Rooflights for a loft conversion
In a loft conversion the roof windows are usually ordered late and they should be decided early. Their positions are fixed by the floor structure, the purlins, the stair and the escape requirement, and every one of those is settled in the first fortnight of a conversion while the windows are still a line on a schedule. By the time somebody is standing in a stripped loft asking where the glass should go, most of the choices have already been made for them by decisions taken for other reasons. This page is about how rooflights fit into a conversion as a project: the structure they have to work around, the insulation build-up they sit inside, where they fall in the programme, and what changes on the particular roofs this county is made of. How many units a given room needs is worked through on how many rooflights do I need in a loft conversion, and the installation itself is on the roof window installation page.
Rooflights against dormers, and the volume allowance each one spends
The first decision in any conversion is whether the roof stays as it is or grows. Permitted development allows an addition of up to 40 cubic metres of roof volume on a terraced house and 50 on a semi or detached, and a dormer eats into that allowance while a rooflight spends none of it.
The reason is the 150mm rule. An alteration to the roof of a house must not project more than 150mm beyond the plane of the existing slope, and an in-plane roof window comfortably satisfies that while a dormer plainly does not. So a rooflight-only conversion generally proceeds without a planning application, keeps the roof line intact, and costs a fraction of what dormer construction costs. What it gives up is floor area with full head height, because the room stays inside the existing triangle. On a terrace in Gloucester where the volume allowance is small and the conservation area status is uncertain, that trade is often decided in the rooflights’ favour before anything else is discussed.
Head height, and where the room really exists
Building Regulations no longer impose a minimum ceiling height for a habitable room, which surprises people. What does apply is Part K for the stair: 2.0 metres of headroom above the pitch line, which may be reduced to 1.9 metres at the centre of the stair width and 1.8 metres at the side where the stair rises into a sloping roof.
Beyond the stair, usability sets the standard rather than the regulations. A room is comfortable where there is around 2.1 to 2.3 metres at the ridge, and the floor area worth counting is the part with 1.5 metres or more above it. Work that out on a section drawing before positioning windows, because a roof window placed where somebody can stand up in front of it is worth twice one placed over a zone that will end up as eaves storage. On a typical 1930s Gloucester semi with a 32 degree hipped roof, the standing zone is a strip about 2.4 metres wide down the ridge, and the windows should be looking into that strip.
The floor comes first, and it takes head height away
A conversion almost always gets a new floor. The existing ceiling joists were sized to carry a plasterboard ceiling and a water tank, not people and furniture, so new joists are installed spanning between load bearing walls, often deeper than the originals and usually sitting above the existing ceiling line on wall plates or in hangers.
That new floor typically raises the finished level by 50 to 150mm, and every millimetre of it comes off the head height above. It also changes where the roof windows have to sit, because cill heights are measured from the finished floor and not from the joist tops. Ordering windows against the old ceiling level and then discovering the floor has come up 120mm is how a cill ends up at 680mm and outside the escape range. The floor design is settled, the finished floor level is written down, and the window heights are set from that number.
Knee walls, and the cill height that follows from them
A conversion is usually built with knee walls, low stud partitions running along each side of the room where the roof gets too low to use, typically at 1.0 to 1.5 metres high, with the space behind them used as storage.
A roof window cill set below the top of the knee wall is a window you cannot see out of and cannot reach, and a cill set well above it wastes the lowest and most useful glass. The comfortable arrangement puts the cill just above the knee wall head, somewhere around 900mm to 1.1 metres above the finished floor, which also happens to sit inside the 800 to 1100mm band an escape window has to meet. That coincidence is convenient and it is worth designing for deliberately rather than hoping for. Where the knee wall is higher because the roof is shallow, the window climbs with it and the escape figure becomes the binding constraint.
Purlins, and the two zones they divide a slope into
Almost every cut roof in Gloucestershire has a purlin running horizontally across each slope at roughly mid height, carried on struts down to a load bearing wall or on the gable walls. In a conversion the purlin is usually retained, sometimes exposed, sometimes boxed in, occasionally replaced by a steel.
It divides the slope into two zones, and a roof window has to live entirely in one of them. Below the purlin, on a typical interwar semi, there is perhaps 1.2 to 1.4 metres of slope, which takes a 780 by 980 unit and not much more. Above the purlin the run is shorter still and the window is high in the room. Where the design genuinely needs a taller window spanning the purlin line, the purlin is being replaced or reinforced, which is a structural design and a Building Control matter rather than a window decision. That work is set out on trimming rafters for a pitched roof rooflight.
Insulation between rafters, and what it does to the opening
A converted roof has to meet a thermal standard, and the usual target for the sloping element is around 0.15 to 0.18 W/m2K. Existing rafters are commonly 100mm deep, which is nowhere near enough on its own.
