Can you fit a flat rooflight to a felt roof?

Which felt you actually have up there

Three things get called felt and they behave differently. A modern built-up roof is two or three layers of polyester-reinforced bitumen membrane, torched or bonded, with a mineral-faced cap sheet. It is tough, it takes a new upstand well, and it welds to new material readily.

An older pour-and-roll roof is oxidised bitumen felt bedded in hot bitumen, often on a chipboard deck. It goes brittle with age, and it does not always accept a torched weld to new modern membrane without an intermediate layer. Then there is single-layer mineral felt nailed to a deck on a shed or garage, which is not a roof to build a rooflight into at all. Establishing which one you have is the first ten minutes of any survey.

How felt turns up the kerb

The principle is the same as everywhere else on the roof: the covering is the waterproofing, and the rooflight frame is a weathering layer sitting above it.

The membrane is carried across the deck, up the outside face of the timber kerb, and terminated near the top of it. On a built-up system that means a base layer taken up the upstand and welded to the base layer on the flat, then a cap sheet over the top with staggered laps so no two joints coincide. A reinforcing angle fillet, usually a triangular timber or insulation fillet, sits in the internal corner so the membrane turns through an eased angle instead of a right angle. Bitumen membrane bridged across a sharp internal corner will crack there within a few years.

Corners, where felt work is won or lost

Every rooflight has four external corners on its kerb, and each one is a place where two runs of membrane meet at a change of plane. Felt is a sheet material; it cannot be stretched round a corner.

The correct approach is a cut and welded gusset: the sheet is slit at the corner, dressed round both faces, and a separate piece of cap sheet is welded over the joint to give a continuous covering with no pinholes. Done well it is neat and completely flat. Done badly it is a wrinkled bunch of material with a void behind it that fills with water. When we look at an old felt rooflight that has failed, the failure is at a corner four times out of five.

Cutting a new opening into a sound felt roof

Where the existing membrane is in good order, a new unit can be let into it without recovering the whole roof. The sequence matters and it is not the obvious one.

The opening is set out and cut through the covering, insulation and deck, and the joists are trimmed with new timbers to carry the cut ends. The kerb is built and fixed down. Then the existing cap sheet around the opening is stripped back by a generous margin, typically 300mm to 500mm on all four sides, so new membrane can be lapped onto sound base layers rather than onto a weathered mineral surface. That stripped margin is why a small rooflight in an existing felt roof takes more felt work than people expect.

When the roof is too far gone to cut into

Sometimes the honest answer is that you should not be putting a rooflight into this roof at all until the covering is dealt with, and we will say so.

The signs are consistent: crazing across the surface of the cap sheet, mineral loss down to the bitumen, blisters that stay soft in warm weather, laps that have lifted at the edges, and a deck that gives underfoot. Cutting a hole into that and welding new material to it produces a good rooflight detail surrounded by a poor roof, and the first failure will be somewhere in the field rather than at the kerb. In those cases we would rather quote the unit alongside a new covering, or step aside and let a flat roofing contractor recover it first.

Hot work on an occupied house

Torch-applied felt means an open gas flame within inches of timber, and that has practical consequences for how the day runs. Around a rooflight kerb the risk is higher than in the open field of a roof, because there are corners, timber and insulation all close together.

The controls are ordinary and non-negotiable. No torching directly against bare timber at the upstand, self-adhesive or cold-applied material used for the last section instead, a fire watch kept for an hour after the torches go off, and the work stopped early enough in the day for that watch to happen in daylight. Some insurers of listed and older Oxford properties restrict hot work entirely, in which case a cold-applied or self-adhesive system is specified from the start.

Falls, and the water the felt will be holding

Felt roofs in Oxford are frequently older builds with modest falls, and a rooflight kerb dropped into a shallow-fall roof creates an obstruction that water has to get round.

Water crossing a felt roof spreads sideways when it meets a kerb and gathers at the two upslope corners, which is exactly where the hand-formed welds are. That is designed out with tapered insulation cut to divert flow round the unit, or a formed gutter behind the upslope face on a wider one. The upstand height is then measured from the water level, not from the deck. The falls, drainage and ponding page covers how those levels are set.

How the finished detail should look

Stand on the roof after we have left and there are five things to look at. The fillet in the internal corner should be continuous with no gaps. The membrane should sit flat on the upstand with no wrinkles or voids you can press in. The cap sheet laps should be neatly welded with a small even bead of bitumen showing at the edge, not scorched or starved. The termination near the top of the kerb should be mechanically held, not relying on adhesion alone. And the four corners should look like deliberate pieces of work.

Felt sits alongside single ply and GRP as one of three common coverings, and the comparison between them is on single-ply, felt and GRP roofs around a rooflight. We install and replace rooflights across Oxford and up to twenty five miles by road, the roof is never left open overnight, and the workmanship carries a ten-year guarantee.