Water tanks
What to check on an overhead or sump tank before it leaks.
Overhead tanks and underground sumps sit out of sight, so seepage tends to go unnoticed until a wall is stained or the water level keeps dropping. A few checks will tell you whether it's time to have the tank looked at properly.
Reviewed 21 September 2026 by PremiumWalls operations.
At a glance
How do I know if my water tank needs waterproofing?
Look for damp patches, stains or dripping on the outside of the tank or on the walls and ceiling near it, cracks or flaking in the lining, a level that falls more than normal use explains, and moss or dampness at the base. Any of these calls for inspection, and the material used inside must be suitable for storing drinking water.
Water Tank Waterproofing: What to Check
Signs of seepage on the outside
Walk around the tank, or the walls and slab that hold it. Damp patches, water stains, efflorescence, paint peeling near the base and a constantly wet floor beside a sump are all worth noting. On an overhead tank, look at the ceiling and walls of the floor below as well, since water often shows there first.
What to look at inside
When the tank is safely empty, look for cracks, flaking or peeling lining, rough patches, exposed reinforcement or stains, and check the joints at the floor and walls and around pipe entries. These junctions are where many tank leaks begin. Take care around a tank, and don't enter a sump or closed tank without proper safety arrangements.
A level that keeps dropping
If the water level falls more than your use would explain, the tank may be losing water through a crack or joint. Check taps, valves and float fittings first, since they're a common and cheaper cause. If those are sound, the tank structure is the next thing to look at.
Why materials for drinking water matter
Anything applied to the inside of a tank is in contact with the water your household drinks or cooks with. Before any system goes on, confirm that it's suitable for storing drinking water, and ask for the maker's information showing this. We'd advise against using anything on a potable tank that you can't verify.
Why tanks are emptied and dried before treatment
Coatings and repair materials need a clean, sound, dry surface to bond, and they can't be applied under water. So the tank is emptied, any growth and loose material are cleaned away, cracks and joints are repaired, and the surface is left to dry fully before the system goes on. That also means planning where your water will come from meanwhile, which is worth sorting out with your building or society.
Water Tank Waterproofing: What to Check — your questions.
Can a tank be treated without emptying it?
Materials that bond to the inside of a tank generally need a clean and dry surface, so emptying is normal. Whether a temporary or partial measure is possible for a particular leak is something an inspection can say.
Who decides which material is safe for drinking water?
The maker's documentation should state whether a product is suitable for potable water storage, and you're entitled to see it before work starts. If a contractor can't show it, ask again or look elsewhere.
Is the tank cleaned as part of the work?
Preparation includes removing growth, loose lining and deposits so the new surface bonds. Ask for what's included to be written into the quotation, so it's clear who's doing what.
How often should a tank be checked?
A look at the outside every so often, and inside whenever the tank is emptied for cleaning, is sensible. We offer a free colour and wall inspection with a written itemised quotation, which can cover walls near a tank.
Let us look at your walls first.
Tell us about the property and we will set up a free colour and wall inspection. You get a written, itemised quotation afterwards, with no obligation to go ahead.
- We call you back the same day to get a picture of the property and the surfaces.
- We agree a time and come out for the free inspection.
- You get the written scope and itemised quotation.
- Nothing starts until you approve it.