Every autumn the calls change. The first cold, wet fortnight arrives and someone notices a tide mark above the skirting in the front room, a bloom of black mould in the corner of a bedroom ceiling, paint lifting low on a chimney breast. By the time they ring us, they've often had a damp-proofing company round already, and they've often been told the same thing: rising damp, a chemical injection along the base of the walls, a metre of plaster hacked off and replaced with a waterproof render.
Sometimes that's the right answer. In the houses we work on, it usually isn't. This is how we work out where damp is actually coming from in a Victorian or Edwardian terrace, and why the cause is more often cheap, plain and on the outside of the house.
How these walls are meant to work
A Victorian terrace is solid brick: nine inches of London stock, no cavity. These walls were never meant to be dry the way a modern wall is dry. They take on moisture when it rains and give it back when it stops, through the brick face outside and the lime plaster inside. They have managed their own moisture that way for a hundred and thirty years.
A wall like that stays healthy as long as two things hold. The amount of water reaching it stays modest, and nothing stops it drying out. Almost every damp problem we find is one of those two things going wrong. We've written more about how these walls behave once they're opened up, including what that means for insulation.
Water getting in
The commonest cause is the simplest. A gutter full of leaves. A cast-iron downpipe cracked along its back, where nobody standing in the garden can see it. A hopper that overflows in heavy rain and runs straight down the face of the brick. One failed joint in a downpipe can soak a strip of wall from eaves to ground, and the stain shows up inside in exactly the place a damp survey will point to as rising damp. Autumn is when it shows first, because autumn is when the gutters fill.
The second is ground level. Most late-Victorian terraces were built with a damp-proof course, often a course of slate or dense brick a little above the ground. Over a century the ground outside tends to climb to meet it: a front path relaid on top of the old one, a back patio poured against the wall, a flower bed banked up against the brick. Once the outside ground sits above that course, water simply steps round it. Lowering the ground, or opening a gap between the paving and the wall, is a less impressive fix than an injection. It is also the one that works.
The third is pointing and render. In a solid wall, the mortar joints are meant to be the softest part: lime that lets moisture out and takes the weathering so the brick doesn't have to. Repoint in hard cement and that reverses. Water gets behind the joint and can't get out, so it leaves through the brick instead, and London stocks, a thin fired skin over a softer body, start to lose their face. On Acacia Road we repointed the whole front in lime mortar mixed for that wall, not the modern cement that traps moisture in the brick face. On Cemetery Road we repointed where the joints had failed and left them alone where they hadn't. A cement render over the plinth at the base of a wall does the same damage lower down, and it's often hiding the very problem it was put on to cure.
Water that can't get out
Under a suspended timber ground floor there's a void, ventilated through air bricks in the front and back walls. Block those air bricks, with a raised path, a rendered plinth, insulation boards fixed over the top or simply decades of soil, and the void stops breathing. Damp sits under the floor and finds the timber. In these houses joist rot concentrates at the wall ends, where the joists are built into the brickwork, and that's where we look first.
Inside, the same logic applies to finishes. Original lime plaster lets a wall dry into the room. Replace it with gypsum, coat it in a plastic emulsion, line it with impermeable boarding, and the moisture doesn't disappear. It moves to wherever it still can, usually the next stretch of original wall along, or the timber built into it. That doesn't mean every modern finish is wrong. It means what goes on a solid wall is a decision, not a default.
Condensation, which isn't the wall's fault
Black mould in the top corner of a bedroom. Streaming windows first thing. A patch behind the wardrobe on the coldest outside wall. That is almost always condensation, not water coming through the brick. Solid walls run colder than cavity walls, and warm, wet air from cooking, showers and washing dried indoors turns back to water the moment it meets them.
Why is damp worse in a Victorian house in winter?
Two reasons that arrive together. Autumn and winter bring the heaviest rain, which finds any blocked gutter or cracked downpipe. And cold weather means windows shut and washing dried indoors, so condensation forms on the coldest solid walls. Proper extraction in kitchens and bathrooms, ducted to outside, does more for winter mould than anything applied to the wall. It's something we design into every bathroom in a period house for exactly this reason.
Why the diagnosis matters more than the treatment
The hand-held meters used in a lot of damp surveys are designed for timber. On brick or plaster they give a relative reading at best, and the salts left behind by old damp can make a dry wall read wet. A reading is a place to start looking, not a diagnosis. It's also worth knowing whether the person doing the survey sells the treatment they're recommending.
Is it rising damp?
Sometimes, but far less often than you'll be told. Genuine rising damp, moisture drawn up from the ground through the masonry, does happen where a damp-proof course has failed or been bridged. But most of the walls we're shown with that diagnosis are getting wet from leaking rainwater goods, raised ground, hard pointing or condensation. Fix those, and the rising damp usually stops rising.
Do I need a chemical damp-proof course?
Rarely as a first step. An injected damp-proof course treats one possible cause. If the water is coming from a downpipe, bridged ground or cement pointing, the injection won't stop it, and the waterproof replastering that usually comes with it can hide the problem for a few years while the wall stays wet behind. We'd always find the cause before treating the symptom.
So we start outside, not in. Gutters, downpipes and hoppers, ideally in the rain. Ground levels against every wall. Air bricks. Pointing. Render. Then inside: where the damp sits, the shape it makes, whether it changes with the weather or with the heating. Most of the time the cause is plain by the end of the walk round.
Where we come in, and where we don't
Clearing a gutter or replacing a cracked downpipe isn't a job for us, and we'll tell you so. A good roofer will sort it in a morning, and we'll usually know one. Where damp is part of a bigger piece of work, repointing a front elevation in lime, taking cement render off a plinth, lowering a path, replacing rotten joist ends while the floor is up, it sits inside our external restoration and home renovation work. External restoration starts from £25,000, and most of our damp-related work happens as part of a wider renovation. The fuller picture of where that fits is in our guide to what a renovation actually costs in East London.
What can I check myself before calling anyone?
Quite a lot. Watch the gutters and downpipes in heavy rain and look for overflow or staining down the brick. Check the air bricks are clear. Keep soil, paving and planting below the damp-proof course, usually visible as a thin line of slate or darker brick a little above the ground. Ventilate kitchens and bathrooms properly. If the patch dries out over the following months, you've found the cause.
A damp wall in a Victorian terrace is usually telling you something specific. The job is to listen to it before treating it. Nine inches of London stock has looked after itself since the 1880s. Mostly, it just needs the conditions to do that again.