Rooflights in a loft conversion get specified twice. Once early, badly, when someone picks two windows off a price list to fill a line on a quote. Then again on site, when it turns out the sill lands at knee height, the escape unit is 60mm too small to satisfy Building Control, and the head of the glass sits so low that the back third of the room stays gloomy at four in the afternoon. The second specification is the expensive one, because by then the structural design is fixed and the openings are cut.
The short version of this page: decide the rooflight positions and sizes at the same time you decide the structural design, not after. Work from where people will stand and sit, set the sill between 800 and 1100mm above the finished floor, get the head of the glass up towards 2m, confirm which unit is carrying the Part B escape duty before you order anything, and give the builder the exact opening dimensions in writing so the trimmers get built to them first time. Everything below is the detail behind that.
This page covers specifying and programming. The hub page on pitched roof skylight installation covers how the unit is physically fitted, the flashing kits and the collars. If the question you actually have is whether to use rooflights at all or build a dormer, read rooflight or dormer for a loft conversion first, because that decision comes before this one.
Start with the room, not the roof
Most loft rooflight layouts we are asked to price started life as a symmetrical pair drawn on an elevation because two windows look tidy on a drawing. Symmetry is a fine outcome and a poor starting point. What decides a good layout is where the usable floor is.
A loft has a distinctive geometry. Full standing height runs in a band along the middle under the ridge, and the floor tapers away to nothing at the eaves. Furniture goes in the low zones, because that is where you cannot stand anyway. People stand and walk in the central band. So daylight is worth most where it falls into the standing zone and onto the far side of the room, and worth least where it lands on the top of a wardrobe.
Three positioning rules we work to at survey:
- Push units up the slope, not down it. The higher up the slope a rooflight sits, the deeper into the room the light throws, because the light is arriving at a shallower angle relative to the floor. A window low in the slope lights the floor immediately under itself and very little else.
- Two apart beat one large. Above roughly 14m² of floor, two separated units give better light than one big one of the same total glass area. Two sources from different points cancel each other’s shadows. One source puts a hard bright patch on the floor and leaves the corners dim.
- Light both slopes if the roof allows it. On a room that runs under a double-pitched roof, one unit in each slope produces genuinely even light and gives you cross ventilation as a free consequence. It usually costs no more than two units in the same slope.
The counterweight to all of that is the rafters. Standard rooflight widths of 550, 660, 780, 940 and 1140mm exist because they suit rafters at 400 or 600mm centres, and a unit that suits the room but lands badly across the timbers gets expensive fast. We measure the actual rafter positions from inside before recommending anything, and where a 100mm narrower unit lands cleanly between existing members we will show you both prices. The detail on how spacing constrains size sits on rafter spacing and rooflight sizes.
Sizing against floor area
There is no single legal minimum glazed area for a habitable loft room in England, whatever a builder tells you on site. What exists is a long-standing design guide of glazing at around 20 per cent of floor area for a room that will feel properly lit, plus the Part L requirement that whatever you install performs thermally.
Twenty per cent is a target, not a ceiling, and it behaves differently in a loft than it does downstairs. A rooflight in a sloping roof sees a much larger area of open sky than a vertical window of the same size sees, so it delivers considerably more daylight per square metre of glass. The flip side is that the same geometry delivers more solar gain in July, which is why south-facing loft glazing above about 20 per cent of floor area wants solar control glass rather than clear.
| Room floor area | Target glazed area | Usual arrangement | Notes |
|---|---|---|---|
| Up to 8m² | 1.4 to 1.6m² | One 780 x 1400mm unit | A single unit is genuinely enough at this size. Position it high in the slope. |
| 8 to 12m² | 1.6 to 2.4m² | One 940 x 1600mm, or two 780 x 1180mm | Two is better if the room is long. If it is nearly square, one larger unit is fine. |
| 12 to 18m² | 2.4 to 3.6m² | Two units placed apart, 780 x 1400mm each | Separation matters more than total area past this point. |
| 18 to 25m² | 3.6 to 5.0m² | Two to three units, or one in each slope | Opposing slopes give the most even light and cross ventilation. |
| Loft bathroom or ensuite | 0.6 to 1.2m² | One 550 x 980mm or 660 x 1180mm | Obscure or laminated glass, and mechanical extract regardless of the opening. |
| Landing or stair head | 0.6 to 1.0m² | One 550 x 980mm | Often the single highest-value unit in the whole conversion. A dark stair makes a loft feel like an attic. |
Glazed area is the unit’s overall frame size, which is how the 20 per cent guide is normally applied. Clear glass area is roughly 15 per cent less.
