Can you fit a flat rooflight to a felt roof?
Yes, and felt is one of the more straightforward coverings to work with at a rooflight, because the membrane itself is dressed up the kerb and terminated under the frame. There is no flashing kit to match to a tile profile and no soakers to weave between courses. What decides the job is not the felt at the rooflight, it is the age and condition of the felt everywhere else, and the state of the deck underneath it.
This page covers how the detail is formed, how to judge whether an existing felt roof is worth cutting into, and the sequencing question that comes up on nearly every job. The general order of work is set out on the flat roof rooflight installation hub.
Three generations of felt, and how to tell them apart
“Felt” covers three quite different products, and the one you have changes the conversation.
Traditional built-up bitumen felt, laid in three layers with hot bitumen and finished with stone chippings, went onto flat roofs across Essex from the fifties through the eighties. It is organic or glass fibre based, it hardens with age, and after twenty five years it crazes and splits at the arrises. Pour and roll is the same family, laid by mopping hot bitumen ahead of the roll.
Torch-on felt, properly called polymer modified bitumen, arrived in the eighties and is the current standard. It has an SBS or APP modified bitumen coating on a polyester carrier, which keeps it flexible at low temperature and tolerant of movement. A two layer torch-on system with a mineral surfaced cap sheet has a service life in the region of twenty to thirty years.
Judging the deck before anything is cut
The felt is the surface. The deck is the thing that has to hold a kerb and a sheet of glass, and it is the part nobody can see.
Most domestic flat roofs of this era are timber: nineteen millimetre softwood boarding on older work, or eighteen millimetre WBP plywood and later OSB. Where water has been getting in for a while, plywood delaminates and OSB swells at the joints and loses its edge strength. A deck that feels soft underfoot, that gives at the joints, or that shows staining on the ceiling below is not going to take a fixing pattern reliably.
The survey lifts a small area of covering where there is any doubt. Finding a failed deck before the price is fixed is far better than finding it with the roof open.
How the membrane terminates at the kerb
The principle is simple and the execution is where the skill sits. The felt is carried up the vertical face of the timber kerb in a continuous run, not butted against it, and it terminates at or near the top of the kerb underneath the rooflight frame. Water reaching the base of the kerb is on top of the membrane, not behind it.
At the internal angle where the roof surface meets the kerb, a fillet is formed, traditionally a timber angle fillet or a bitumen based cove. Bitumen membranes do not like being folded through a sharp right angle. The fillet turns one ninety degree bend into two forty five degree bends and roughly halves the strain on the sheet at exactly the point where it would otherwise fatigue.
Torch-on detailing around an upstand
The corners are the test. Each corner of a rooflight kerb needs a gusset piece, cut and welded in so the membrane is continuous around the change of plane rather than pleated.
Laps run with the fall so water crosses them rather than into them, and a torched lap should show a small continuous bead of bitumen squeezed out along its edge. That bead is the visual confirmation that the compound reached melting temperature across the full width. No bead means the lap is mechanically pressed together rather than welded, and it will open. On a mineral surfaced cap sheet the granules are burned off the lap zone first, because bitumen does not bond through granules.
Old built-up felt, and the honest assessment
Cutting a new opening into thirty year old three layer felt is possible, and the new detail around the kerb will be sound. The problem is that the surrounding covering has a few winters left and the new work has twenty years in it.
The signs that a covering has reached the end of its life are consistent: surface crazing that looks like dried mud, blistering across the field, chippings washed into drifts leaving the cap sheet exposed, splits at the perimeter upstands, and laps that have curled at the edges. Where several of those are present, laying the new covering and setting the rooflight into it at the same time is the sensible order, and it costs less than doing the two jobs a few years apart.
Coverings compared at an upstand
| Covering | How it meets the kerb | Corner detail | Typical service life |
|---|---|---|---|
| Built-up bitumen felt | Dressed up with an angle fillet, bonded in bitumen | Cut and bonded gussets | 15 to 20 years |
| Torch-on polymer modified | Dressed up and heat welded, mineral cap burned back at laps | Welded gussets | 20 to 30 years |
| Single ply, PVC or TPO | Hot air welded to a coated metal trim on the kerb | Prefabricated welded corners | 25 to 35 years |
| Liquid applied or GRP | Continuous laminate over the kerb, no laps at all | Formed in place, monolithic | 20 to 25 years |
Chippings, reflective coatings and what happens near the glass
Stone chippings on an older felt roof do two useful things: they shield the bitumen from ultraviolet and they add thermal mass. They also migrate, and they collect against any vertical face they meet.
Around a rooflight kerb that matters, because a drift of chippings holds water against the upstand and hides the condition of the membrane behind it. Where chippings are retained, a clear margin around the kerb, formed with a proprietary edge or simply kept swept, keeps the detail visible and keeps drainage moving. Solar reflective paint is the cheaper alternative on smaller roofs and needs recoating every few years, so it wants a kerb it can be cut in against cleanly.
Kerb construction on a felt roof
The kerb is built off the deck and the trimmers, not off the felt. It is usually a treated softwood box, insulated on the outside face where the roof is a warm deck so the timber does not become a cold bridge, and fixed down into the trimming joists around the opening.
Height is measured from the finished surface of the covering to the top of the kerb, which means the chippings or the mineral cap count. The industry figure of one hundred and fifty millimetres is discussed in detail on how high a rooflight kerb should be. What is specific to felt is that the kerb has to be tall enough to take the membrane, the fillet and the lap without the top edge of the felt finishing proud of the frame seating.
Sequencing on an Essex flat roof
The commonest version of this job in the county is a felted rear addition or a single storey extension in Chelmsford, Braintree or one of the Harlow and Basildon new town estates, where flat roofs are everywhere and the original covering is on its second or third life.
Where the felt is sound and recent, the rooflight goes in and the covering is dressed up the new kerb and welded into the existing field with a full lap. Where the felt is tired, the covering is renewed and the kerb built in as part of it, which gives a single continuous membrane and a single point of responsibility. Sunspire installs new rooflights and replaces units that have reached the end of their working life, and the roof is closed and watertight at the end of every working day either way.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
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- Building Control notification handled
- New installations and replacements