Kumar IronUpdated
In this guide
Positive side and negative side
These two terms describe which face of the wall or slab the coating sits on, relative to the water pressure.
Positive side waterproofing is applied on the face that the water touches, usually the outside of a basement wall or the outside of a tank. It is the stronger option, because the water is pushed against the coating. But it needs excavation, so it is practical mainly during construction.
Negative side waterproofing is applied on the inside face, where the water comes through. It is the option when you cannot dig out the outside. The water pushes from behind the coating, so the coating must bond well and the cracks must be sealed first.
Water tank: treat it right
For an overhead or underground tank, the basic order is the same as any other surface: clean, repair, coat, cure, flush and refill. The one extra rule is that the product must be fit for drinking water.
- Empty and clean the tank. Remove silt, algae and loose plaster.
- Chip out weak areas, fill cracks and rounded joints with mortar, and let them set.
- Apply the coating in the number of coats and at the coverage rate on the data sheet.
- Cure for the time the data sheet gives. Some epoxy systems ask for about a week or two before the tank is used for drinking water.
- Flush the tank with clean water as the data sheet says, then drain and disinfect before refilling.
- Keep the lid closed and check the inlet and overflow are sealed.
Checking the data sheet for potable water
Not every waterproofing product is safe for drinking water, and a bucket label does not tell you. Before you buy, read the technical data sheet and check these points.
| What to check | What good looks like |
|---|---|
| Potable water statement | The data sheet says the product is suitable for potable water in those words |
| Certification | Named certification such as NSF/ANSI 61 or CFTRI, if the maker holds it. Ask for the certificate |
| Cure period | A stated cure time before water contact, followed exactly |
| Flushing | Stated flushing and disinfection steps |
| Smell | A strong solvent smell during or after cure is a warning sign |
| Substrate | Suits concrete, plastered masonry or metal, as used in your tank |
If the data sheet does not say potable water, assume the product is not suitable and do not use it in a drinking water tank. The national drinking water quality standard is IS 10500, and your local water authority can advise on tank cleaning and testing.
Crystalline waterproofing
Crystalline products are cement based powders or slurries. When they are applied to damp concrete, their chemicals react with the moisture and water in the concrete and form crystals that block the pores and fine cracks. This is why crystalline coats are often used inside basements and tanks.
They need a damp surface and damp curing to work. They are good at sealing hairline cracks and porous concrete. They are not a cure for a large crack or a flowing leak, which must be stopped with injection or repair first. Brand claims differ, so read the data sheet for the area and water pressure it covers.
Basement tanking basics
- Find the water source first: ground water, a crack, a joint, a drain or a blocked gutter outside.
- Fix drainage. Slope the ground away from the walls, clean the drains, and keep the gutters and downpipes working.
- If you are building new, plan a positive side membrane on the outside before backfilling.
- For an existing basement, use negative side options: crystalline or cementitious coats, PU or epoxy injection at cracks, and waterproof rendering.
- Where water keeps coming in, a sump and pump may be needed to keep the basement dry.
- Work in dry weather, and keep the floor and walls dry before coating.
When to call an engineer
Get an engineer or a waterproofing consultant if water is flowing through a wide crack, if the wall has shifted, if the basement floor is heaving, or if the water table is high. In those cases, the coating is only part of the answer, and the drainage or structure must be checked too.
Frequently asked questions
- Is crystalline waterproofing safe for drinking water tanks?
- Only if the data sheet states potable water suitability and gives the certification. Do not assume it. Check the product's own sheet for each tank.
- How long should a tank wait after coating before use?
- It depends on the product. Some epoxy systems need about a week or two, while cement based systems may need less. Follow the cure and flushing steps on the data sheet.
- Can I waterproof a basement from inside?
- Yes, when the outside cannot be reached. Negative side systems work best with the cause of the water fixed first, such as drainage and cracks.
- Which is better, positive side or negative side?
- Positive side is stronger, but needs excavation. Negative side is practical for existing basements. The right choice depends on the access, the water pressure and the stage of the building.