Single-ply, felt and GRP roofs around a rooflight

The rooflight itself is a sealed unit. The glass, the frame and the gaskets are made in a factory to a tolerance you cannot match on a roof. What you build on site is the part underneath: the kerb, also called the upstand, a raised timber box that lifts the rooflight above the water sitting on the roof. The covering is then dressed up the outside of that kerb and terminated near the top, and the rooflight caps it off. Water that lands on the glass runs off onto the covering. Water that lands on the covering runs to the outlet and never reaches the glass at all.

So the question in the title is really a question about upstands. Single-ply, felt and GRP all do the same job, which is to carry a waterproof surface off the flat and up the side of the kerb, but they do it with different materials, different tools and different weak points. The height matters more than the material. A rooflight upstand wants to stand about 150mm above the finished covering, and skimping on that height is the single most common reason a flat-roof rooflight lets water in. Below 150mm, driven rain and standing water start to find the top of the kerb.

Single-ply means one layer of flexible sheet: EPDM synthetic rubber, or a thermoplastic such as TPO or PVC. The field of the roof is one large sheet, bonded or mechanically fastened to the deck, and that is a genuine advantage over anything laid in strips, because there are far fewer seams to begin with.

At the kerb the sheet is taken up the outer face and bonded to it. EPDM uses prefabricated internal and external corner pieces, moulded to shape, because rubber does not weld and a hand-formed corner in a stiff rubber sheet is asking a lot of an adhesive. TPO and PVC are hot-air welded instead, so the corners and the upstand can be fused into one continuous skin with a welding gun. A good weld is as strong as the sheet either side of it. That welded detail is why single-ply is often the covering of choice on a roof with several penetrations close together, where felt would need a lot of careful cutting and bossing.

Membranes move. They expand and contract with temperature, and a well-detailed single-ply upstand allows for that movement rather than fighting it. The termination near the top of the kerb is usually a bonded overlap or a mechanically fastened bar hidden under the rooflight frame, so nothing that holds water sits proud of the surface. EPDM in particular suits a large, simple extension roof where you can lay one sheet with almost no joints and only the rooflight kerb to dress.

Built-up felt is the traditional flat-roof covering, and modern felt is a long way from the pour-and-roll bitumen of forty years ago. Today it is two or three layers of polymer-modified bitumen, SBS or APP, torched or bonded together into a single mass. It is the covering many south Essex extensions already have, and it is still specified new because it is forgiving to lay and does not need a perfectly rigid deck under it.

The upstand is where felt asks the most of whoever lays it. A separate skirting piece of cap sheet is dressed up the kerb, bonded to the main covering at the base, and the corners are cut and bossed by hand so the bitumen folds cleanly without a gap. A drip or a check at the top throws water clear. Bitumen is stiffer than a single-ply sheet and it does not weld, so the quality of a felt upstand comes down to the skill of the hand that formed the corners and the heat that was in the material when they did it. Done well it lasts. Done in a hurry, cold, the corners are the first place it opens up.

Felt gives you the widest range of price and quality of the three. A basic two-layer job is the cheapest way to cover a flat roof. A three-layer SBS system with a mineral cap sheet is a considered specification with a working life to match. If the felt on your extension has reached the end of its life, re-covering the field of the roof is a job for a flat-roofing contractor, and it is worth having the new rooflight and the new covering planned together so the upstand is formed once, to the right height, for both.

GRP, glass reinforced polyester, is fibreglass. It is laid wet: a catalysed resin worked into a chopped-strand glass mat over a rigid deck, usually 18mm OSB3, which cures into a hard, jointless laminate. The whole roof becomes one continuous shell with no joints anywhere, and that is exactly why GRP is so strong at a rooflight upstand.

On a felt or single-ply roof there is always a junction where the flat part meets the kerb, and a junction is a detail that can be got wrong. On a GRP roof the laminate runs straight off the deck and up the kerb in one pass, with no seam at the base of the upstand at all. The corners are wrapped in the same continuous glass and resin. Purpose-made trims, a raised edge trim at the perimeter and drip trims at the eaves, are bedded into the laminate, and the top of the kerb is glassed over ready for the rooflight to sit on. For a small or medium extension roof carrying a lantern or a run of skylights, that jointless detailing around every corner is the reason a lot of people choose it.

GRP has two conditions attached. It needs a rigid, well-supported deck, because once cured the laminate is hard and a deck that flexes or deflects underfoot can crack it over time. And it has to be laid dry and reasonably warm. The resin will not cure properly below about 5 degrees or in damp conditions, so GRP is a fair-weather covering to install, which in practice means planning the work around the forecast rather than the diary. In this exposed corner of the estuary, that is a real scheduling consideration, not a footnote.

These are industry-typical figures for common flat-roof coverings around a rooflight. Exact numbers vary by product, by installer and by how the roof is built, so read them as the shape of the choice rather than a specification. What matters is how each covering behaves at the kerb.

Read across the seams column, because that is the column that decides how a covering copes with a rooflight. GRP has no joint at the base of the upstand. TPO and PVC have a welded joint as strong as the sheet. EPDM has a bonded joint helped along by moulded corners. Felt has hand-formed corners whose quality depends entirely on how they were laid. None of these is wrong. They are different answers to the same junction, and the right one depends on the deck you have, the shape of the roof and how many things poke through it.

