LAST UPDATED: 9th January 2023 by The Editorial Team
Summary: Step by step guide and video showing how to deal with rising damp, how to fit or replace a damp-proof membrane or replace a damp-proof course.
How to treat rising damp
Damp patches on floors and around the bottom of walls could indicate a problem with rising damp. Other indicators of this problem include mould and peeling wallpaper. If you suspect rising damp, you could engage the services of a professional surveyor to determine the precise cause. Alternatively, you can use a damp meter to check the moisture level of the walls. These instruments are widely used in the building industry and provide accurate readings.
Assessing the problem
- Inside the building take readings at regular intervals along the entire length of the wall and from the floor up to a height of about 1m.
- If the meter registers a high moisture content reading up to this level and then drops sharply above it, this is a strong indication that you have a problem with rising damp.
Concrete floors should be built with an integral damp-proof membrane (DPM), while walls should have a damp-proof course (DPC) between layers of brick near ground level. These are impervious layers built into walls and under floors that act as a barrier to moisture.
If the damp-proof course or damp-proof membrane have become damaged you will need to install new ones.
Fitting a damp-proof membrane
You can lay a new DPM over the concrete floor and construct a floating timber floor on top.
- First remove the skirting board from the room.
- Dry out the floor as much as possible with dehumidifiers and heaters.
- Insulation foam board should then be placed directly onto the concrete covering the whole floor.
- A DPM is then laid over the insulation board. Any joins should overlap by a minimum of 300mm and sealed with a waterproof adhesive tape. Around the edges of the room the membrane must lap up the wall higher than the level of the floor you are going to lay.
- Panels of chipboard can be laid directly onto the DPM. The chipboard panels have tongue and groove edges which when glued together create a strong join, so fixing with screws or nails is unnecessary.
- Having laid the chipboard floor re-fit the skirting board. This will secure the edges of the board around the room and cover the DPM that is lapping up the bottom of the wall.
Replacing a damp-proof course
Firstly, ensure there is nothing against the exterior of the wall such as a flowerbed, garden refuse or building materials, allowing moisture to soak into the wall above the damp-proof course.
If this is not the case, you will have to install a new damp-proof course by injecting a waterproof silicone-based compound into the wall to form an impervious layer. This can be done from just the outside of the wall or from both sides. Some products work very effectively being injected only into the external wall, but always check with the manufacturer’s instructions or your supplier that this is the case.
- Damp meter
- Damp proof membrane (DPM)
- Insulation foam board
- Waterproof adhesive tape
- Wood adhesive
Injecting the silicone-based compound
- Using a power drill fitted with a masonry bit 18mm – 25mm in diameter, drill holes in the external wall in the mortar course al least 150mm above ground level. If you are only drilling into the outside wall the holes should be 115mm apart and 190mm deep. However, if you’ve decided to drill holes in both sides of the wall a depth of 75mm will be sufficient.
- You can either drill squarely into the mortar course or into the brickwork directing the drill slightly down at an angle.
- Once all your holes are drilled inject the silicone-based compound. This is done using a sealant dispenser, which can be loaded with a cartridge containing silicone-based damp-proof compound.
- Insert the nozzle and fill the holes with the compound, leaving the last 10mm of the hole to the surface empty.
- Fill the holes with mortar.
- Power drill
- Masonry bit
- Sealant applicator
- Damp-proof compound
Sealing the interior wall
With the damp problem now fixed outside, you should seal your walls from the inside.
- With a bolster and club hammer, chip off the plaster to the height of the damage, usually no more than 1m from the floor.
- Clear the wall of any dust and then apply a coat of diluted Polyvinyl acetate (PVA) sealant solution – three parts water to one part PVA.
- Patch the wall using a waterproof render and when dry apply the finishing plaster to the wall, carefully tying it in with the existing plaster.
- Club hammer
Treating rising damp video
Rising damp is caused by ground moisture rising up a masonry wall. A symptom of rising damp is dampness appearing significantly above the ground water line. Damp is able to rise up the walls because most construction materials contain tiny pores. These pores ‘suck’ water from the ground, causing the moisture to rise.
- Polyvinyl acetate (PVA)
- Finishing plaster