the numbers the height. The widths are chosen deliberately to sit between rafters at common spacings, so a unit can often be fitted without cutting any structure at all.

That is the practical value of the coding system. If your rafters are at 400mm or 600mm centres, there is a width that drops between them, and a job that touches no structure is quicker, cheaper and does not need a trimmer. Sizing is covered properly on what size roof window should I get.

What it does for a room that a wall window cannot

Glass in a roof faces the sky rather than a fence, a wall or a neighbouring house, so it collects light from the whole hemisphere above it. A roof window of a given area delivers substantially more daylight to a room than a vertical window of the same area, and it delivers it deeper into the plan, away from the external wall.

It also ventilates better on a still day. Warm air collects at the ceiling, and an opening at the highest point of the room lets it leave, drawing cooler air in at the doors and low windows. That stack effect is why a roof window in a loft or a bathroom does more work than the same opening in a wall.

Where a roof window is the wrong choice

If the slope faces a road in a conservation area, a standard white framed unit standing on the tiles may not be acceptable and a flush conservation pattern unit is the honest answer. If the roof is flat, it is the wrong product. If you want a large expanse of glass over a kitchen extension, you are looking at a lantern or a fixed flat rooflight, not a roof window. And if the room below has no headroom at the eaves, a window positioned where you cannot reach it is a window you will never open.

We install and replace roof windows and rooflights across Oxford and up to twenty five miles by road, we have been at it for fifteen years, and the workmanship carries a ten-year guarantee. We fit VELUX, Fakro, Keylite and Roto units, we handle the Building Control notification where one is needed, and the roof is never left open overnight.