The Same Rooflight in Four Different Roof Build-Ups

The kerb record describes the constant part. An opening removes the deck across that area and every layer the deck was carrying, and the kerb takes those duties over: it supports the unit that will sit on it, gives the waterproofing something to turn up and terminate against, and carries the insulation and vapour control around an aperture colder than the roof surrounding it.

It also describes the structural consequence as separate from the weathering one. Trimmers frame the hole and carry the loads the removed deck used to take, redistributing them to the members either side, and they establish the position and squareness of everything that follows because the kerb can only sit where they allow it to. Trimming a roof deck changes how loads are carried, which the record sends to a qualified structural professional.

On a warm deck every layer above the deck has to be turned up the kerb in the order it occurs in the field. The record notes that an aperture is where that order is most often broken, because the opening is frequently formed after the field layers are already complete, and that when the aperture is formed is a design question rather than a programming one.

The consequence is specific. The vapour control layer stopped at the deck, as often happens for that reason, turns the aperture into a route by which internal air reaches cold construction from behind, where nobody is looking for it. The warm deck record describes the same junction from its own side: the kerb has to receive the membrane, carry the insulation up its outside face and let the vapour layer be closed against it, all before the rooflight itself is considered.

Where a void below the deck is ventilated, the record states that an opening interrupts the path air was taking across it, and that the kerb becomes an obstruction inside the ventilated zone as well as above the deck, which is a separate problem from the weathering of the aperture.

The cold deck record describes what that does to the roof as a whole. Structure and stored material zone the void without anyone intending it, and upstands for rooflights are named among the things that divide it into bays. A perimeter arrangement that ventilates the roof as a single space is not ventilating a roof that has quietly become a set of separate compartments.

In a pitched roof the aperture is cut through insulation held between and under the rafters, so the record says the trimming, the insulation return and the underlay diversion all have to be resolved within the depth of the rafter rather than above a flat deck. That is a much tighter space than a kerb on a flat roof, and the layers that have to be brought to a stop are the same ones.

The rafter-line record adds the thermal half. An opening in an insulated slope interrupts rafters, insulation and control layer together, and puts a reveal around the hole where each has to be brought back to a defined stop. It also notes that where insulation sits above the rafters, the kerb grows with it, which ties this case to the covering plane rather than only to the opening.

The composite panel record is blunt about this one. Cutting a panel interrupts the member that was spanning between purlins, so the structural consequence is as real as the weathering one, and the record names treating a cut panel as a weathering problem alone among its common misunderstandings.

It also describes what the opening does to the panel's other duties, which arrive bonded together. An opening removes all of the panel's bonded duties at once, so the kerb has to weather the roof, continue the thermal layer and close the air path in components that were not made together. On a built-up twin-skin roof the same opening stops the liner, the insulation and the outer sheet, each needing its own closure, and the record notes that the liner's air control duty is the one most often forgotten.

The kerb record devotes as much space to condensation as to rain, and it is the part most often misread. A glazed unit is usually the coldest surface in the room below, so air reaching it deposits moisture on the glass and on the frame, and that moisture runs downward. Manufactured units generally deal with what forms on their own surfaces, but what forms on a cold uninsulated kerb behind the lining is not their concern and has nowhere to go except the reveal.

That is why the record names two components answering the same problem from opposite ends: the insulation returned around the aperture, which limits the amount of cold construction ringing it, and the condensation channel at the foot of the unit, which collects what runs off and discharges it clear of the lining. It also names the misreading directly, that a rooflight leak is usually assumed to be water from outside although condensation forming on cold construction around the aperture behaves the same way.

A kerb standing in a roof plane obstructs the flow across it, so the record states that anything upslope has to be led around rather than allowed to pond against the turn-up, and that kerb position and roof falls cannot be decided independently. An aperture placed where water naturally converges hands the weathering a job the same detail elsewhere on that roof would never face.

On a metal roof the position question is different again. The standing seam record notes that an opening interrupts the tray, so the roof above it needs a diverting form and the tray lengths either side change, and that the kerb must sit clear of the drainage surface and must not pin the metal against the deck around its perimeter. On a twin-skin or panel roof, rooflight positions are coordinated with the purlin layout and the sheeting module.

The record closes on maintenance, and treats it as a design outcome rather than an operational one. How high and where the kerb stands governs whether the glass can be reached for cleaning, whether an opening light has room to swing, and whether the upstand can be worked on without disturbing the roof covering, and none of those can be improved later without disturbing the covering around it.

It also names the responsibility line that appears on nobody's drawing by default. The kerb belongs to the roof but its top face belongs to a manufactured unit, so recording where one scope ends and the next begins is what lets either party see the gap before it is built.