Can a walk-on rooflight be set into decking?

What setting glass into a deck involves

A walk-on rooflight is a sheet of laminated, toughened glass in a frame, designed to be stood on and to let daylight through to the room below. Setting one into a deck means the glass becomes part of the walking surface, sitting in a cut-out in the boards with the same finished height as the timber or composite around it.

The part that decides whether this works is not the glass. It is the build-up underneath: the structure that carries the frame, the roof covering that keeps the room dry, and the route water takes once it has fallen between the deck boards. A deck over a flat roof is two layers at once, a decorative surface above and a weathertight roof below, and a rooflight has to be right for both.

Carrying the frame on the structure

The frame and the glass must bear on the structure of the roof, usually a kerb or upstand built up from the roof deck and trimmed into the joists. Deck boards are not designed to take that load. If the frame rests on the decking joists or on battens above the roof, the glass moves when people step near it and the seals are put under strain.

The usual arrangement is a separate load path. The rooflight kerb stands on the roof structure, and the deck frame is built around it, with its own bearers and joists held clear of the kerb so that nothing presses on the glass frame. Where the deck sits on adjustable pedestals over a flat roof covering, the pedestals stop short of the kerb and the boards are supported on both sides of the cut-out.

A structural engineer or the project architect usually confirms the joist trimming for the opening. We send the dimensions for the unit chosen on paper, so that whoever is designing the structure has the real dimensions rather than an estimate.

Getting the glass level with the boards

A rooflight that sits proud of the deck is a trip hazard, and one that sits below it collects debris and standing water. The aim is for the top of the glass to finish level with the boards, with a narrow, even gap around it that lets the deck expand and contract through the seasons.

That depends on the build-up, which is why the make and thickness of the glass unit, the kerb height and the thickness of the boards are all fixed before the deck is built. Timber boards move more with moisture than composite boards, so the gap is planned for the material actually used. Cutting boards to a clean line around the frame, with the cut ends sealed or capped as the board maker advises, keeps the edge neat for years.

The glass itself should carry a slip-resistant finish suited to a surface that gets wet. Our page on whether walk-on glass is slippery explains the options, and the glass and load ratings guide covers the loadings the glass is made for.

Drainage under the deck

Water passes through the gaps between deck boards and runs onto whatever is below. Over a room with a rooflight, that has to be the roof covering, laid to falls so water travels to an outlet, gutter or drain. The kerb must stand high enough that water running past cannot get over the top, and the roof covering must be turned up the kerb and finished by the roofer to the maker's instructions.

Debris is the other risk. Leaves, grit and dirt collect beneath a deck and can block an outlet or sit against the kerb. A deck that can be lifted in sections, or a gap kept clear along the edge, lets you rinse the roof surface from time to time. We raise this at survey because it affects how the deck is built, not just how the rooflight is fitted.

Who does what, and in what order

Three trades touch this detail, and the order matters. The roofer lays the membrane or covering and turns it up the kerb. We fit the rooflight onto the kerb once the covering is in. The deck builder then lays the frame and boards around it. If the electrician is running lighting for the room below, that first fix is done before the ceiling is closed.

The glass is made to order, so we order it to arrive as the roof nears readiness, and the homeowner and builder both receive the confirmed delivery date. The maker gives the delivery period and the quote states it, which stops the roof sitting open while glass is awaited.

If the deck is a terrace on a larger roof, our guide to walk-on rooflights on roof terraces goes through the wider layout, and the walk-on rooflights hub has the rest of the range.

What to ask for in the quote

Ask for kerb and opening dimensions in the quote, the glass specification and its rated loading, the finish on the walking surface, the maker's delivery time and the preparation the builder must finish before fitting day. A quote that names these is one the deck builder can work to.

For cost, the size of the glass, the number of layers and the finish on top set most of the price, and access to a roof terrace sets much of the rest. The walk-on rooflight cost page explains this, and our rooflight costs guide covers how a quote is built.

Fixed units suit a deck best. If you are wondering whether the glass could also open, see the page on opening walk-on rooflights.

Related questions

Can a rooflight be added to an existing deck?

Often yes. The boards and frame are opened up around the planned position, the structure beneath is trimmed to suit, and the deck is made good around the new frame. The roof covering underneath is checked at the same time, because the kerb has to be weathered into it.