Why rendered walls are harder than brick
On a brick wall the top edge of a flashing slots into a mortar joint, which is already a natural groove. Render covers that groove. It is a skin of cement or lime mix over the wall, and it presents a flat face with nowhere obvious to tuck the lead into.
So the roofer has to make the slot, and that is where the quality of the job begins. A flashing is the lead strip that seals the line where a roof slope meets a wall, and its top edge must be held firmly in the wall, never merely stuck onto it.
The render matters in two ways. It is a hard, brittle layer, so cutting into it needs the right tools and some care to avoid breaking away more than intended. And it is a water-shedding skin which may hide what is behind: crumbling brickwork, a failed older flashing or damp that has been working in for years.
That second point is the main reason a flashing on a rendered wall deserves a proper look. A neat render surface says nothing about the state of the junction beneath it.
How the lead is fixed in a chase
The usual method runs in these stages:
- A chase, a straight slot, is cut across the wall at the right height above the roof line, through the render and into the masonry behind it.
- The top edge of the lead is folded into a tight return and slid into the slot.
- Lead wedges, small folded strips, are tapped in at intervals to hold the edge so it cannot drop or lift.
- The chase is filled, either with a flexible sealant or with mortar pointed up, depending on the wall and the specification.
- The lower edge of the lead is left to lie over the roof covering, free to move.
The point of the wedges is that the lead is held mechanically, with the sealant playing a sealing role and not a structural one. A flashing held on sealant alone will drop away when the sealant ages.
The Royston Lead brochure gives Code 3 to soakers alone. The thickness of a wall flashing comes from the specification, and it should appear in your quote.
How render hides a failed flashing
When a flashing fails behind or beside render, the signs can be slow and misleading.
Water gets in at the top edge where the sealant has split, or where the render has cracked above the chase. It then runs down the back of the render and into the wall. Inside the house you may notice a damp patch on a ceiling or an internal wall. From outside there may be nothing, apart from a faint stain or a bulge in the render.
Look for the following when you check:
- Hairline cracks running from the top of the flashing up through the render.
- Sections of render that sound hollow when tapped, or that feel loose.
- A dark line or stain just below the flashing.
- Sealant that has pulled away or turned brittle along the chase.
If you spot any of these, do not repaint or fill over them. Cosmetic cover can seal the problem in. A roofer can open a small area to see what is behind, and a renderer may be needed for the finish once the lead is sorted.
Doing it again properly
A replacement on a rendered wall usually means three trades' worth of care, even if one person does the work: the old lead and sealant come out, the render edge is made good, and the new lead goes in with a fresh chase and wedges.
Points to ask about before the work starts:
- Will the chase be cut into the wall behind the render, and not just into the render?
- Are lead wedges being used, with sealant as the seal?
- Who will repair the render afterwards, and to what finish?
- If the wall is damp inside, has the cause been checked before new lead covers it?
A flashing is also only one half of the junction. If the roof covering beside it is lifted or the gutter below is overflowing, the new lead will be working in poor conditions, and it will fail for reasons that have nothing to do with the metal.
The render and the wall are looked at together before the total is set. For related work, see stepped lead flashing and lead flashing repairs, both within lead roofing.
