Rooflights and Part L for a replacement
Taking out an old rooflight and putting a new one in is controlled work under the Building Regulations, and Part L sets the thermal standard the new unit has to meet. For a replacement roof window in an existing dwelling that standard is 1.4 W/m2K. For a rooflight it is 2.2 W/m2K, quoted in the vertical plane, which is a distinction that catches people out and is explained below. Any current manufactured unit clears both comfortably, so the number is rarely the difficulty. What causes trouble is everything attached to it: which document applies, what counts as a controlled fitting, the ventilation requirement that arrives alongside, the escape opening in a loft room, and the relaxations available on a listed building. This page works through all of it. The performance side of the same question sits on the energy efficient rooflights hub.
The standard a replacement has to meet, stated plainly
Approved Document L, Volume 1, covering dwellings, sets limiting standards for replacement controlled fittings in existing homes. A replacement roof window is held to 1.4 W/m2K, or the equivalent window energy rating band. A replacement rooflight is held to 2.2 W/m2K expressed in the vertical plane.
Those are limiting values, not targets. They describe the worst that is permitted, and a well specified current unit will be well inside them. A manufactured roof window from VELUX, Fakro, Keylite or Roto with a modern low-emissivity double glazed unit typically lands between 1.0 and 1.4, and a good made-to-size flat roof rooflight can reach around 1.0. The realistic conclusion is that meeting Part L on the glass is straightforward, and the effort belongs elsewhere.
Which document applies, and which edition
The regulation itself is Part L of Schedule 1 to the Building Regulations 2010, headed conservation of fuel and power. Approved Document L is the statutory guidance on how to satisfy it, and it is published in two volumes, the first covering dwellings and the second covering everything else. A house, a flat and a maisonette are all Volume 1.
The current edition is the 2021 edition, which came into force in June 2022, with subsequent amendments. It is worth checking the edition rather than assuming, because the limiting values for fittings have moved over the years and a figure quoted on an older website may be out of date. Building Control will confirm which edition applies to a particular commencement date.
Roof window against rooflight: why there are two numbers
The two words are used loosely in ordinary speech and precisely in the regulations. A roof window sits in the plane of a pitched roof, in line with the covering, and is the kind of unit that comes in manufactured sizes with a matching flashing kit. A rooflight is a unit for a flat or nearly flat roof, sitting on a kerb built up above the roof surface.
They are held to different standards because they are tested and used differently. The roof window is a window in the ordinary sense and is measured like one. The rooflight sits nearly horizontal, its kerb forms part of the assembly, and the industry convention for declaring its U-value is different. The 2.2 figure is not a lower expectation of the product so much as a different way of measuring it, which brings us to the plane.
The vertical plane declaration, and the figure that looks worse than it is
A sealed glazing unit performs differently depending on the angle it is set at. Lay it flat and the gas in the cavity convects more freely between the panes, because the warm side is underneath and the cold side is on top, which is the ideal arrangement for a convection loop. Stand it vertical and that loop is suppressed. The same unit therefore has a worse U-value horizontal than vertical.
The European product standards for rooflights declare U-values in the vertical plane, so that is what appears on the certificate, and Part L’s 2.2 figure is quoted on the same basis so the two can be compared. A unit declared at 1.2 in the vertical plane might be nearer 1.4 to 1.6 installed at five degrees. Nothing is wrong and nothing is being hidden, but the two numbers should not be mixed. When comparing quotations, check that both are quoting the same plane, and treat a horizontal-plane figure as the more demanding test rather than the worse product. The industry body for rooflight manufacturers publishes guidance on the conversion, and manufacturers will supply both figures on request. More on how the numbers are built up is on U-values and glazing specification.
