In the UK we see a massive 25,000 gallons of rainwater a year, so it is no surprise that one of the most common problems in our homes is penetrating damp. Damp is usually fairly easy to fix, but can sometimes be an indication of a more serious problem. This guide aims to help you find and solve the main causes of damp.
Penetrating damp is by far the most common type of damp problem in UK homes, particularly in older houses with solid walls. As the name suggests, penetrating damp is the result of water somehow permeating the fabric of the house from outside. It is almost always as a result of rain and will often almost disappear after a few dry days. Signs of penetrating damp include isolated stained patches on walls and ceilings as well as areas of mould. The cause of penetrating damp is often easy to fix and, if caught early, should not pose long-term problems.
Rising damp is caused by water seeping up from the ground and into the solid floors and walls of the house. Solid floors should have a DPM (Damp-Proof Membrane) to stop this happening, whilst walls should have DPC (Damp-Proof Course) built into the walls just above ground level. If the DPC or DPM has been bridged, broken or if your house does not have them (some old houses might not have either!) water will be able to soak up into your home. Rising damp, if not prevented, can cause major problems.
Dealing with Penetrating Damp
The main causes, symptoms and solutions of penetrating damp.
Broken or Blocked Downpipe
A crack or hole in the rear of a downpipe (facing the wall) will quickly soak the wall in one spot.
Symptoms – Outside: Mould and Mildew growing in one place behind the pipe. Inside: Isolated damp patches or stains, often halfway up a wall.
Solution – Seal the hole or crack. Replace the whole downpipe or broken section with new.
Broken or Blocked Gutter
A blocked gutter will cause water to soak the wall in one area, as will a crack or ill-fitting joint.
Symptoms – Outside: Mould or Mildew growing in an isolated place behind the gutter. Inside: Isolated damp patches or mould growth at the top of the wall.
Solution – Clear leaves and silt from the gutter and check that water runs smoothly along it. Repair any cracks or splits and seal leaking joints with waterproof sealant. Alternatively, replace the broken section of gutter.
Loose, missing or broken roof tiles
Broken or missing roof tiles will allow water to leak down into the roof space and then down into the room below.
Symptoms – Damp patches or stains on top floor ceilings.
Solution – Replace missing or broken tiles with matching tiles.
Bricks in good condition are basically waterproof, but once the face has been damaged, the brick can become porous and act a bit like a sponge, eventually effecting the surrounding bricks as well. Very old bricks can also be porous even when the face seems in good condition.
Symptoms – Large areas of damp on the inner face of exterior walls, increased mould growth on exterior walls.
Solution – Treat the wall with a clear water-repellant solution or paint it with waterproof exterior paint. Remove and replace a single broken brick as soon as you notice it to avoid the problem spreading. Read more on Replacing Bricks and Re-pointing.
Loose or Missing Pointing
Over time, mortar between bricks can dry and fall out. This can allow water to penetrate and spread through the surrounding mortar and into the house.
Symptoms – Small areas of damp on the inner face of exterior walls. Small patches of mould growth.
Solution – Repoint the joints of the damaged area and then treat the whole wall with water-repellant solution.
Cracked or Blown Render
Cracked or blown render allows water to seep behind and soak the brickwork underneath. The water will be trapped and absorbed into the wall. If left, the damage may spread as water effects the surrounding good render.
Symptoms – Isolated damp patches or areas of mould on the inner face of exterior walls.
Solution – Cracked render can be filled and repainted. Blown render should be removed and the surrounding render taken back to good. Patch with new render and then repaint with waterproof exterior paint.
Damaged or missing Coping Stone
If the coping stone on the top of a roof parapet wall is damaged or missing, water will be able to soak down through the bricks underneath and then into the fabric of the house.
Symptoms – Damp patches around the outside edge of the room, initially concentrated at the top of the walls.
Solution – Replace the broken or missing coping stone with a new one, making sure to bed it down properly. Take care to ensure the joints are properly filled.
