Why concrete tanks and sumps leak
- The base joint: the line where the wall meets the floor is where two concrete pours met, and it is the first place a tank lets go.
- Plaster instead of a coating: cement plaster inside a tank is not waterproofing. It holds for a few years, softens, and then water reaches the concrete.
- Pipe entries: inlet, outlet, overflow and washout all pass through the wall, usually packed with mortar that loosens around a vibrating pipe.
- Filled too early: a new tank put into use before it has cured never reaches its designed strength and stays porous.
- Ground water pushing in: a sump can leak inwards as well as outwards, especially where the water table rises in the monsoon.
How to tell your tank is leaking
On an overhead tank, look for damp patches or salt marks on the outside walls, a wet ceiling in the room below, or water marks around the pipe entries. For a sump, the simplest check is a level test: fill it, mark the water level, stop all use and drawing for a day, and see whether the level drops. If water comes in when the sump is empty, the problem is ground water pushing in, which is treated differently.
How a tank is waterproofed
The work is done from the inside, because that is the face holding the water. It follows this order:
- The tank is emptied, the sludge taken out, and loose old plaster removed down to a sound surface.
- The base joint and each pipe entry is cut into a groove, cleaned and packed with a fast-setting plug mortar that works even against seeping water.
- The wall-to-floor line and the vertical corners are packed into rounded coves so the coating has no sharp edge to crack along.
- Two coats of a coating certified for potable water are brushed on, one across the other, each dried before the next.
- The coating is cured, the tank is filled, the level is marked and checked again after 48 hours.
Is it safe for drinking water?
It should be, and you should ask. Inside a drinking water tank we only use coatings certified as non-toxic for potable water, and we tell you which product is going in before we start. Be wary of anyone who suggests painting the inside with an ordinary roof or wall coating.
When ground water is coming into a sump
If water is entering the sump from the soil, it is under pressure and will push any coating off. It has to be stopped first, usually by pressure grouting the joint or crack it is coming through. Only once the surface is still do we cove and coat the inside. The same principle applies to cellars and basements.
How long the tank is out of use, and what affects cost
Plan on three to four days for a typical household tank: a day to empty and clean, a day for the joints and coves, a day for the two coats, then curing. Arrange a temporary supply for that window. The quotation depends on the internal surface area, how much old plaster has to come off, how many joints and pipe entries need sealing, whether ground water has to be stopped first, and how easy the tank is to reach. Full details are on water tank and sump waterproofing.