The stair head is the one people leave out and then regret. A conversion stair usually rises into the least generous part of the roof, and a 550 x 980mm unit over the top few treads changes how the whole floor reads when you walk up to it. It is also, at the smaller sizes, one of the cheaper units on the job.
Sill height, head height and the bit no one measures
Height in the slope is where most of the regret lives, and it is decided by two numbers.
The sill
Set the bottom of the glass between 800mm and 1100mm above the finished floor. That range is not arbitrary. Below about 800mm you have a low glazed panel in a sloping roof at floor level, which is a guarding question under Part K and a poor view besides. Above 1100mm you cannot see out when seated, the window becomes a light shaft rather than a window, and if it is your escape unit it fails outright, because Part B puts the bottom of the openable area no more than 1100mm above the floor.
Note the words finished floor. A loft conversion floor is very often built up above the existing joists, sometimes by 60 to 100mm once new structural joists, insulation, deadening and boarding are in. If the rooflight position is set out against the old joist tops, the sill ends up that much lower than intended, and an escape unit that measured 1080mm on the drawing can end up compliant or not depending on which surface someone measured from. We ask for the finished floor level in writing before we set out.
The head
Get the top of the glass to roughly 2m above finished floor where the roof allows it. The head height governs how far light penetrates horizontally into the room, far more than the width does. This is also where roof pitch bites: on a shallow pitch of 20 to 25 degrees, a unit needs to be physically much taller to gain the same vertical height, which is why 1400mm and 1600mm units are standard on shallow roofs and 1180mm is usually enough at 45 degrees.
The head-height trap on a shallow pitch. A 780 x 1180mm unit in a 22-degree roof gains only about 440mm of vertical height across its whole length. Put the sill at 900mm and the head lands at roughly 1340mm, well below eye level for anyone standing. The room gets light on the floor and a view of nothing. On shallow pitches specify tall units and accept the extra cost, or the conversion will disappoint every day it exists.
Reach
Whoever lives there has to operate it. A handle at the top of a centre pivot sash is reachable up to about 2.2m above floor. A bottom handle on a top hung unit needs the sash within about 1.9m or you want a control rod or an electric actuator. Decide this before first fix, because actuator wiring goes in before the reveals are boarded and retrofitting it means opening finished plasterwork. That side of it is covered on electric skylight installation.
Escape provision: which unit is carrying it
This is a requirement, not a preference, and on a loft conversion it drives the specification more than anything else on this page.
Where a loft conversion creates a habitable room a storey above the ground floor and the design does not provide a fully protected stairway route down to a final exit, an escape window is required from that room. Where the conversion adds a second storey above the ground floor, a protected stair becomes the route and the rules change again. Your builder or designer decides which strategy applies, and Building Control signs it off. What we need from that decision is a single answer: which rooflight is the escape window.
Once a unit is carrying escape duty, it must meet all of the following:
- A clear openable area of at least 0.33m². Clear means what a person can actually pass through, not the frame size.
- A minimum clear opening of 450mm in both height and width. Both. A tall narrow unit can meet the area and fail the width.
- The bottom of the openable area no more than 1100mm above the finished floor.
- An unobstructed opening. A standard centre pivot rotates on a mid-height bar that sits across the middle of the opening and generally cannot serve, unless it is a model with a specific escape function that swings the sash clear.
- It must open without a key from inside.
The practical consequences are worth stating plainly. A 550 x 780mm unit does not achieve 0.33m² and never will. Escape units are ordered as escape units, top hung or side hung, from the outset. They cost more than the equivalent standard unit and have longer lead times. And the escape unit’s 1100mm sill limit frequently sets the height for every other unit in the room, because a room with one window at 900mm and one at 1500mm looks like a mistake, which it is.
We check this at survey and we will tell you if the window you want cannot legally serve the room you are creating. Building Control checks it at inspection whether or not anyone checked earlier, and finding out at that point means a new unit, a re-cut opening and a re-flashed roof. The full dimensional detail is on fire escape rules for loft rooflights.
Structure: the openings are part of the design
A loft conversion is a structural project. New floor joists span between load-bearing walls, the roof is often propped or partially rebuilt, and steels frequently go in at floor level. The rooflight openings are part of that structural design, not an afterthought bolted onto it.
That is genuinely good news, because it means the trimming around your rooflights gets designed and calculated alongside everything else, rather than being a judgement call on a Tuesday morning. What it demands from you is timing. The engineer needs to know the opening positions and sizes before the calculations are issued.
What we hand over for the structural design:
- Structural opening dimensions for each unit, which are not the same as the unit’s frame size. Every manufacturer publishes a required rough opening, typically 40 to 60mm larger in each direction.