Cracked Mortar Seals around Windows or Doors
Over time, timber window and door frames can shrink and warp, cracking or shifting the mortar seals between the timber and the wall. Water can then penetrate the gaps and effect both the wood and the wall.
Symptoms – Damp patches around window and door frames. Rotting frames, both inside and out.
Solution – Repair the wooden frames (if needed) and then seal the gap with an exterior grade mastic.
Bridged Drip Groove
The underside of window sills should have a shallow groove running along the full length. This drip groove stop water running back along the underside of the sill and soaking the wall. If the drip groove is bridged with layers of paint, moss or debris, rainwater will quickly soak the wall beneath the window.
Symptoms – Outside: mould or mildew growing directly beneath the window sill. Inside: Damp patches or mould directly below the window. Rotten timber sills.
Solution – Scrape out the drip groove and then repaint the sill if it is still sound. Alternatively, nail a thin wooden batten along the full length of the underside of the sill. This will deflect water running back along the sill.
Dealing With Rising Damp
The main causes, symptoms and solutions of rising damp. Unlike penetrating damp, solving the cause of rising damp is often difficult and can be expensive.
No DPC or DPM
Although it is rare, some houses are built without a Damp-Proof Course or Damp-Proof Membrane. In these cases, water is allowed to soak up from the ground and effect the sub floor and base of the walls.
Symptoms – Surface damp on concrete floors, as well as widespread damp at the foot of the walls.
Solution – There is not an easy fix to this. You will need to have a DPM and DPC fitted. Fitting them yourself is possible, but not the easiest of tasks.
Broken DPC or DPM
Building work carried out after the house is built can cause damage to the DPM or DPC. Water will then be able to penetrate the fabric of the house at that point.
Symptoms – Initially isolated damp patches on concrete floors or the base of walls. Damp could start spreading quickly if the problem is not dealt with.
Solution – Replace or repair the DPM or DPC.
Bridged Damp-Proof Course
Your DPC can be bridged in several ways. Debris can fall into the cavity of the wall, exterior render taken too low down a wall can bridge the DPC and building materials or garden refuse piled up against a wall will do the same. All of these scenarios could cause a bridge and allow water to penetrate the fabric of the house.
Symptoms – Large areas of damp at the level of the skirting (moisture will spread inside the cavity and effect a large area).
Solution – If the DPC is bridged by debris, you will need to remove bricks and clear the cavity. For render bridging the DPC, you will need to remove the render up to a level above the Damp-Proof Course. If garden refuse is the cause, simply move it away from the wall.
Other Causes of Damp
If you notice localised damp (in one area of one wall for example), often the cause is a leak. Check the guttering and downpipes in the affected area first, these are classic causes of localised damp. Make sure they are clear and running smoothly, as well as that all joints are tight. If not, seal or replace. If the damp is below a flat roof, or a roof channel, check that the felt is sound. Finally check flashing and tiles (around the chimney stack if the damp is on a chimney breast). If the damp is below a window, check the windowsill and window frame. Badly fitting frames can let damp penetrate the room, as will a bridged drip groove on a windowsill.
Dripping water, even when it is not raining, is a pretty good sign that you have a leaking pipe close to the drip. Locate the area of the drip and remove the floorboards above it to have a look. If the water is coming from the ceiling of a top floor room, the leak could be from the pipes or tanks in the loft.
Damp might not always be coming from outside your house. Condensation is another classic cause of damp problems, especially in rooms without adequate ventilation. Condensation is caused when warm air meets a cold surface (such as a metal window frame of badly insulated wall). If you cannot improve ventilation, you might need to improve insulation. Condensation on walls can be fairly easily prevented by wallpapering the wall in question with polystyrene-backed paper, fixed with fungicidal adhesive. You can also use a polystyrene lining paper. Make sure the wall is as clean as possible before starting this job. Is the condensation is on windows, a cheap option is secondary glazing. This is cheaper and easier that getting all new UPVC windows. There are plenty of plastic kits available.