Can a flat rooflight be fitted to a green roof?

A taller kerb

The kerb has to clear the whole green roof build up: drainage layer, filter fleece, substrate and vegetation, plus room for the plants to grow.

Where a normal kerb is around 150mm above the finished surface, a green roof kerb is often 300mm or more depending on the substrate depth. That is designed from the build up rather than assumed.

An edge restraint

Substrate migrates. Without a physical restraint around the rooflight, soil and vegetation creep towards the glass, hold moisture against the frame and eventually grow across it.

The usual detail is a gravel margin held by an edge trim, giving a clean band between the planting and the unit. It also gives a route for maintenance access.

Drainage

Green roofs drain through the substrate into a drainage layer below. That layer must not be interrupted by the kerb in a way that dams water against it.

The gravel margin usually doubles as the drainage path around the unit.

Maintenance

Green roofs need occasional attention, and a rooflight adds a reason: keeping vegetation off the glass and out of the margin.

Worth agreeing at handover who does that, because a rooflight in the middle of a sedum roof is easy to forget until it has disappeared.

Weight

A saturated green roof is heavy, and the structure is designed for it. Forming an opening in that structure is a structural question and it needs an engineer where the span is anything but modest.

Yes, and the kerb is much taller

Green roofs and rooflights work well together. The one detail that changes fundamentally is the upstand height, because the effective surface level is not the deck: it is the top of the growing medium.

On a conventional flat roof, 150mm above the finished surface is the working minimum. On a green roof, the kerb has to clear the substrate depth plus that margin, so on a sedum roof with 80mm of growing medium you are looking at 230mm or more, and on a deeper biodiverse or intensive roof considerably more again.

A kerb built to the conventional figure and then buried in substrate is a rooflight sitting in wet soil, which is the fastest way to destroy a perimeter seal.

The other details

A vegetation free zone. A margin of gravel or paving around the kerb, typically 300mm or more, keeps roots and growing medium off the upstand, allows the flashing to be inspected, and gives somewhere to stand when cleaning the glass.

It is not optional. A green roof planted right up to the rooflight is a maintenance problem waiting to happen.

Root resistant membrane. Standard on green roof build ups and it has to continue correctly at the upstand rather than stopping short.

Drainage. Green roofs hold water by design, which is part of their point. The drainage layer has to work around the kerb rather than damming against it, and the outlet has to cope.

Weight. A green roof is heavy before you add glazing, and the structure has to carry both. On a retrofit this is a structural question rather than a detail.

Keeping a planted roof off the frame

More than on a conventional roof, and it is the thing people underestimate.

Growing medium migrates, plants spread, and organic debris collects against anything that stands up out of a green roof. The vegetation free zone needs clearing, and the glass needs cleaning more often because a green roof generates its own dust and pollen.

Expect a couple of visits a year rather than one.

The payoff

A rooflight in a green roof looks better than one in a bare membrane, the substrate moderates temperature swings at the perimeter, and the vegetation reduces reflected glare and heat around the unit.

It is a good combination when it is detailed properly, and a difficult one when the kerb was built to a conventional height.

Design it together

Green roof and rooflight belong on the same drawing. Retrofitting a rooflight into an established green roof means lifting substrate, cutting membrane and rebuilding a taller kerb, which is considerably more work than getting it right at the outset.

What the green roof changes

A green roof adds a substrate layer, a drainage layer, a filter and a root barrier on top of the waterproofing. All of that has depth and weight, and the rooflight has to come up through it.

The consequence is a taller kerb than a bare membrane roof would need. The kerb has to rise above the finished planting level by a real margin, so that soil and vegetation do not sit against the frame and so that water within the substrate cannot reach the junction.

Skimping on kerb height here is the classic error, and it puts wet growing medium permanently in contact with the rooflight upstand.

The root barrier

Roots find any gap and enlarge it. The root barrier has to be continuous, dressed up the kerb, and terminated properly, with no interruption at any corner.

This is the layer that protects the waterproofing, and a rooflight is the place it is most likely to be compromised, because it involves a change of direction and a junction with a different material.

Drainage and maintenance

A green roof drains through its substrate to a drainage layer, and the rooflight kerb is an obstruction in that path. The drainage layer needs to carry water around the unit rather than into a dead area beside it.

A gravel margin around the kerb, free of planting, is standard good practice. It keeps vegetation off the frame, gives access for maintenance, and provides a visible zone where blockages can be seen.

Weight and access

A green roof build up is heavy, and that load is a structural matter that has to be designed for regardless of the rooflight.

And the rooflight will need cleaning, which means getting to it across a planted roof. Plan a route rather than trampling the planting.