The limiting standards in one place
| Situation | Element | Limiting standard | Note |
|---|---|---|---|
| Replacement in an existing dwelling | Roof window | 1.4 W/m2K | Or the equivalent window energy rating band |
| Replacement in an existing dwelling | Rooflight | 2.2 W/m2K | Declared in the vertical plane |
| New opening in an existing dwelling | Roof window or rooflight | Same limiting values apply | Plus the area limit on new glazing where relevant |
| New extension | All glazed openings | Assessed together against the extension’s overall standard | Total glazed area is normally limited relative to floor area |
| Listed building or a building in a conservation area | Any fitting | The best achievable without unacceptably altering character | A judgement made with the conservation officer and Building Control |
| Sealed glazing unit changed within a sound existing frame | Glass only | Treated differently from a whole fitting | Confirm the position with Building Control for the specific job |
What a controlled fitting is, and what triggers the requirement
The Building Regulations define a controlled fitting as a window, door, rooflight or roof window, along with the associated frame and glazing. Taking a whole unit out and putting a new one in is therefore replacing a controlled fitting, and it engages Part L whether or not the opening size changes.
That is true of a like for like replacement into the existing structural opening, and it is true of a unit being upgraded to a larger one. The threshold is the fitting, not the scale of the work. It is also worth being clear that this is a regulation about energy, so it applies even where the old unit was performing perfectly well and is being changed for appearance or operation.
Whole unit against glass alone
There is a real distinction between changing the complete fitting and changing only the sealed unit inside a frame that is staying. Where a timber roof window is sound, the gaskets are in good order and the misted glass is a standard size, a new sealed unit can go into the existing sash. Where the frame is twenty years old, the flashing kit is the same age as the glass and the whole assembly is at the same stage of its life, the complete unit is normally the better outcome and not much more work.
The regulatory position differs between the two, and the safe course is to ask Building Control which route the specific job falls under before starting rather than after. Whichever way it goes, the safety glazing requirements still apply: a toughened outer pane, and a laminated inner pane wherever there is habitable space, a stairwell or a bath underneath. The practical comparison between the two routes is on replacing a failed or misted rooflight.
Part L does not stop at the edge of the glass
The requirement is conservation of fuel and power, not conservation of fuel and power through the glazing. Insulation continuity around the new fitting is part of the same requirement, and it is the part most often skipped, because it is buried and nobody sees it.
What that means on site is insulation carried tight and continuous around the frame with no void behind the plasterboard, a kerb insulated through its depth rather than left hollow, and the roof insulation lapped onto the kerb wrap rather than butted loosely against it. The heat lost through a badly detailed perimeter can exceed the heat lost through the glass it surrounds, which makes it the most valuable part of the job to get right. That is set out in full on thermal bridging at the rooflight perimeter.
Part F arrives at the same time, and it is the one people forget
Replacing a window or rooflight in an existing dwelling also engages Part F, ventilation. The principle is that the work must not make the ventilation of the building worse than it was, and the guidance goes further: where an existing unit had a ventilation opening, the replacement should have one at least as effective, and where the existing had none, background ventilation should still be provided to the current standard.
In practice that means a trickle vent or a background vent flap in the head of the new unit, and it is a common cause of a job being questioned at inspection. It is also the requirement that most directly affects how the finished room behaves, because a sealed replacement in a kitchen or bathroom raises the internal humidity and moves the condensation somewhere else. The mechanism is covered on condensation, ventilation and rooflights.
Part B, and the escape opening trap in a loft room
Where the existing unit forms the escape opening from a loft room, the replacement has to keep doing that job. The clear openable area must be at least 0.33 square metres, with a minimum of 450mm in both height and width, and the bottom of the opening between 800 and 1100mm above the floor.
The expensive mistake is putting a fixed unit, or a smaller opening unit, into a position that was serving as the escape route. It clears Part L easily and fails Part B outright, and it is discovered at inspection rather than at order. Any loft room replacement should start by establishing whether that unit is the escape opening, and if the room has only one roof window, it almost certainly is.
Listed buildings, conservation areas, and the flexibility that exists
Gloucestershire has an unusual concentration of both, so this matters more here than in most counties. The guidance recognises that in a listed building, a building in a conservation area or a scheduled monument, meeting the full energy standard may unacceptably alter character or appearance. In those circumstances the requirement becomes the best that is achievable without doing so.
