Walls are designed to get wet and dry out again. What they are not designed for is one particular strip of wall getting a concentrated flow every time it rains, which is what happens when something at roof level is delivering water to a single point instead of into a gutter. The stain tells you a great deal if you read it properly, because the top of the mark is almost always directly below the fault. Start at the top of the stripe, not at the bottom where the damage is showing.
Start at the top of the mark
Walk out and look up. A stain narrows towards its source, so following it upwards usually brings you to a joint, a gutter union, a downpipe collar, the end of a verge, an overflow pipe or the corner of a roof. That point is the fault, and everything below it is consequence.
Green algae growth is a particularly useful indicator, because algae need persistent moisture rather than occasional rain. A green stripe means that spot has been wet regularly for a long time, which distinguishes it from a wall that simply got soaked in one storm.
- Stripe starting exactly at a gutter joint: failed union seal
- Stripe starting at a downpipe: cracked collar or a blocked pipe splitting
- Wide wet band along the top of the wall: water behind the gutter
- Stripe below the gable edge: verge failure
The gutter and downpipe versions
Gutter unions fail quietly. The seal hardens, a drip forms, and it lands on the same square of brick thousands of times a year. Because it is a drip rather than a torrent, people rarely notice until the wall is stained or the plaster inside starts to blow.
Downpipes fail differently. A split at the back of a pipe, which is the side you cannot see from the garden, throws water straight at the wall. So does a blocked pipe that has cracked at a collar under the pressure of ice. The classic test is to run a hose into the gutter above and walk around the pipe looking at it from the side.
At the bottom, a downpipe that discharges onto a paved surface rather than into a gully, or a gully that has silted up, soaks the base of the wall and produces damp inside at skirting level.
The verge and gable version
At the gable end of a roof, the covering is finished at the verge, traditionally bedded in mortar and increasingly with a dry verge system. Where mortar has cracked and fallen away, water runs off the end of the tiles and down the gable wall in a well defined stripe. On an exposed gable in Norfolk, that mortar takes a beating from wind driven rain and it is one of the first things to fail on an older roof.
The tell is the position: a stain that runs down the gable wall a few inches in from the edge of the roof, with no gutter anywhere near it. Repointing or fitting a dry verge system cures it, and it is a modest job compared with the damage it prevents.
The wall head version, which is the serious one
Water getting behind a gutter, rather than over it, lands on the top of the wall and on the ends of the rafters. Outside, that shows as a broad damp band along the top of the elevation rather than a narrow stripe. Inside, it shows as damp at the top of an upstairs wall or in the corner of a bedroom ceiling.
It is the version worth acting on quickly, because unlike a stain on the face of the brick this one is wetting structural timber. Rafter feet, wall plates and the ends of joists are all in that zone, and they are expensive to put right once they have softened.
The usual cause is an underlay that finishes short of the gutter or has perished at the eaves, sometimes combined with a missing eaves protection tray.
What the water is doing to the wall
Repeated saturation on one spot does three things. It carries damp through to the inside face, particularly on solid walls without a cavity, which many older Norfolk properties have. It accelerates frost damage, because saturated brick spalls when it freezes. And it feeds algae and moss, which hold yet more moisture against the surface.
On rendered walls it does something worse, because water getting behind render has nowhere to go and pushes the render off in sheets. A hollow sounding patch of render below a stain is a sign the fault has been running for years.
Fixing the cause rather than the symptom
Cleaning or painting the wall achieves nothing while the water keeps arriving. The order of work is always the same: stop the water at roof level, let the wall dry out over a season, then deal with the cosmetic damage and any repointing or render repairs that are needed.
Most of these faults are small pieces of work. A gutter union, a length of downpipe, a section of verge, an eaves detail. It is the delay that makes them expensive, because timber and masonry both cost far more to repair than the roofline component that soaked them.
We look at the whole roofline during an inspection rather than the single component that is obviously wet, and the inspection and written quotation are free with no obligation.
Common questions
What causes a green stripe down an outside wall?
Persistent moisture in one line, almost always fed from roof level. Algae need regular wetting rather than the occasional downpour, so a green stripe usually means a dripping gutter joint, a split downpipe or a failed verge has been running for a long time.
Is a stained wall a sign of rising damp?
Rarely, if the stain starts high up. Rising damp shows near the base of a wall with a fairly level tide line, whereas a stripe that begins at the eaves, a gutter joint or a verge is water arriving from above.
Should I have the wall treated or the roof fixed first?
The roof or roofline every time. Treating, sealing or painting a wall that is still being soaked traps the moisture and the problem reappears, often worse, within a year.
How do I find a split downpipe?
Run a hose into the gutter above and look at the pipe from the side rather than head on. Most splits are on the back face where they are invisible from the garden, and water will run out clearly once the pipe is flowing.
Can you repair verges without redoing the whole roof?
Yes. Verges can be repointed or converted to a mechanically fixed dry verge system as a standalone job, and it is a common repair on exposed gables in this part of the country.