Do walk-on rooflights leak?

No upstand

Every other rooflight is protected by a kerb standing proud of the surface, so the joint between roof and unit sits well above the water line.

A walk-on unit is flush by definition, because anything else is a trip hazard. So the joint is at surface level, in the path of water crossing the terrace.

Which makes drainage everything

The perimeter drainage channel takes water away before it reaches the frame junction. Falls run away from the panel, not towards it.

Where that is designed properly, water never sits at the junction and the unit stays dry.

What goes wrong

No channel at all. Water reaches the flush junction and sits. It finds a way past eventually and sealant does not fix it.

A blocked channel. The most common cause of a leak on a working installation. Leaves, grit and moss fill a channel nobody thinks to clean, and a blocked channel is worse than none because it holds water against the unit.

Falls laid towards the unit, usually because the paving was set out from the building rather than from the drainage.

Maintenance

Clear the channel two or three times a year, more under trees. That is the single thing that keeps a walk-on installation dry, and it is frequently forgotten because the channel is hidden under a decking board.

Agree at handover who does it.

Access for inspection

Pedestal paving lifts, so the waterproofing and drainage can be checked without dismantling anything. A bonded deck cannot, which is worth weighing at design stage.

Not if the drainage was designed

Walk on units are among the most reliable rooflights when detailed correctly, because the glass build up is heavy, rigid and well sealed. When they do leak, it is almost always about water rather than about the unit.

Why they are more exposed than other rooflights

A walk on unit sits flush with the surrounding surface, because a raised kerb in the middle of a terrace is a trip hazard. That means the usual defence, a 150mm upstand lifting the seal clear of standing water, is gone.

Everything therefore depends on the surrounding surface draining properly and on a perimeter channel carrying water away to a working outlet.

The three failure modes

A blocked perimeter channel. By a wide margin the commonest. Leaves and silt collect in the channel, water backs up, and a pond sits against the seal after every shower. A seal designed to shed water is not designed to be submerged.

This is a maintenance failure rather than an installation one, and clearing the channel twice a year on a plot with trees prevents it entirely.

A surface that falls the wrong way. Where the terrace drains toward the unit rather than away from it, no channel will keep up. That is a design fault and it is corrected in the surrounding surface rather than at the rooflight.

An outlet that does not run. A perfect channel discharging into a blocked gulley is a pond. Check the outlet before assuming the rooflight is at fault.

The thermal problem that reads as a leak

A flush walk on unit in a terrace usually has heated space below, so the perimeter is a thermal bridge in exactly the place where cold bridging shows as condensation on the ceiling.

That is not water coming in. It is warm moist air condensing on a cold surface, and the fix is insulating the perimeter and ensuring the vapour control layer is continuous and sealed to the frame, which is done at installation rather than afterwards.

What to check first

Whether the water follows rain or cold weather. Whether the perimeter channel is clear and the outlet running. Whether water sits on the terrace after rain, and where.

Between those three you will usually identify the cause without anybody coming out, and two of the answers are things you can put right yourself.

What we detail at installation

A surrounding surface that falls away from the unit, a perimeter drainage channel that discharges into a working outlet, insulation to the perimeter, and a continuous vapour control layer sealed to the frame.

Those four are what separate a walk on installation that works for twenty five years from one that becomes a maintenance problem in three.

When they do, it is rarely the unit

A well made walk on unit is a factory bonded assembly, and the glass to frame bond is made under controlled conditions. That part is reliable.

What fails is the interface between the unit and everything around it, and on a trafficked installation there are more trades meeting at that interface than on almost any other detail.

The four causes, in order

Falls pointing the wrong way. The deck laid so water runs towards the glass rather than away from it, usually because the paving was set to a level datum without a drainage strategy that included the rooflight. The commonest error by some margin.

A perimeter channel with no outfall, or one that has blocked. A channel that connects to nothing is a trough holding water permanently against the seal. One without a removable grating fills with grit and leaves within two seasons and becomes the same thing.

The membrane terminated over the unit rather than dressed up the upstand and into it. Wrong sequence, lap running the wrong way, and it cannot be corrected without lifting the deck.

A sealant joint that has been worked to failure. Either sized wrongly, so it was always beyond its movement range, or flexed by a deck that moves underfoot.

The one that is not a leak

Water on the underside in cold weather without rain is condensation on a cold unit above a humid space, particularly a basement. Different problem, different remedy, and no work on the surface will fix it.

What prevents all of it

Drainage designed on a drawing rather than resolved on site, a drained and ventilated frame system, correct membrane sequence, and one party made responsible for the whole detail in writing.