The standard answer is to deepen the rafters with a batten or a timber scab, fit rigid insulation between them while keeping a clear 25 or 50mm ventilation gap above it, and add a further insulated board across the underside before plasterboard. The finished construction is often 200 to 250mm thick where the original roof was 100. At a roof window that thickness becomes the depth of the reveal box, and a deep reveal on a pitched roof throws real shadow unless the sides are splayed. The reveal geometry is covered on rooflight reveals and why splaying them matters.
Warm rafter build-ups, and where the frame ends up sitting
A different approach puts the insulation over the rafters rather than between them, which removes the cold bridge at every rafter and keeps the timber warm and dry. It also raises the level of the battens and the covering by the thickness of the insulation, commonly 60 to 100mm.
That matters enormously for the roof windows, and it gets missed. A roof window frame is designed to sit at a specific height relative to the batten line, and if the battens have risen 80mm the frame has to rise with them or the covering will sit proud of the glass and the flashing will have nothing to work with. Manufacturers supply installation brackets that allow the frame to be set higher, and the correct bracket position is calculated from the finished batten level rather than adjusted by eye. Where an over-rafter system is being used, the roof window fixing height is confirmed in writing before the units are ordered.
Where the rooflights fall in the conversion programme
The sequence on a well run conversion is consistent, and the windows go in earlier than most people expect.
| Stage | What happens | Where the rooflights sit |
|---|---|---|
| 1. Survey and design | Structural design, floor and stair set out, Building Control application | Positions, sizes and escape unit fixed on the drawings |
| 2. Scaffold and access | Scaffold to the working elevations | Access to the slope confirmed, delivery route agreed |
| 3. Structure | Steels, new floor joists, trimming of the roof openings | Openings trimmed to the unit sizes already ordered |
| 4. Roof windows and weathering | Frames set, flashings and membrane detailed, covering reinstated | This is the point at which the shell becomes watertight |
| 5. First fix | Stud walls, electrics, plumbing, insulation | Insulation taken tight to the frames, vapour layer taped |
| 6. Second fix and finish | Plasterboard, reveals, plaster, decoration | Reveal boxes formed and splayed, blind fixings anticipated |
The important line is stage four. Until the windows are in and flashed, the roof has holes in it and nothing internal can start. Delays to a window delivery therefore stop the entire conversion, which is why the units are ordered against surveyed sizes at stage one rather than chosen later.
Who fits the windows, and the handover between trades
On a conversion run by a builder, the roof windows are sometimes fitted by the builder’s own team and sometimes by a specialist brought in for two days. Either can work. What causes trouble is an unclear boundary.
Four things need an owner: who confirms the structural opening dimensions against the unit, who forms the trimmers, who details the underlay and flashings, and who takes the reveal insulation and vapour layer tight to the frame. The last of those is the one that falls between trades most often, because the roofer has gone and the plasterer arrives to find insulation stopping 30mm short of the frame with nobody’s name against it. Agreeing those four in advance costs nothing. Severn fits roof windows into openings formed by others as readily as forming the openings as well, and the split is written down at survey.
Escape from the new room, and the unit that has to satisfy it
A loft conversion creates a new storey and with it a fire escape requirement, and on a two storey house becoming three the answer is usually a protected stair enclosure plus an escape window in the new room.
The window has to give a clear openable area of at least 0.33 square metres with a minimum of 450mm in both height and width, and the bottom of that opening between 800 and 1100mm above the floor. A fixed unit cannot do it, a centre pivot only does it with an escape hinge that swings the sash clear, and several popular sizes do not make the area once the frame is deducted. This is the constraint that most often determines which unit is bought, so it is checked first and the rest of the glazing is arranged around it. The detail is on roof window escape openings and the rules.
The stair, the landing, and the daylight nobody budgets for
The new stair takes floor area out of the room below and arrives in the loft at a point dictated by head height, which usually means it lands along the ridge rather than where anybody would choose.
The top of that stair is nearly always the darkest place in the finished conversion, because it sits in the middle of the plan with a wall on one side and a door on the other. A small roof window directly over the stair head, or a sun tunnel where the roof geometry is tight, changes it from a dim corner into a lit one and it costs very little because the scaffold and the trades are already there. Where the unit is above a stairwell and out of reach, an electric opener with a rain sensor makes it a working vent rather than a fixed pane, covered on electric rooflight installation, and the tube option is on sun tunnel installation.
Solar gain in a west facing loft, and the glass that answers it
A loft room is the hottest room in the house in July, and the physics is unfriendly. The glass is overhead, the sun is high, the roof void above the plasterboard is warm, and heat rises through the house into the space anyway.
Three things are worth specifying at order stage rather than adding afterwards. A solar control coating on the outer pane brings the g-value down from around 0.5 to roughly 0.25 to 0.3, which is the single most effective change available. An external awning blind, fitted outside the glass, stops the heat before it enters and outperforms any internal blind by a wide margin. And a top hung sash on the highest window gives a stack effect, drawing warm air out at the top of the room. A west facing rear slope in Cheltenham or Gloucester will take all three and still be warm in a heatwave. Doing none of them produces a bedroom nobody sleeps in from June to August. The glazing numbers are on energy efficient rooflights.
