Rooflights and Part L for a replacement

The requirement

A replacement rooflight must achieve a whole unit U-value no worse than 2.2.

Undemanding. A modern double glazed unit with a low emissivity coating, argon fill and a warm edge spacer lands around 1.2 to 1.4.

Whole unit, not centre pane

The regulation uses the whole unit figure, including frame and spacer. Marketing frequently quotes centre pane, which ignores exactly the parts where heat escapes and is always better.

When comparing quotations, compare whole unit.

When it applies

Replacing a rooflight: yes, even like for like.

New opening: yes, and notifiable for structure too.

Repair, such as a handle or a seal: no.

Extensions are assessed differently

Where a rooflight forms part of a new extension, it is assessed as part of that extension’s overall performance rather than in isolation. A poor rooflight then has to be compensated for by better performance in the walls or roof.

Getting the glass right first is cheaper than compensating elsewhere. There is also a limit on total glazed area as a proportion of floor area, so a heavily glazed extension may need a calculation.

The number nobody checks

The kerb. Part L expects continuity of insulation, and an uninsulated upstand does not provide it. There is no U-value on a quotation for it, and it frequently loses more than the glass.

Who notifies

Where notifiable, we handle it and the certificate comes to you.

The number is 2.2

For a replacement rooflight in an existing dwelling, Part L expects a whole unit U-value no worse than 2.2 watts per square metre kelvin. That is the compliance threshold rather than a target.

Any current double glazed unit with a low emissivity coating, argon fill and a warm edge spacer lands around 1.2 to 1.4, so the requirement is cleared with a wide margin and it rarely causes anybody trouble.

The figure the regulation asks for

This is the point that matters and it is where quotation comparison goes wrong.

Centre pane measures the glass away from the edges, ignoring the spacer bar and the frame, which is exactly where heat escapes. It is always the better figure and it is what marketing quotes.

Whole unit includes glass, spacer and frame weighted by area. It is what the building experiences and what the regulations use. It is always worse, frequently by 0.4 or more, and on a small unit with a lot of frame relative to glass the gap is wider still.

Ask for the whole unit figure in writing on the quotation.

Why a replacement is caught at all

A rooflight is a controlled fitting, so replacing one engages Part L even though you are not altering the structure. Repair does not: renewing a flashing or replacing a broken pane in an existing frame is maintenance rather than building work.

The distinction the regulations draw is between repairing something and replacing it.

How it is notified

Through a competent person scheme, where the installer self certifies and notifies afterwards and you receive a certificate, or through a building notice to the local authority.

Either way it is the installer’s job and it should be a named line on the quotation.

What Part L does not cover

The installation quality. A product certificate shows the unit meets the standard. It says nothing about whether the insulation collar was fitted, or whether the kerb on a flat roof was insulated on its outer face.

Those decide whether the installation performs as the certificate suggests, and a 1200 by 1800 flat rooflight on an uninsulated kerb has close to a square metre of poorly performing envelope around its perimeter regardless of what the glass achieves.

Where the standard bites harder

On an extension rather than a replacement. There the rooflights form part of the calculation for the whole structure, so a poor unit forces better performance somewhere else in the build, and there is a limit on total glazed area as a proportion of floor area.

Getting the glass right first is cheaper than compensating for it in the walls.

What triggers the requirement

Replacing a rooflight is work on a controlled fitting, and the replacement has to meet the standard applying to replacement work at the time it is carried out.

That standard is expressed as a maximum U value for the whole unit. It is not the standard the original was built to, which on a twenty five year old window will be far worse, and it is not the same as the standard for a new opening in a new building.

In practice, any current mainstream double glazed roof window comfortably meets it. This is rarely a constraint on product choice.

Where it does bite

On a listed building or in a conservation area, where the visual requirement and the thermal requirement can pull in opposite directions. There is provision for relaxation where compliance would unacceptably alter the character of the building, and that is a conversation with building control rather than an assumption.

And on an obsolete size, where the temptation is to source any unit that fits rather than one that complies.

Notification

Replacement of a rooflight is generally notifiable work. It is either notified to building control directly, or carried out under a competent person scheme where the installer self certifies and the certificate is issued to you.

That certificate matters at sale. A missing one turns up in conveyancing enquiries and is far more awkward to resolve retrospectively than to obtain at the time.

Ask which route the installer is using and make sure you receive the paperwork.