Does leadwork need an underlay?

What an underlay is under lead

In leadwork the underlay is a sheet laid on the deck before the lead goes on. The deck is the surface the lead sits on, normally timber boards or plywood forming a gutter bed or a flat roof. The underlay is not a waterproof layer in its own right. Its job is to separate, cushion and protect, and the lead above it does the waterproofing.

The usual choices are a felt or building paper, or a synthetic geotextile, a soft, non-woven fabric. Whichever is used, the principle is the same. It must be clean, smooth, laid flat, and lapped so there are no gaps where lead could touch bare timber. It must also be chosen to suit the lead above, because some bitumen-based products can stain or attack lead and are poor partners for it. The roofer reads the manufacturer's advice and the lead supplier's guidance before ordering.

This is a different thing from the underlay on a pitched roof, which is laid under battens and slates to catch wind-blown rain. The lead underlay is a slip layer, and it lives only where sheet metal is laid flat or in a gutter.

Why lead is not laid straight onto the deck

Lead is a soft, heavy metal that moves a great deal. It expands in sun and contracts again in the cold, and a sheet several metres long moves measurably between a hot afternoon and a frosty night. If the metal grips the deck below, it cannot slide, and the stress goes into the lead itself. Repeated over years, the result is cracking at the points where the sheet is held, usually at fixings and at the bottom of a drip.

The underlay does four jobs.

• It lets the lead slide. A smooth layer lets the sheet move as one piece rather than tearing against the boards.
• It guards against the surface. Nail heads, ridges, splinters and the edges of boards would otherwise press into the metal and wear thin spots.
• It keeps the lead off the timber. Damp timber can give off acids, and some woods are known to be hard on lead. A layer between the two keeps contact to a minimum.
• It stops lime and mortar fouling the underside. On masonry the layer keeps wet lime and cement from touching the metal.

None of this is dramatic, and that is the point. The failures it prevents are slow ones, and they show up as a split in a gutter years later, long after anyone remembers what was laid below.

What goes wrong without one

When a roof comes up for repair and the lead has been laid bare, the same story repeats. The underside of the sheet is dull, corroded or pitted where it touched wet timber. A white powdery crust, lead carbonate, sits in patches. Pale scuff marks show where the metal has dragged over the boards, and cracks run out from the points where it was held tight.

The damage is not instant. Lead is forgiving, and a gutter laid without an underlay may keep out the weather for a number of years. But the metal under attack is on its underside, where nobody can see it. The first sign is often a leak, and by then the thin areas are widespread rather than local, which is why a patch seldom lasts.

It also affects repair choices. If the lead is cracked because it was stuck fast to the deck, patching the split does nothing about the cause, and the neighbouring sheet will crack next. Our page on blocked hoppers shows how a gutter that holds standing water ages, and an unprotected sheet suffers sooner for it. Where the cause is the missing layer, the honest fix is to lift the lead and relay it over a proper underlay.

Where it matters most

Every lead job benefits, but a few places depend on the underlay far more than others.

In a parapet or box gutter, the lead lies in a long narrow channel, often with standing water after rain. The boards beneath are among the timbers most likely to stay damp, so the separating layer carries the heaviest load there. A secret gutter, tucked under the roof covering, is the same, with the added problem that you cannot inspect it from outside.

On a lead flat roof, the sheets run in long bays between rolls and drips. These are the raised timber and step details that let the sheets move and shed water. The underlay lets each bay slide between them as the temperature changes, and the lead flat roofs page shows how the bays are set out.

Flashings, the lead strips turned over the junction between roof and wall, mostly sit on masonry and need less of this, but a lead soaker or apron laid on a timber deck still wants it. The rule is simple: wherever metal rests on a surface it can drag across, a layer goes between.

Getting it right on the job

The correct sequence is short, and every step is checked on the day. The deck is made sound first, with boards fixed flat, any raised nail heads punched down and the surface cleaned of dust and debris. The falls, the gentle slope that drains water to the outlet, are confirmed before any metal is cut. Then the underlay goes down in lapped sheets, trimmed square, and the lead is laid over it, fixed at its upper edge only so the rest can slide.

On a repair, we look for the underlay as part of the diagnosis. If a gutter has been relaid over a dried-out felt that has crumbled, or over nothing at all, we tell you, and show it in the photographs from the survey. A relay then costs more than a patch, but it addresses the cause, and the quote we give in writing sets out which approach we recommend and why.

Underlay is a small line on a quote and a long-lived decision on the roof. If you are comparing prices, ask what is going under the lead, and whether the lead code and the layer beneath it suit the job. Gutters, valleys and flat roofs in the metal are covered under lead roofing. If the lead is being relaid over winter, ask how the deck will be kept dry between stripping and laying, as trapped damp under a new underlay undoes the benefit of fitting it.