Roof Window Installation

What gets delivered is a frame, a sash with the sealed unit already in it, and a flashing kit chosen for the covering the window is going into. The flashing kit is a separate order line, it is specific to the profile of the tile or slate, and ordering the wrong one is the single most common way a delivery goes back.

Alongside those there are usually three collars: an underlay collar with a moulded drainage channel that carries water clear of the frame, an insulation collar that wraps the perimeter with a continuous layer, and a vapour barrier collar that gets taped to the frame on the room side. All three come folded flat in the box. All three are routinely left in the box by installers working to a price, and every one of them is doing a job that cannot be inspected once the plaster is on.

The frame and sash carry the manufacturer’s warranty. The perimeter work carries the ten-year workmanship guarantee, because that is the part the workmanship controls.

Two figures govern where a roof window sits vertically. The bottom of the frame wants to be roughly 900 to 1100mm above finished floor level, which is the height at which somebody sitting in the room can see the ground outside rather than only sky. The top of the frame wants to be at least 2 metres above the floor measured vertically, so that the head of the unit is above head height and the reveal is not something people duck under.

On a 45 degree roof those two figures are about 1.1 metres apart along the slope, which fits a 980 or 1180 tall unit neatly. On a 25 degree roof the same vertical difference stretches to about 2.6 metres along the slope, so a taller unit is both possible and desirable: a 1400 or 1600 in a shallow roof does the daylight work that a 980 does in a steep one. On a very steep roof the arithmetic runs the other way, and a 1600 tall unit puts its cill at knee height with the head barely at two metres.

Where a loft floor has been built up over the existing joists, which is common in a conversion, the finished floor level is 100mm or more above the level the survey first measured from. That gets recorded on the day rather than assumed, because it moves both figures at once.

Once a rafter is cut, the load it was carrying has to go somewhere. Doubled trimmers are fixed across the head and the cill of the opening, housed or hung on hangers rather than skew nailed, and the load from the cut rafter passes through them into the rafters either side. Over a single bay that is routine carpentry. Across two or three bays it is a calculation, and the answer is a deeper timber, a flitch plate or a small steel, submitted with the Building Control application.

The frame then goes in on brackets, usually four, screwed to the rafters and the trimmers and adjusted to bring the frame level and square. Squaring is checked on the diagonals, not by eye against the tile courses, because the tile courses on a hundred year old roof are not square either. A frame racked by 5mm across the diagonal will still take its sash, and the sash will bind on one corner for the rest of its life and wear a gasket flat on the other.

On trussed roofs the webs and the chords stay whole, because they are part of a triangulated frame held with punched nail plates. An opening either fits between two trusses or the affected truss is carried on a designed member spanning to the trusses either side. Across the newer estates at Quedgeley, Bishops Cleeve, Ashchurch and Yate that is why the narrow 550 wide units exist and why two of them in adjacent bays is so often the better answer than one wide unit and a structural submission.

Older Gloucestershire lofts are full of services that were installed when nobody imagined a window going in. The cold water storage cistern in a gravity system usually sits central on the rear slope, propped on boards across the joists, precisely where the daylight is wanted. Moving one is plumbing work with its own consequences for the system pressure downstairs, so the sensible sequence is to design the window position around it, or to establish early that the household is going to a mains-pressure system anyway.

The soil vent pipe rises through the roof close to the bathroom, and a bathroom is one of the rooms most often getting a roof window. The pipe cannot be moved sideways without opening the wall below it, so the window goes to one side and the flashing keeps clear of the pipe’s own weathering slate. Older lead-covered vent pipes are worth photographing for the survey notes.

Cabling, ancient television feeds, and the occasional buried junction box get traced before the saw comes out. And where a chimney breast rises through the slope, the stack has its own apron, side soakers and back gutter, all of which need their falls and laps left alone. A window is kept a clear tile width plus the flashing from a stack, and on a narrow terrace slope in Tredworth or Barton that single constraint can decide the whole layout.

Where a roof window is the escape and rescue opening from a room formed in the roof, the clear openable area has to be at least 0.33 square metres, with a minimum of 450mm in both height and width, and the bottom of that openable area between 800 and 1100mm above the floor of the room. A fixed unit in that position does not satisfy it, and neither does a small opening one.

