In depth

A roof lantern in an existing flat roof: how does it work?

Fitting a roof lantern in an existing flat roof is usually quite possible, provided the structure allows it.

Own factory in Barneveld
Lead time from 2 weeks
Quote within 24 hours

Google-rating: 5.0★★★★★from 2 reviews

Build your quote

Put together your quote

A free, no-obligation quote tailored to you in just a few clicks. You finish on the quote page, where everything is already filled in.

Lean-toGable

123456

Step 1 of 6

Which roof lantern are you looking for?

Feel free to choose more than one option.

What are the dimensions?

The outside size of the opening or upstand, in centimetres. A good estimate is fine.

Width (cm)

Length (cm)

I do not know yet, I will send a photo or you can come and measure

Where will the roof lantern go?

So we know what to take into account.

When would you like the roof lantern?

An estimate is fine.

Add a photo, sketch or drawing? (optional)

A photo of your roof or a sketch with dimensions helps us make an accurate quote. You can also skip this.

Add photos

Where should we send the quote?

You will receive the quote in your inbox within 24 hours.

Name

Company (optional)

Phone number

Email address

Project address

Free and no obligationDone in 1 minuteQuote within 24 hours

In depthReading time: 3 minutesUpdated: September 2026

The short answer

Fitting a roof lantern in an existing flat roof is usually quite possible, provided the structure allows it. An opening is cut, the joists around it are trimmed and the roof lantern sits on an upstand that is built into the roof build-up. A builder or structural engineer checks beforehand whether the roof can take it.

Structure first: can the roof take an opening?

A flat roof usually rests on timber joists or a concrete deck. For a roof lantern you remove part of that roof. The joists you cut have to transfer their load to the joists beside them. That is called trimming: a cross member picks up the interrupted joists.

Whether that is possible and how heavy it needs to be is assessed by a builder or structural engineer. With a concrete roof an opening is more drastic and a structural engineer is almost always needed. Do this before you order, so the size of the roof lantern matches what is structurally possible.

Check joist direction and span
Trimming around the opening
With concrete, almost always a structural engineer
Only then fix the final size

Building into the roof build-up

From bottom to top, a flat roof consists of the structure, a vapour control layer, insulation and the roof covering. The upstand of the roof lantern is built into that build-up and rises above it. That lets the roof covering be turned up against the upstand, keeping the water out. More on height and insulation is in the upstand of a roof lantern.

Pay attention to the insulation all round. A cold edge between upstand and roof insulation causes condensation on the inside. The DagLichtDesign installation manual includes a step-by-step plan for upstand, insulation and sealing.

Roof lantern in an existing flat roof: who does what?

DagLichtDesign makes the roof lantern to measure in its factory in Barneveld, as a mono-pitch, wall-abutting lean-to, gable or hipped version. Your own builder or contractor cuts the opening, builds the upstand and fits the roof lantern. Installation by a regular installation partner is available on request, as a separate line in the quote.

The roof covering is connected to the upstand. On a rubber roof, see how that works in connecting to EPDM; on a bitumen roof, in connecting to bitumen. Measure the opening accurately, see measuring up for a roof lantern.

Made-to-measure roof lantern from our own factory
Opening and fitting by your own builder
Installation on request through a regular partner
Connection details for EPDM, bitumen and zinc

Frequently asked questions

Can an old rooflight dome come out and a roof lantern go in?

Yes, that is a common job. The existing opening is often smaller than the roof lantern you want, so the hole is usually enlarged. Read more in replacing a rooflight dome with a roof lantern.

Do I need a permit for a roof lantern in my existing roof?

Usually not for the roof lantern itself. If in doubt, for example with a listed building or a protected townscape, check with your local council.

When should the opening be cut?

Delivery takes from 2 weeks after approval, up to 8 weeks for complex made-to-measure work or special glazing. Only schedule the opening once the delivery date is fixed, so the roof is not left open for long.

Want to know what your roof lantern costs?

Send a photo or sketch with dimensions. Within 24 hours you receive a quote with glazing options, lead time and price.

Request a quote Get in touch