- The position of each opening along and up the slope, dimensioned from a fixed datum such as the party wall or the gable.
- Which rafters are cut and how many.
- Whether the roof is trussed. On trussed roofs, and that is most housing built after about 1970, including the newer stock at Grange Farm, Sproughton, Martlesham and Kesgrave, nothing gets cut without a design. Trusses are engineered as complete units.
- Unit weight, which matters on the larger and triple-glazed units.
On older Suffolk stock the structure is the interesting part. Victorian and Edwardian roofs across central Ipswich and the Norwich Road terraces have cut rafters of irregular section, purlins in inconvenient places, and a hundred and thirty years of settlement. A purlin running across your intended opening is a common and entirely solvable problem, but it is solvable at design stage and awkward once the plasterboard is up. Timber-framed and historic roofs across the inland villages need an engineer comfortable with them rather than a standard trimming detail. How the opening itself is formed is covered on forming the structural opening in a pitched roof.
Where we sit in the programme
We are one trade among several on a conversion, and the sequencing is straightforward once everyone knows it. Rooflights go in at the weathering stage, after the structure is formed and before the roof is insulated and boarded. Fitting earlier than that means fitting into structure that may still move. Fitting later means opening up finished work.
| Stage | Who | What we need or do |
|---|---|---|
| Design and structural calculations | Designer and engineer | We supply structural opening sizes, positions, weights and the escape unit decision. |
| Order | Apex | Placed once sizes are frozen. Standard units one to two weeks. Escape, conservation, triple-glazed and made-to-size units run longer. Nothing starts until stock is physically in. |
| Structural works and openings formed | Main contractor | Trimmers built to the issued dimensions. We check them before we arrive with glass. |
| Rooflight installation | Apex | Frames set, underfelt collars formed, flashing dressed. Roof watertight at the end of this stage. |
| Insulation and vapour control | Apex around the frames, contractor elsewhere | Insulation collar fitted tight to each frame, vapour layer taped to the frame on the warm side. |
| Boarding, reveals and plaster | Main contractor | Reveals splayed, vertical at the head and horizontal at the sill. |
| Second fix and blinds | Apex | Sashes adjusted, actuators commissioned, blinds fitted. Cheaper now than retrofitted. |
Two units on a conversion that already has scaffold and open structure is typically one to two days of our time, split across two visits. That is much less disruptive than the same two units retrofitted into a finished roof, and it is why doing it as part of the conversion is cheaper per unit. Access is the other reason: your scaffold is already up, and scaffold is a real line on a standalone retrofit.
What we need from the contractor is honest notice. Loft conversion programmes move, and we would far rather be told a week early that structure has slipped than turn up to openings that are not formed. Where a contractor wants us on a fixed date, we hold it.
Glazing choices that matter in a loft
A loft room sits directly under the roof with a large area of sky in view, so the glazing specification does more work here than it does anywhere else in a house.
- Overheating. South and west-facing loft glazing gains heat quickly. Solar control glass at a g-value of 0.28 to 0.38, with light transmission still above 60 per cent, removes most of the problem without making the room dim. It costs less than the blinds and air conditioning that get bought instead.
- Noise. Rain on glass is louder in a loft than anywhere else because there is no ceiling void above you. Laminated outer panes cut it noticeably. On the coastal belt, where driving rain is a normal winter condition rather than an occasional one, this is worth specifying.
- Safety glass. Any glazing overhead in a habitable room should have a laminated inner pane. If it fails, it holds together rather than coming down on the bed.
- Triple glazing. Adds roughly 15 to 25 per cent to the unit cost and about a third to the weight. Worth it in a bedroom on an exposed slope, marginal elsewhere. Tell the engineer if you specify it.
- Blinds. Order at the same time as the units. Fitting them on the same visit is meaningfully cheaper than a return visit, and blackout blinds in a loft bedroom are not optional in June.
Cold, draughty loft rooflights are almost always a collar and vapour control failure rather than a glazing failure, and that is worth understanding before spending on triple glazing to fix the wrong problem. See draughty, cold loft rooflights.
Conservation constraints in Suffolk and north Essex
Loft conversions in central Ipswich, Lavenham, Aldeburgh, Southwold, Dedham, Coggeshall and Clare run into conservation and listing far more often than they do in most of the country, and a rooflight layout designed on daylight alone can be undeliverable.
The pattern is usually this. Rear-slope units, not visible from a public highway, are frequently acceptable in a conservation area. Front or otherwise visible slopes need conservation pattern units sitting flush with the roof plane, with a central glazing bar and dressed lead flashing, and are sometimes refused regardless. On a listed building, consent is required for any rooflight anywhere, including a rear slope nobody can see.
