Can a walk-on rooflight be retrofitted?

The easiest candidates are flat roofs that people already walk on: the roof of a rear extension used as a terrace, a balcony over a kitchen, or a flat section of roof beside a loft conversion. A pane of structural glass set into that surface lets daylight drop into the room below while the space above stays usable.

It also works in solid or timber ground floors above a cellar or semi-basement, and in landings that sit over a dark hallway. In a Georgian terrace in Southernhay or St Leonards, a lower-ground room that has always been gloomy can be lit from the garden level above. In a 1930s semi in Heavitree, a flat-roofed rear addition is a more likely home for one.

Retrofitting is harder where the surface above is pitched, where the structure is unknown, or where the load path is hidden. A pitched roof is not a walking surface, so the question there is really about a different product, and our conservation and flat-rooflight ranges are the ones to look at.

The glass is the simple part. The structure around it is what needs proving. On the day of the survey we look at:

• the direction and spacing of the joists or rafters, and their size and condition;
• whether the opening can be formed between existing members or needs one cut;
• what the surface build is (decking, paving, single-ply membrane, asphalt), and how deep;
• what the ceiling or floor below is made of, since that is where the opening will be seen from;
• how high the new glass will sit relative to the surrounding finish, so water runs away from it.

In an older house, particularly cob or stone with irregular timber, the engineer may need to open up a small area to see what is really there. It is better to find that out at the start than with the roof open.

A walk-on unit is heavy compared with an ordinary rooflight, and it has to carry people, not just rain. Where an opening is cut through a joisted floor or roof, the cut members are replaced by a trimmer, a short beam across the opening, and the load is carried round to the joists either side. The new trimmers and any doubled joists are sized by the engineer for the span and the loading the glass is specified for.

Walk-on glass is built from several laminated layers and is specified against structural loading guidance, which is a different standard from a pitched rooflight. The size you can have depends on the supports as much as on the glass, so it helps to read about walk-on rooflight sizes and supports and the glass specification before settling on a layout.

A new opening in an existing floor or roof is structural alteration, so it needs Building Control approval. Where the opening is in the roof or an external wall of the insulated envelope, the new unit also has to meet Part L, which sets limits on heat loss. We handle the Building Control notification as part of the job. The detail for this product is on our page about whether a walk-on rooflight needs Building Control.

Planning is usually simpler. Rooflights in a house roof are generally permitted development, within the limits on projection and height. A walk-on rooflight sits flush with a flat surface, so those limits are rarely a problem. What does change things is the setting. A listed building needs listed building consent for external changes, a conservation area may have an Article 4 direction removing the usual rights, and a flat or maisonette has no householder permitted development rights to begin with. If any of that applies to you, get an answer from the council planning team before you commit to the work.

This is the part most quotes skip. A walk-on rooflight is the only kind you look at from underneath and from above, so the opening has to be finished on both faces. From the room, the edge of the glass meets your ceiling or floor structure, and whatever happens around it is on show.

In a lath and plaster ceiling, cutting the opening needs care so the surrounding plaster does not crack away from the lath. Beneath a plasterboard ceiling, the opening is simpler to form, but the trim, the bead and the paint line still decide how it reads. Where the glass is set deep, a lining with a neat reveal makes it look planned. Our finish schedule accompanies the quote and notes how the opening will be formed and finished, which means the ceiling detail is agreed on paper before anything is cut.

On a flat roof, the glass has to be waterproofed into the existing covering, not just set on top of it. A raised upstand or a carefully formed edge detail keeps water from tracking under the frame, and the membrane around it is dressed up and sealed to suit whatever is already there, whether that is single-ply, asphalt or GRP. If the roof is walked on, the decking or paving around the glass should finish at a sensible height so it does not trap water against the frame.

Insulation needs the same attention. The opening interrupts the insulation layer, and a walk-on unit is a thermal weak point if it is poorly specified. The sealed unit and frame have to meet Part L, and the surrounding insulation should be returned neatly to the edge of the opening so there is no gap behind the lining. It is dull work, and it is what stops a cold edge and condensation showing up in the ceiling later.

Start with a few photographs: the surface you would like to put the glass into, the room or space below it, and the loft or void if there is one. Tell us what the surface will be used for, since a terrace with chairs asks more of the glass than a small plant-room window. From there, we arrange a survey, look at the structure and the ceiling, and write a quote that includes the structural work and the finish.

If you would like to see how a retrofit might sit in your house, ask for a quote and send the photographs with it. For more on this product, the walk-on rooflights page covers the options in full.