Can a Walk-On Rooflight Sit Flush With Paving?

A flush unit has its top surface at the same level as the paving or decking around it. There is no raised kerb to step over, so the glass reads as part of the floor of the terrace or path. Some units use a slim frame around the glass. Others use a frameless edge, where the glass meets a narrow joint and the paving runs up to it.

The visual effect is the point of the choice, but it comes with a technical cost. A raised rooflight sheds water because it stands proud of the surface. A flush one has to be taught to do it.

Flush units come in two broad types. A framed unit has a narrow upstand of metal at the edge that the paving butts against. A frameless one has glass edges bedded in sealant, which gives the cleanest line but demands the most accurate levels. The survey says which suits the surface around the opening.

The roof terrace guide covers where these units usually go, and the rest of this answer covers the flush detail itself.

Water is the main design issue. Rain landing on paving must run away from the glass edge, and any water that gets past the joint must be able to leave.

• Fall. The paving around the unit is laid to a gentle slope away from the glass, so surface water moves outward.
• Joint. The gap between glass and paving is sealed with a flexible material that allows for movement.
• Channel. Some systems include a drained perimeter channel that collects water at the edge and carries it away.
• Waterproofing layer. Under the paving, a membrane is dressed up to the frame so water that passes the joint is caught.

The cleanest results come from planning the drainage before the paving is laid. Altering a terrace after it is finished to add a fall is slow and costly.

Leaves, soil and grit collect in a flush joint more easily than around a raised unit. A flush joint should be brushed clear a few times a year so that water still has a route out, and the channel, where fitted, should be kept open.

The unit and the paving have to meet at the same height, so the depth of the glass stack has to be matched by the build-up of whatever surrounds it.

The glass stack is deep, as described in our answer on glass thickness, and it needs a recess to sit in. So the finished paving level is measured at the survey. A difference of a few millimetres shows as a lip, a trip or a puddle.

The structure under the unit must take the weight of the glass and the people on it. In a terrace over a room, that means checking the deck or slab, and any strengthening is described on the quote.

Where the unit sits over a room below, the waterproofing under the paving doubles as the roof covering for that room. The membrane detail at the frame is the place where the roof and the rooflight join, so it is fitted and checked before any paving goes down.

Different surfaces meet the glass in different ways.

• Porcelain or stone slabs. Cut neatly to the unit's shape and laid on supports, with a narrow sealed joint at the glass.
• Timber decking. Boards finish short of the glass with an edge trim, and the timber needs to drain freely.
• Resin or poured surfaces. The edge is formed against the frame, which needs the frame to be set precisely.
• Paved garden path over a basement light. The unit is bedded in a ring of supporting masonry, with drainage around it.

Each surface moves with temperature and moisture a little differently, so the joint material is chosen to suit. A flexible sealant between glass and paving takes that movement without cracking.

The survey records the finished level of the surface, the planned build-up, the clear opening and the support condition. We also record the fall available and where a drain or outlet is. Those figures go on the measured survey sheet, and the glass is ordered to match them.

If the paving is to be laid by another trade, the sheet tells them the exact level and the fall to leave around the unit, so the glass and the paving meet cleanly. The wider service is laid out under walk-on rooflight installation.

Slip finish matters more on a flush unit outdoors, because it sits in the walking path. The slip resistance answer covers the finishes and the ratings.