That is a judgement, made between the conservation officer and Building Control, not a blanket exemption. The sensible order is pre-application advice with photographs of the roof and a clear note of what is proposed, before anything is ordered. Officers in this county have seen a very large number of rooflight proposals and will usually indicate early what would be acceptable, which is far cheaper than a refusal after the scaffold is up.
What a conservation rooflight actually achieves thermally
The relaxation exists because of a genuine tension. A conservation rooflight has a slim cast or pressed steel frame with a single central glazing bar, designed to sit almost flush in a thick Cotswold stone slate roof. That slim frame is the whole point of the product, and a slim steel frame conducts heat rather well.
Current conservation units are much better than the originals they imitate, with modern sealed units, low-emissivity coatings and thermal breaks where the profile allows. Many now reach the replacement standard outright. Where a particularly slim profile is required by a conservation condition, the honest answer is that the unit may land above 1.4 and that this is precisely the situation the flexibility is written for, provided it is agreed in advance rather than discovered at inspection. Sizes and finishes are on conservation rooflight installation.
Notification, inspection, and the certificate that ends the job
The work is notified to the local authority Building Control, the inspections are booked and attended, and a completion certificate is issued in the homeowner’s name at the end. Severn submits the notification and closes it out as part of the job rather than leaving it with the customer.
Keep the certificate. It is the document a buyer’s solicitor asks for, and its absence is discovered at the worst possible moment, when a sale is already agreed. Retrospective regularisation is possible and it is slower, more expensive and more intrusive than doing it in order, because the work has to be opened up for inspection.
Where the rules change: extensions and brand new openings
A brand new opening in an existing roof is held to the same limiting values as a replacement, so the glass specification does not change. What changes is that the structural work is now clearly notifiable, and that in some circumstances there is a limit on how much new glazed area can be added without a compensating calculation elsewhere.
An extension is a different exercise again. The extension is assessed as a whole, the glazed area is normally limited in proportion to its floor area, and a large lantern or a run of rooflights can push it over. Where that happens the usual remedy is a calculation showing the extension performs at least as well overall as a notional one built to the standard values, which allows more glass in exchange for better performance elsewhere. That is a design stage decision, not a site one.
Gloucestershire’s older stock, and where Part L bites hardest
The county has a very large quantity of glazing installed between the late 1970s and the late 1990s that is now at the end of its service life, and it is all arriving at replacement at roughly the same time. Acrylic domes on flat rear extensions across Gloucester, Longlevens and Hucclecote. Early timber roof windows in interwar semis at Warden Hill and Charlton Kings. First-generation loft conversion glazing on the Quedgeley and Abbeydale estates.
The gap between what those units achieve and what the current standard asks for is large. A 1980s acrylic dome on an uninsulated softwood kerb performs somewhere around 3.5 to 5.0 W/m2K taken as a whole assembly. A 1990s double glazed roof window sits around 2.8 to 3.0. A current unit is 1.0 to 1.4, and the kerb underneath it is insulated rather than hollow. Meeting Part L on these is not a hurdle, it is the main benefit of the work, and the rooms below them feel different afterwards in a way that has as much to do with the perimeter as the glass. Sizing, sequencing and what a replacement day looks like are on flat roof rooflight installation and roof window installation, and the process for getting a figure is on costs.
Next on this subject.
- Guide U-values and glazing specification A U-value is the rate at which heat crosses one square metre of a finished rooflight for every degree of…
- Guide Solar gain, overheating and shading Overhead glazing collects far more solar energy in July than in January, and the ratio is not close. A rooflight…
- Guide Condensation, ventilation and rooflights Surface condensation on a rooflight is a whole-house ventilation problem showing up at the coldest point in the building. The…
- Guide Triple glazing in a rooflight and when it pays Triple glazing in a roof is worth more than triple glazing in a wall, and for a reason that has…
- Guide Warm-edge spacers and where condensation starts Watch any rooflight through a cold week and the water always arrives in the same place first: a band following…
- Guide Low-emissivity coatings and argon fills Roughly three quarters of the heat crossing an uncoated sealed unit travels as infrared radiation from the warm inner pane…