The short answer
The roof covering is turned up the wall as an upstand, and a lead flashing is pushed into a slot cut along a brick course, then dressed down over the top of the upstand. The covering keeps water out below, the lead sheds it above, and the groove is pointed so no water gets behind. Royston Lead lists code 4 among the weights for flashings.
Where an extension, porch or garage roof meets the house, two materials have to work as one. This junction decides whether the room underneath stays dry.
The upstand and the flashing, in order
Think of the junction as two layers. The lower layer is the roof covering itself, felt, EPDM or GRP, which is carried up the face of the house wall as an upstand. The upper layer is a lead strip, the flashing, which covers the top edge of that upstand and throws rain away from it.
How does the flashing stay in the wall?
A groove, called a chase or a raglet, is cut into the brickwork along a mortar joint, using a cutter and not a hammer, so the bricks are not cracked. The upper edge of the lead is bent into the groove and wedged with small lead wedges, then pointed with mortar or sealed. Lead on its own would slump, so the wedges and pointing keep it tight against the wall. The lower part hangs over the upstand like a cap.
Often the flashing is made in short lengths that overlap, so the metal can expand and contract in sun and frost without tearing. A strip stuck on the face of the wall with only mastic is not the same thing. That kind of fix often fails inside a couple of seasons.
The same junction repeats along the side of a roof where it steps against a sloping wall.
Which lead, and how long it lasts
Lead is sold by code, which is a number for its thickness and weight. According to the Royston Lead brochure, Code 4 is 1.80 mm thick and 20.41 kg per square metre, and is used for soakers and flashings, among other jobs. Code 5 is 2.24 mm and suits flashings and gutters. Code 3 is for soakers only.
Is Code 4 lead enough for a flat roof flashing?
For a wall flashing on a house extension, Code 4 is one of the listed weights. A heavier code is used where the lead is exposed, long, or acting as a gutter. The roofer should state the code in the quote, so ask.
MyJobQuote describes lead flashing as something that "can last for over 80 years" (Sep 2026), which is a floor, not a promise. In practice it is the laps and the pointing that give up first, not the metal. A sound lead flashing with failed pointing is cheap to put right, so a quick look at the pointing every few years is time well spent.
Lead is a different material from the roof covering. Its fall rules are separate, and BS 6229 covers only flexible-membrane flat roofs. You can read more about the covering itself on the flat roofing page.
Why this junction leaks first
A wall is a vertical surface that collects rain, and everything that runs down it ends up on the roof edge. Brickwork also moves differently from a timber extension, so the two slowly part company at the joint.
What goes wrong at a wall abutment?
The usual faults are visible. The chase has been left unpointed, or the mortar has fallen out. The lead has been fixed with a bead of mastic that has dried and cracked. The upstand is too short and rain splashes over its top edge. Or the covering has been folded into the corner, not cut and sealed, leaving a creased line where water collects.
Signs inside include damp on the internal wall just below the ceiling, a brown stain that grows after rain, and peeling paint on the house wall in the room under the roof. Outside, look for a lifted strip of lead, a gap of daylight between flashing and brickwork, or moss along the line of the joint.
This is why we check the abutment first. Repairs are often small: re-pointing the chase, replacing a short run of lead, or bonding a loose upstand.
Getting the junction right on a new roof
On a replacement, the wall junction is one of the first things decided, and the survey is where it gets looked at properly. It controls where the new covering finishes, how thick the insulation can be and whether the old flashing can be reused.
Should the old flashing be reused?
Old lead that is thick, flat and undamaged can sometimes be lifted and refitted, but lead cracked along a fold, or patched with sealant, is better renewed while the roof is open. Fitting new lead costs little against the scaffold and labour already there.
Extra insulation raises the finished surface, so the upstand has to be taller to compensate. That is why the junction is settled before materials are ordered, not on the day. Where the extension sits against a listed house or in a conservation area, the way the flashing looks from the garden can matter, so when in doubt, ask Somerset Council's planning team before any lead is ordered.
For the related details at edges and kerbs, read flat roof trims and upstands. A photo showing wall and roof together, taken from the ground, is a good starting point if you want a junction checked.
More on wall junctions
Can the gap at the wall be sealed with silicone?
Silicone or mastic can close a hairline gap for a short while, but it dries, shrinks and cracks as the wall and roof move. A lead flashing set into a cut chase and pointed is the lasting repair. Sealant is best kept as a minor finishing touch, not the fix.
Does a new flat roof against a house need approval?
The work against the wall is part of the roof, so it falls under the same checks. A replacement membrane is renovation under Approved Document L, and the Planning Portal's re-roofing test decides whether building control is involved. Somerset Building Control can be reached on 0300 303 7790.
The full picture: Flat roofing in Taunton ยท More short answers