The figure that catches people is that the clear area is measured on the actual sash and its hinge geometry, not on the frame size printed in the brochure. A centre pivot sash blocks its own opening with the pane, so the clear area is much smaller than the frame. That is why an escape position takes a top hung unit, and usually a purpose-made escape version whose sash swings out towards 90 degrees. The manufacturer publishes a clear openable area for each unit, and that published figure is what goes to Building Control rather than a calculation off the frame dimensions.

The 800 to 1100mm cill band interacts with everything in the section on height above. On a steep Forest of Dean roof at 45 degrees, an escape unit set to put its openable area in that band sits low in the slope, which is good for the view and bad for furniture, and the room layout gets drawn around it. On a shallow rear addition roof the band may be unreachable within the run of slope available, in which case the escape provision has to be solved elsewhere in the design.

Blinds fit to the frame, not to the plaster, and every manufacturer’s blind system relies on a groove or a bracket position in the frame’s top cover. If the plasterboard lining runs tight over that groove, the blind cannot be fitted later without cutting the finished reveal back. So the decision about blinds is made before the plastering, even if the blinds themselves are bought two years afterwards, and the lining is stopped short at the correct offset.

The same applies to an insect screen, which runs in a track at the head of the frame and pulls down over the opening. Anyone planning to have a window open on summer evenings in a vale village near standing water will want one, and it needs the head of the reveal to leave the track accessible.

Electric blinds need a feed. A low voltage supply run to the head of the frame while the reveal is open costs almost nothing; running one afterwards means lifting a floor or chasing a wall. Solar powered blinds avoid that entirely and are the sensible choice on a unit high in a slope. Where the window itself is going to be electrically operated, the wiring is planned at the same time, and that is covered on the electric rooflight installation page.

Central Cheltenham’s Regency terraces are largely mansard: a steep lower slope in Welsh slate that you can see from the pavement, and a shallow upper slope behind a parapet that you cannot see at all. Lansdown, Montpellier, Pittville and the Bath Road terraces are full of them.

The shallow upper slope is frequently below the 15 degree minimum a roof window is rated for, sometimes down at 5 to 10 degrees, which makes it kerb-mounted rooflight territory rather than roof window territory. The steep visible lower slope will take a roof window technically, and on a listed building or in a conservation area it is exactly the elevation where a unit is most likely to be resisted.

That combination is why these buildings need the consent conversation before the product conversation. Listed building consent is required for an alteration whether or not it can be seen, and visibility strongly affects what an officer will accept, so a hidden flat behind a parapet is a very different proposition from the face above the front door. It is worth getting a camera up on a pole first, because a good number of these roofs already carry Victorian or Edwardian openings on the flats that nobody on the street has ever noticed.

Forming a new opening in a pitched roof is notifiable work, and the notification is submitted, the inspections booked and the completion certificate closed out as part of the job. Four parts of the regulations bear on a roof window: structure, where trimming rafters or carrying a truss needs a calculation; conservation of fuel and power, where the limiting standard for a replacement roof window sits at 1.4 W/m2K and any current unit is comfortably inside it; fire safety, where the escape figures above apply if the window serves a room in the roof; and safety glazing, covering the toughened outer and laminated inner panes. The certificate comes back in the homeowner’s name, and it is the document a buyer’s solicitor asks for years later.

Most roof windows in a house go in under permitted development, because an in-plane unit projects less than the 150mm limit beyond the plane of the existing slope. Flats and maisonettes have no permitted development rights for roof alterations, Article 4 directions remove rights street by street in parts of Gloucester and Cheltenham, and the local planning authority is the only body that can confirm the position for a specific address.

To start, email info@severnskylightsgloucester.co.uk with a photograph of the roof from the garden or the street, a photograph of the room from inside, what the covering is if you know it, and roughly where you want the daylight to land. That produces an indicative range by email within a day or two, then a survey, then a fixed price that holds. The costs page sets out what moves a figure and why there are two of them, the areas pages cover the thirty miles by road, the guides answer individual questions in more depth, and about explains how the company works and what the fifteen years have been spent on.