Why a metal roof changes the detail
Lead and zinc move with every change in temperature, so they have to be allowed to slide rather than being glued to the rooflight. That single fact shapes the upstand, the flashings and where the opening can go.
On an EPDM, GRP or single-ply roof, the waterproof layer is bonded or welded straight up the face of the kerb and finished under the rooflight frame. The guide to flat rooflights and roof coverings covers those membranes. A metal roof works differently. Lead sheet expands and contracts more than almost any other roofing material, and zinc is not far behind, so each bay is fixed along one edge and left free to move at the others. If the metal is nailed or stuck tight around a kerb, it will fatigue and split over a few seasons of hot summer afternoons and frosty nights.
A metal roof is also laid in pieces. Lead is divided into bays by wood-cored rolls running down the slope and drips stepping across it. Zinc is laid in long trays joined by standing seams. The rooflight has to sit within that pattern, not across it at a random point, which is why the position of a skylight on a metal roof is agreed with the roofer before the joists are trimmed.
How the upstand is weathered in lead
The kerb is built as normal, then the lead bay is turned up its sides, a separate cover flashing laps over that upturn, and the high side gets a small gutter to carry water round the unit.
The timber upstand is framed off the trimmed joists and stands at least 150mm above the finished lead surface, the same minimum as any flat roof; the answer on how high a flat rooflight upstand should be sets out how that is measured. On a lead roof the rolls and drips add height, so the kerb is often set a little taller to clear them comfortably.
The Lead Sheet Training Academy guidance describes the usual pattern. The bay around the opening is bossed or lead-welded into an upturn against the kerb, not fixed through its face. A cover flashing, usually its own piece of lead, is fixed to the top of the kerb and dressed down over the upturn with enough lap to shed wind-driven rain. On the upslope side a back gutter, formed with its own small fall and a drip, catches the water running down the bay and turns it round the rooflight so it never stands against the frame. The downslope side is simply the upturn and apron. Once the metal is dressed, the rooflight itself is fixed to the kerb through its own frame, and its perimeter trim or seal closes the top.
- Joists, with doubled trimmers either side of the opening to carry the loads of the cut joists around it.
- Deck (usually 18mm ply or OSB) with a vapour control layer laid on it.
- Insulation boards above the deck (a warm roof), often tapered to give the roof its falls.
- Waterproof membrane dressed up the upstand and over its top, so the joint is lapped, not sealed.
- Upstand, or kerb: insulated timber, finishing at least 150mm above the finished roof surface to keep standing water and splash off the joint.
- Rooflight unit fixed over the kerb, its frame skirt shedding water outside the membrane turn-up.
- Plastered reveal lining the opening from the ceiling up to the frame, finished white to bounce the light down.
- Roof falls to an outlet: designed at about 1:40 so the finished roof never falls less than 1:80.
Zinc and standing seam roofs
Zinc is formed in a workshop or on the roof into trays with folded upstands, and the rooflight is best placed so its kerb lines up with the seams rather than cutting through a tray.
Standing seam zinc can run at a surprisingly low pitch, with manufacturers such as VMZINC and RHEINZINK giving minimums of around 3 degrees for standard seams, but the glass on the rooflight still wants roughly 5 degrees of fall to clear water; the answer on the minimum pitch for a flat rooflight explains why. Around the kerb, the tray below is folded up the side, the tray above ends at a saddle or back gutter, and the side trays are welted or soldered to purpose-made upstand pieces. The zinc is held with sliding clips so it can still move.
| Point | Lead | Zinc |
|---|---|---|
| How bays join | Rolls and drips | Standing seams |
| Upturn at kerb | Bossed or welded | Folded or soldered |
| High side | Back gutter | Saddle or back gutter |
| Deck | Ventilated, on underlay | Ventilated or matting |
These are general patterns from the lead and zinc trade guidance. The roofer's own drawings and the manufacturer's details decide the exact joints on a particular roof.
Materials that must not touch
Rooflight frames are usually aluminium, and the metals around them have to be kept compatible, both where they meet and where rain runs off one onto the other.
Zinc should not sit under water that has run off copper, and it does not like contact with acidic timbers such as untreated western red cedar or oak, so kerb cladding and fixings are chosen with that in mind. Aluminium, stainless steel fixings and zinc sit together without trouble in normal conditions. Lead is more forgiving, but fresh lead sheds a white carbonate in its first months of rain. On a rooflight that sits downhill of a lead bay, that run-off can streak the glass, so the lead is treated with patination oil when it is laid. Both metals also need air beneath them: lead and zinc laid over a warm, unventilated deck can corrode from the underside, so the roof build-up is checked before a new opening goes in. The warm deck answer and the one on vapour control layers go into the layers below.
Where the light lands on a metal roof
The bay and seam pattern narrows where a rooflight can go, so the light plan works within it, choosing the bay that puts the daylight where the room is used.
In Cambridge, lead appears on the flat tops of bay windows and dormers, on the small back roofs of older houses in areas such as Newnham and the central conservation areas, and on some flat-roofed Victorian rear additions. Zinc is more often found on architect-designed rear extensions and new homes, where its grey finish sits well against gault brick. On both, the choice of bay may shift the opening by a few hundred millimetres. That small move changes where the sun patch falls on a south-facing room, or whether a north light reaches the worktop or the floor in front of it. We check the options against the room at the survey, then confirm the size against the guide to flat rooflight sizes. On a lead roof, a unit sized to fit one bay usually gives the neatest detail.
Questions people also ask
Can an old lead roof take a new rooflight without relaying it?
Often it can, if the lead is sound and the bays around the opening can be lifted and re-dressed. Where the lead is thin, cracked or brittle, the affected bays are renewed as part of the job. The survey looks at this, and the answer on fitting a flat rooflight to an existing roof covers the structure side.
Is a framed or frameless rooflight better on zinc?
Both work. A frameless unit gives a clean edge against the seams, while a framed unit has a deeper perimeter that covers the metal upturn neatly. The comparison of framed and frameless flat rooflights sets out the differences.
Can I replace an older rooflight on a lead roof with a larger one?
Yes, but a larger unit means a new kerb and new lead around it, and possibly trimming joists. We plan the new size, the metalwork and the light together, and it is all written into the quote. Read more on our flat roof rooflight installation page.
How do I find out what my roof needs?
Book a light survey. We look at the roof covering, the deck and the room below, and the written light plan that comes with the quote names the position, size, glass and the way the metal will be detailed. Book a light survey or message us on WhatsApp.
