Roof Window Installation

The flashing kit decides the job

A roof window is watertight because a set of pressed metal sections lap the covering correctly on all four sides: an apron at the bottom, soakers at the sides, a head flashing at the top. Those sections come in different profiles depending on what they have to lap.

• Slate and other flat coverings. A flat profile kit. The sections sit tight to the covering because the covering is flat.
• Plain clay tile. Also relatively flat, but the tiles are small, so the soakers work differently and more tiles need cutting.
• Concrete interlocking and pantile. A deep profile kit, because the covering is not flat and the flashing has to bridge the roll of each tile.

Fitting a flat kit to a profiled tile leaves a gap under every roll. It will not leak on the day. It will leak in the first driving rain from the wrong direction, which on this coast is a matter of weeks.

Rafters, and what can be cut

Most standard roof windows are sized to fit between rafters at typical centres, which is why the size codes exist. A window that fits between rafters needs no structural work at all.

Go wider than one rafter bay and a rafter has to be cut and trimmed, which means a trimmer above and below and the load carried to the neighbouring rafters. That is routine on a traditional cut roof.

It is not routine on a trussed roof. A trussed rafter is an engineered component and cutting one without a replacement design compromises the whole truss, not just that member. On a modern estate roof, that is a structural engineer’s decision and it changes the cost of the job materially. We check the roof type at survey, because finding out at installation is far too late.

Sizes and codes

Roof window sizes are coded rather than described, and the codes are consistent within a manufacturer. A code gives the frame size, and the frame size is what has to fit the opening. The glass area is always smaller than the code suggests, which is worth knowing when you are working out how much light you will actually get.

Cill height and the view

Where the bottom of the window sits decides whether you can see out of it sitting down, standing up, or not at all. The usual guidance is a cill around 900mm to 1100mm above the finished floor, which gives a view from standing and from most seated positions.

Higher than that and the window becomes a light source rather than a window, which is a legitimate choice in a bathroom and a disappointment in a bedroom. It is worth deciding with a tape measure in the room rather than from a plan.

Escape windows

A habitable room in a loft conversion generally needs a means of escape, and a roof window can provide it if it meets the clear opening requirements and sits within reach of the floor. The rules are specific about clear opening area, minimum dimensions and the height of the cill above the floor.

An escape window is a different unit from a standard one, usually top hung so the sash swings clear. It cannot be retrofitted by changing the handle. If the conversion needs one, it has to be in the specification from the start.

What the installation involves

1. Survey. Roof covering, pitch, rafter type and centres, room use, and whether escape is required.
2. Fixed written price, including the correct flashing kit for the covering and Building Control where notifiable.
3. Fit. A single window in an existing rafter bay is usually a one day job. Cutting a rafter adds time and sometimes scaffold.
4. Inside. Reveal formed and splayed where depth allows, plaster made good.

What it costs, and what moves it

Roof windows have the widest cost spread of anything we fit, because the same window in two different roofs can be two very different jobs.

Whether it fits between rafters. If it does, there is no structural work at all and this is the cheapest useful job in this trade. Standard size codes exist precisely to suit common rafter and truss centres, usually 400mm or 600mm.

Cut roof or trussed roof. Trimming a traditional cut roof is routine carpentry. Cutting a trussed rafter needs a structural engineer’s replacement design, and that changes both cost and timescale materially.

The covering. Slate and plain tile are straightforward. Profiled tiles need a deep profile kit. Where tiles are obsolete or fragile, sourcing replacements for the ones cut adds cost, and we check availability at survey rather than with the roof open.

Access. A single storey rear slope may be reachable from a tower. A two storey front slope over a road generally means scaffold.

Internal finishing. Forming and plastering the reveal is a real part of the job, and a splayed reveal is more work than a square one and throws noticeably more light.

The false economy

Choosing a larger window that does not fit between rafters, to gain a modest amount of glass, and paying for structural work to get it. On a trussed roof that decision can double the cost of the job. Going taller within the same rafter bay usually gains more usable light for nothing.

Cill height, and the view you actually get

Where the bottom of the window sits decides whether it is a window or a light source, and it is the dimension most often left to chance.

Between about 900mm and 1100mm above the finished floor gives a view standing and from most seated positions. Below about 800mm the sash starts to interfere with furniture and with anybody standing at it. Above about 1200mm it stops being a window you look out of, which is legitimate in a bathroom and disappointing in a bedroom.

Pitch changes all of it. On a steep roof a window at a given cill height sits close to vertical and the view is like a normal window: horizon, garden, street. On a shallow roof the same cill height angles the window towards the sky, and you see cloud rather than garden. Neither is wrong; they are different products in practice and people are regularly surprised.

It is worth deciding in the room with a tape measure rather than from a plan. Ten minutes doing that prevents the most common regret in this trade.

Blinds, and deciding early

Blinds are an accessory rather than part of the window, designed to locate into brackets already in the frame. Fitting them at installation is neater and usually cheaper than a separate visit later, and if the window is above reach an electric blind wants its power feed run at first fix.

In a bedroom, blackout is not optional. An east facing loft window is light before five o’clock in late June, and a blackout blind only blacks out if it runs in side channels holding the fabric against the glass. A roller blind hanging in front of a sloped window leaks light down both sides and sags away from the glass.

Blinds do not solve overheating. An internal blind sits behind the glass, so the solar energy has already entered the room and the blind absorbs it and re-radiates it as heat. Specify solar control glass for temperature and use blinds for glare, privacy and blackout.

Maintenance

Five things once a year, four of which cost nothing.

Clean the filter behind the ventilation flap. It clogs with dust and pollen, background ventilation drops to nothing, and condensation appears on cold mornings that never used to happen. It is the most neglected part of a roof window and it takes two minutes.

Wipe the seals. Grit stops them compressing, which lets draughts and eventually water past.

Lubricate the hinges. A stiff window gets forced, and forcing is what breaks handles.

Check the timber finish, particularly at the bottom of the sash where condensation runs. Recoating a sound frame is straightforward; replacing a rotten one is not.

Clear the head flashing of leaves and moss, which dam water where it should be flowing away and cause a class of leak that gets blamed on the window.

What goes wrong

Nearly always the flashing. The single most common defect in this trade is a flat profile kit fitted to a profiled tile, leaving a gap under every roll.

After that: the underlay collar omitted or badly dressed, so water travelling on the membrane goes into the roof rather than around the opening. Battens left unsupported over the opening, which drop and take the tiles with them, opening a gap right at the head of the window. And the insulation collar left out, which is not a leak but produces condensation at the reveal that reads exactly like one.