Rain noise, when the room below is a bedroom
Glass overhead in a bedroom is a different acoustic proposition from glass in a wall, and heavy rain on a large pane is audible in a way that surprises people who have only had roof windows over a landing.
A laminated inner pane makes a genuine difference, because the interlayer damps the panel rather than letting it ring. Manufacturers also offer specific acoustic glazing options with an asymmetric pane thickness, which detunes the panel further. Where a conversion is putting a large area of glass over a child’s bedroom in the Forest of Dean or the Stroud valleys, both of which are wetter than the vale, the laminated inner pane is worth having for the noise even before its safety role is counted. It is also the pane that has to be laminated anyway wherever there is habitable space beneath.
Gloucestershire lofts: hips, trusses and valley cottages
The county gives three recurring conversion problems, and each has a different window answer.
The interwar hipped semi at Longlevens, Hucclecote or Warden Hill has a rear slope that narrows towards the hip and a purlin across the middle, so the usable rectangle is far smaller than the roof appears. Two units of 780 by 980 in adjacent bays generally beat one large unit that will not fit, and side by side arrangements are covered on two rooflights side by side on one slope. A trussed estate house at Quedgeley or Bishops Cleeve cannot have its webs cut, so unless the trusses are attic trusses the conversion needs a designed structure and the windows work to the 600mm module in the meantime, described on rooflights in a trussed roof and what cannot be cut. And a Stroud or Chalford valley cottage often has a steep roof with generous height but hand-carried scaffold and no lorry access, so the constraint is getting the glass to the roof rather than getting it into the roof.
Party walls, and the Gloucester terrace
A conversion in a terraced or semi-detached house that involves cutting into or building off the party wall brings the Party Wall etc. Act 1996 into play, which means notice to the adjoining owner and, if they dissent, a surveyor’s award.
Roof windows themselves rarely trigger it, because they sit in the middle of a slope. What triggers it is the structure the conversion needs: steel beams bearing into the party wall, new floor joists built into it, or a strengthened purlin picking up on it. That is worth knowing at the point the window positions are set, because a position that forces a longer beam span may force a party wall notice that a position 600mm away would have avoided. Notice periods are two months for party structure works, so the timing question is real on a Gloucester terrace where the neighbours are close.
Building Control, and how the rooflight package gets signed off
A loft conversion is a full Building Regulations job and the roof windows sit inside it rather than alongside. Several parts touch them directly.
- Structure. Trimming for each opening, and any purlin or truss work, designed and calculated.
- Fire safety. The escape window’s clear opening area and cill height, and the protected stair enclosure it works with.
- Conservation of fuel and power. A limiting standard of 1.4 W/m2K for the units, plus continuity of insulation around every reveal.
- Ventilation. Purge ventilation of at least one twentieth of the floor area, and background ventilation from the sash flaps.
- Safety glazing. Toughened outer and laminated inner wherever there is habitable space below.
- Stairs. Headroom over the stair, which sometimes moves a window that was going to sit above it.
The inspector will want to see the openings trimmed before they are covered, the insulation continuous around the frames before boarding, and the escape window measured with a tape rather than accepted from a brochure. Where Severn is fitting the windows within someone else’s conversion, the relevant inspection stages are flagged to whoever holds the application so nothing gets boarded over the day before a visit.
What to settle before the conversion starts
A short list, agreed on paper, removes almost every argument that happens later.
- Finished floor level, written as a dimension rather than described.
- Knee wall height, and the cill height that follows from it.
- Which unit is the escape window, and confirmation that its clear opening makes the figures.
- Purlin strategy: retained, boxed, reinforced or replaced.
- Insulation build-up, and whether any of it goes over the rafters.
- Glass specification per window, including solar control on any south or west facing unit.
- Which units are manual, rod operated or electric, decided by reach from the finished floor.
- Blind type, because an external awning needs its fixings before the covering goes back.
- Who does what at the frame perimeter, listed by trade.
Settle those nine and the roof window element of a conversion becomes a two day operation in the middle of a much longer programme, which is what it should be. The route from a first enquiry through a range and a survey to a fixed price is on the costs page, and the coverage across the county is on areas.
Next on this subject.
- Guide Trimming rafters for a pitched roof rooflight Trimming is the carpentry that lets a hole exist in a sloping roof without the roof noticing. A rafter is…
- Guide Rooflights in a trussed roof and what cannot be cut A trussed roof is not a set of rafters that happen to be joined together. It is a series of…
- Guide Soakers, aprons and head flashings explained A flashing set around a pitched roof rooflight is three separate components doing three separate jobs, and confusing them is…
- Guide Rooflight reveals and why splaying them matters The reveal is the plastered box between the frame of a rooflight and the ceiling of the room, and it…
- Guide Two rooflights side by side on one slope Two roof windows side by side on one slope is usually a better answer than one wide unit, and on…
- Guide Rooflights on a shallow pitch below 15 degrees Below fifteen degrees a pitched roof stops behaving like a pitched roof, and the whole approach to glazing it changes.…