Whatever the covering, the kerb underneath has to be right, and there are three numbers that decide it.

The first is height. Around 150mm of upstand above the finished covering is the target, and it is worth holding to even when the roof build-up is deep and the kerb starts to look tall. Water on a flat roof does not always drain the moment it lands. It sits, it gets driven by wind, and the extra height is what keeps it below the top of the kerb where the rooflight seals.

The second is fall. A flat roof is never truly flat. It is built with a slight slope, a minimum designed fall of around 1:40 so that the finished surface still drains at roughly 1:80 once you allow for deflection and workmanship, following BS 6229. The fall has to carry water away from the rooflight kerb, not towards it. A rooflight sitting in a low spot where water ponds against the upstand is a problem no covering fully solves, because you have asked the weakest part of the roof to sit in the wettest part of the roof.

The third is the cold bridge. A bare timber kerb is a line of poor insulation running right around a hole in your ceiling, and in an insulated extension that shows up as a cold stripe and, on a bad day, condensation on the inside of the kerb lining. A properly detailed upstand carries insulation up the kerb, often PIR board, so the thermal line is continuous from the roof insulation up to the rooflight frame. It is the same principle as the glass specification itself: the performance is only as good as the weakest part of the whole assembly.

Two trades touch this junction: whoever lays the covering and whoever sets the rooflight. The order they work in is what makes or breaks the detail, so it is worth understanding before anyone starts.

The kerb is formed first, to the right height and with insulation built in. The covering is then dressed up the kerb and terminated near the top. Only then does the rooflight go on, bedded down onto the finished upstand so that its frame overlaps the top of the covering and sheds water outwards onto it. Done in that sequence, every layer laps the one below it, the way roofing is meant to work. Done out of sequence, with the unit set down before the covering is dressed, you end up chasing the covering around a frame that is already in the way, and that is where flat-roof rooflights start to let water past. When we handle a flat-roof skylight installation, forming the kerb and weathering the covering into it is the part of the job that gets the most attention, because it is the part the weather tests first.

One practical point on the glass above the kerb. A flat rooflight over a flat roof will pond water on the glass and hold dirt, so most flat-roof units are made with a slight pitch built in, commonly around 5 or 10 degrees, or with a low-domed profile, so rain runs off and takes the dirt with it. If you want a properly flat sheet of glass to look at, that is a design decision to raise early, because it changes both the unit and how it drains.

South Essex has an enormous stock of 1930s semis, and the classic addition to one is a single-storey rear extension over a knocked-through kitchen, with a flat or shallow roof and a roof lantern or a run of flat skylights in it. Because Southend faces south across the estuary, a huge number of those extensions have their flat roof facing straight up at the strongest sun of the day.

A flat rooflight is the most exposed piece of glazing on the whole house. It faces the sky directly, so at midsummer noon it collects far more solar energy per square metre than any wall window, and it takes in diffuse light from the entire sky dome on top of that. The covering under it keeps the water out. The covering does nothing at all about the heat coming through the glass. That is the glass specification’s job, and it is the one most quotes leave off the page.

On a south-facing flat roof, the number to ask about is the G-value: the share of the sun’s energy the glass lets through. Clear double glazing sits around 0.6, letting most of that energy in. A solar-control unit brings it down towards 0.3 or below, cutting the afternoon heat roughly in half while keeping the room bright. Getting a first-rate felt, single-ply or GRP job done, and then putting clear glass over a south-facing kitchen, is half a specification. The covering and the glass are two separate decisions, and on a south aspect the glass is the one you will feel. This is where solar-control glazing earns its place, and it applies whichever of the three coverings sits around the kerb.

You can settle most of this in one conversation with whoever is quoting. Run down the list.

• How high is the upstand? You are looking for around 150mm of kerb above the finished covering. Anything shorter is a corner being cut where the roof can least afford it.
• Which way do the falls run? Water should drain away from the rooflight kerb, not pond against it. A rooflight in the low spot is the wrong place for it.
• Is the kerb insulated? A bare timber upstand is a cold bridge. The insulation should carry up the kerb to meet the rooflight frame.
• How does the chosen covering meet the corners? Welded on TPO and PVC, moulded corners on EPDM, hand-bossed on felt, jointless on GRP. Each is fine when it is done properly. Ask how, and ask who.
• Is the deck right for the covering? GRP wants a rigid, well-supported deck of 18mm OSB3 or better. A deck that flexes underfoot is the wrong base for a hard laminate.
• What is the G-value of the glass? On any south or west-facing flat roof, this is the number that decides whether the room is usable on a July afternoon. If the answer is only about the covering, half the specification is missing.
• Who notifies Building Control? A new or enlarged rooflight is notifiable. We make that notification to Southend-on-Sea City Council, or the relevant local authority for your address, as part of the job.

The covering keeps the weather out, and single-ply, felt and GRP all do that well when the upstand is formed to the right height and dressed with care. The glass decides whether you actually want to sit under it. If you are planning a flat-roof rooflight and want the kerb detail and the glass specified together for your own roof, ask us for a specification that names the G-value alongside the covering, for the aspect your roof actually faces.