Rising damp is ground water that soaks up into the bottom of your walls through the blocks and mortar, like water climbing a sponge. In Tanzanian houses it shows as peeling paint, white salty powder and a dark tide mark up to about a metre above the floor. The fix in a new house is simple and cheap: a continuous damp-proof course (DPC) in the wall just above ground level, a damp-proof membrane under the floor slab, and ground that slopes away from the house. In an existing house the fix costs more, but it can almost always be done.
What causes rising damp in Tanzania?
Concrete blocks, burnt bricks and mortar are all porous. If the bottom of a wall sits in wet ground with nothing to stop the water, it rises. The problem is worst in places with a high water table or heavy rain, such as parts of Dar es Salaam, Morogoro and the lake zone, and on plots where rainwater pools against the house.
Rising damp is often confused with other kinds of damp. Before spending money, work out which one you have:
| Type of damp | Where it shows | Usual cause |
|---|---|---|
| Rising damp | Bottom of walls, up to about 1 m, tide mark | No DPC, a broken DPC or soil piled above it |
| Penetrating damp | Anywhere on an outside wall, worse after rain | Cracked plaster, no roof overhang, bad gutters |
| Leaks | Patches near pipes, roofs, bathrooms | Plumbing or roof leaks |
| Condensation | Cold corners, behind cupboards, bathrooms | Poor ventilation, especially in closed rooms |
What is a damp-proof course and where does it go?
A DPC is a waterproof layer built into the wall, usually a strip of heavy polythene or bitumen felt laid along the full width of the wall at least 150 mm above the finished ground level. Below the floor, a damp-proof membrane (a sheet of polythene, normally 1000 gauge or thicker) goes under the concrete slab and laps up to meet the wall DPC. Together they form an unbroken barrier.
The details matter more than the material:
- Keep it continuous. Overlap joints by at least 100 mm and do not leave gaps at door openings.
- Keep it high enough. 150 mm above outside ground level is the minimum. Lower than that and rain splashes over it.
- Do not bury it. The most common way a good DPC fails is someone later builds a raised flower bed, a path or a pile of soil against the wall above it.
- Protect the membrane. Lay sand blinding under the floor membrane so stones do not puncture it, and do not let workers walk on it in boots before the concrete goes on.
How does drainage help stop damp?
A DPC handles water rising from below. You also need to keep water away from the base of the house in the first place. That means:
- Shaping the ground so it falls away from the walls for at least the first 1.5 to 2 metres.
- A concrete apron or paved strip around the house, about 600 mm to 1 m wide, with a small fall.
- Gutters and downpipes that discharge into a drain, not straight onto the soil next to the wall.
- A plinth plastered with a waterproof mix up to the DPC level.
Our guide to stormwater drainage for homes goes into the drains, falls and soakaways in more detail.
How do you fix rising damp in an existing house?
There is no single fix, and anyone who sells you a paint that "stops damp forever" is selling you a cover-up. The usual steps are:
- Lower the outside ground so it is at least 150 mm below the floor level, and fix the drainage.
- Remove damaged plaster up to about 300 mm above the tide mark and let the wall dry out for several weeks.
- Install a chemical DPC. A contractor drills a line of holes along the bottom of the wall and injects a silicone damp-proofing cream that forms a water-repellent band inside the blocks.
- Replaster with a salt-resistant mix, usually 1:3 cement and sand with a waterproofing additive, and leave it to dry well before painting.
- Use breathable paint on the repaired area so any remaining moisture can escape.
As a rough guide, fixing rising damp along the walls of one room costs from a few hundred thousand shillings to over a million, depending on how much plaster has to come off and whether drainage work is needed outside. A whole house can run to several million. These are examples only. A site visit is the only way to price it properly.
Does rising damp damage the structure?
Usually not quickly, but it does real harm over time. Salts carried up by the water break down plaster and paint, mould grows behind furniture, and steel in the lower part of reinforced walls and columns can start to rust if water stays there for years. It also makes rooms smell musty, which tenants notice at once. For the wider picture, see waterproofing roofs and walls.
Frequently asked questions
Is a DPC required in Tanzania?
Good building practice calls for one in every wall that sits on the ground, and your approved drawings should show it. Your engineer should check it is in place before the walls go up.
Can I just paint over damp walls?
You can, but the paint will peel again within months. Find and fix the source of the water first.
How long does a damp wall take to dry?
Roughly a month for every 25 mm of wall thickness in good weather, so a 150 mm block wall can take several months to dry fully.
Does tiling the bottom of the wall stop damp?
No. Tiles just push the water higher up the wall, where it shows above the tiles instead.
Get it right from the foundation
Vika Construction builds every house with a continuous DPC and proper site drainage. Contact us for a new build or a damp repair.
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