The planning constraint interacts with escape provision, which is the part that catches people out. If the only slope where a large escape unit is acceptable faces the rear, the room layout has to follow. Conservation pattern units are also generally available in fewer sizes, which narrows the options for hitting 0.33m² of clear opening. Sort planning strategy and escape strategy in the same conversation, before ordering. More on the units themselves at conservation rooflight installation, and on the consent position at planning permission for a pitched roof rooflight.
What loft conversion rooflights cost
A pair of 780 x 1400mm top hung units fitted during a conversion, with the scaffold already up and the openings formed by the main contractor, typically lands well below the equivalent retrofit because access and structural work are already accounted for elsewhere. A Part B escape unit sits at £2,400 to £3,300 supply and fit as a standalone job. Full published ranges by unit size, covering and access, with everything that moves the number, are on pitched roof rooflight installation costs. We do not repeat them here, because on a conversion the meaningful figure is what changes when the work is bundled, and that comes out of the survey.
Common questions about loft conversion rooflights
How many rooflights does a loft conversion need?
Work to roughly 20 per cent of floor area in glazing and let the room shape decide the arrangement. Up to about 8m², one 780 x 1400mm unit is genuinely enough. Between 12 and 18m², two units placed apart beat one large one, because two light sources from different points cancel each other’s shadows. Above 18m², or in a room under two slopes, one unit per slope gives the most even light and cross ventilation as a bonus. Add a small unit over the stair head almost regardless of the maths, because a dark stair makes the whole floor feel like an attic.
How high should a loft rooflight sill be?
Between 800mm and 1100mm above the finished floor. Below 800mm you have a low glazed panel in a slope at floor level, which raises a guarding question under Part K. Above 1100mm you lose the view when seated, and if the unit is carrying Part B escape duty it fails, because the bottom of the openable area must be no more than 1100mm above the floor. Measure from the finished floor, not the existing joists. A conversion floor commonly builds up 60 to 100mm and that difference has decided compliance before now.
Can a centre pivot rooflight be my fire escape window?
Usually not. A standard centre pivot rotates on a bar at its mid height, and that bar sits across the middle of the opening when the sash is open, so the clear openable area does not meet the 0.33m² requirement. Some manufacturers make centre pivot models with a specific escape function where the sash swings clear of the opening, and those do qualify. Otherwise you need a top hung or side hung escape unit, ordered as an escape unit rather than converted later. It also has to give at least 450mm clear in both height and width, and open without a key from inside.
When in the build should rooflights be fitted?
After the structural openings are formed and before the roof is insulated and boarded out. That is the weathering stage. Fitting earlier means setting frames into structure that may still be adjusted, and fitting later means cutting into finished work. In practice we come twice: once to set frames, form the underfelt collars and dress the flashing, which leaves the roof watertight, and once at second fix for sash adjustment, actuator commissioning and blinds. Sizes have to be frozen well before that, because the order goes in at design stage.
Is it cheaper to fit rooflights during the conversion than afterwards?
Yes, usually by a meaningful margin per unit. The scaffold is already up, which is a real cost on a standalone retrofit. The structural openings are being designed and formed anyway, so trimming is part of the main contract rather than a separate exercise. The insulation and vapour control layers are open, so the collars go in as part of the normal build-up. Retrofitting the same units into a finished, insulated, plastered loft means new scaffold, a separate structural check and making good internally. If a rooflight is even a possibility, form the opening during the conversion.
Do I need an engineer for loft conversion rooflight openings?
On a loft conversion, effectively always, but not as a separate exercise. The conversion is a structural project already, so the rooflight openings are designed alongside the new floor structure and any roof alterations. What matters is timing: the engineer needs the structural opening sizes, the positions dimensioned from a fixed datum, the number of rafters cut and the unit weights before the calculations are issued. If the roof is trussed, and most post-1970 housing is, nothing gets cut without a design at all. Trusses are engineered as complete units.
Get your loft conversion rooflights specified properly
Tell us the room dimensions, the roof pitch, whether the roof is cut or trussed, what the conversion drawings show and whether a protected stair is being provided. We will come out, measure the actual rafter positions and the intended finished floor level, confirm which unit has to carry escape duty, and issue the structural opening sizes your engineer and contractor need. You get one written fixed price with the units, flashing kits, collars and glazing specification itemised. Every installation carries a 10-year workmanship guarantee, and the installers have 25 years fitting rooflights behind them.
Request a fixed quote for loft conversion rooflights in Ipswich, Suffolk or north Essex, or read the rooflight or dormer comparison first if that decision is still open.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.