Water Tank Waterproofing Chemicals and Coatings for Overhead Tanks and Sumps

Water tank leakage: check your tank first

A water level that drops between fills, damp streaks below an overhead tank, wet soil around an underground sump and plaster flaking off the inside faces are the usual signs of a leaking water tank. A tank holds water against its walls every hour of the day, which is a harder duty than a roof, so a coating made for roofs or walls is not automatically right inside a tank. This page covers RCC and cement water tanks, overhead tanks, underground sumps and potable water reservoirs. Five checks decide the water tank waterproofing treatment and the right water tank leakage solution.

Find the right water tank waterproofing method

Match your tank to one route. Each Armstrong lining is a complete answer for its condition, and each route tells you what is done and takes you to its section.

Repair first, line last. A lining over an unsealed pipe sleeve or a running leak fails at that point. On every route the repairs come before the lining.

Water tank waterproofing coating finder

Armstrong makes cementitious, crystalline and food grade epoxy waterproofing coatings for water tanks and sumps. Every one is a sound choice for its condition. They differ in chemistry and in the pressure they are made for, so the pick depends on the tank, the water it stores and whether water presses from outside. Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

Water tank leakage repair: seal leaks, joints and pipe sleeves

Water tank leakage repair is done from inside, with the tank empty. Waterproofing a water tank from inside puts the barrier on the face the water touches. These repairs come before any lining, on every route.

1. 1
   Drain, clean and inspect

   Empty the tank and remove silt, scale, algae and all loose or hollow plaster. Mark every crack, joint, pipe sleeve and honeycombed patch.Why: Leaks and weak plaster show only in an empty, clean tank, and silt and scale block every repair.
2. 2
   Stop any running water

   Plug points where water is coming in with RAPIDSTRONG PLUG 31, which sets in about 30 seconds, or WONDERSEAL PLUG XL, which expands as it sets. Where water runs through a crack or joint, inject RAPIDSTRONG GROUT PU through packers. It reacts with water in 36 to 48 seconds.Why: Water entering the tank washes out repair materials, so it is stopped first with a quick-setting plug.
3. 3
   Grout the pipe sleeves

   Pack around every inlet, outlet, overflow and scour pipe with a non-shrink grout made with RAPIDSTRONG GROUT ADMIX at 250 g per 50 kg of cement. Ordinary cement slurry shrinks and leaves a gap around the pipe.Why: Inlet, outlet, overflow and scour pipes pass through the wall, and non-shrink grout closes the gap round each.
4. 4
   Repair honeycombs and tie-rod holes

   Cut back honeycombed concrete to sound material and rebuild it with ROOFCRETE AR over a bond coat of ROOFBOND SBR 401, applied wet on wet.Why: A honeycomb is a leak path through the wall, and it is rebuilt with bonded repair mortar.
5. 5
   Fill the cracks with ROOFCOAT FLEXI 100 putty

   Open each crack to a 4 to 5 mm V-groove and clean it out. Mix ROOFCOAT FLEXI 100 with cement to a stiff putty and press it firmly into the groove. Tank cracks are filled with this cement putty, not with a sealant, and the lining then goes over it.Why: A crack opened to a V-groove holds the flexible putty, where a flush fill has nothing to hold.
6. 6
   Treat the shuttering joints

   Construction joints between pours, and above all the wall to base joint, are opened to a V-groove, injected where they leak and filled with repair mortar. The full treatment is in the injection grouting section.Why: The joint between pours, and above all the wall-to-base joint, is where tanks leak first.
7. 7
   Seal expansion joints in large tanks

   Movement joints are sealed with ROOFEX-SEAL PS polysulphide sealant, not with mortar. It is certified safe for water-retaining structures, takes joints 6 to 25 mm wide in one application at a width to depth ratio of 2:1, and cures fully in 7 days at 20°C.Why: Movement joints in big tanks are sealed with a flexible sealant, never a rigid fill.

Injection grouting for a water tank in bad condition

When a tank or sump seeps through the walls themselves, with seepage and damp patches across the outer face, porous or honeycombed concrete, or many leaking cracks and joints, surface repair and a lining are not enough. The concrete is first made solid again by injection grouting: grout is pumped under pressure into the wall so it fills the voids, cracks and joints from inside. The lining then goes on a sound structure.

Which injection grout for which water tank leak

Injection grouting procedure for water tanks, step by step

1. 1
   Empty, clean and map the tank

   Remove all loose plaster and mark every crack, shuttering joint, pipe sleeve, honeycombed patch and damp area.Why: Marking every crack, joint and sleeve gives a plan so that none is missed.
2. 2
   Drill the holes

   Drill along each crack and joint, at an angle so the hole crosses the crack inside the wall, and on a grid over porous areas. The usual practice is 12 to 14 mm diameter holes.Why: The holes are drilled at an angle so they cross the crack inside the wall.
3. 3
   Fix the nozzles

   Set injection nozzles or packers in the holes with RAPIDSTRONG QS 113 and seal the crack surface between them so the grout cannot escape.Why: The nozzles are set with a fast-setting compound so they hold the injection pressure.
4. 4
   Inject from the bottom up

   Start at the lowest nozzle. Pump until grout shows at the next nozzle or the wall takes no more, close it and move to the next. Cement-slurry grouting is pumped at about 140 psi.Why: Grout pushes air and water up ahead of it, filling the crack without pockets.
5. 5
   Re-inject where needed

   Any nozzle that takes a large volume is injected a second time once the first pass has set.Why: A nozzle that took a lot may not be full, and a second pass fills what the first left.
6. 6
   Cut off the nozzles and make good

   Cut the nozzles flush and patch the holes and grooves with ROOFCRETE AR over a bond coat of ROOFBOND SBR 401.Why: The nozzles are cut flush and the holes patched, leaving a sound surface for the lining.
7. 7
   Treat joints, form the cove and line the tank

   Finish the shuttering joints as below, then apply the positive-side lining in two coats.Why: The lining then runs continuously over joints and round the cove.

Construction joint leakage in water tanks: treating shuttering joints

Shuttering joints, also called construction joints or cold joints, are the lines where one concrete pour stopped and the next began: the wall to base joint, the horizontal joint between two lifts of wall, and the wall to roof joint. The two pours do not bond fully, so these joints are among the first places a tank leaks. The tie-rod holes left by the shuttering leak in the same way.

1. 1
   Open the joint

   Cut a V-groove along the full length of the joint and clean it out. The usual practice is a groove about 6 mm wide and deep.Why: A V-groove along the full length gives the injected grout and the fill something to hold.
2. 2
   Inject the joint

   Fix nozzles along the groove and inject RAPIDSTRONG GROUT VM 99, which is made for construction joint sealing, or RAPIDSTRONG GROUT PU where water is running.Why: A grout made for construction joints fills the whole length of the joint, and not just the surface.
3. 3
   Fill the groove

   Pack the groove with ROOFCRETE AR over ROOFBOND SBR 401.Why: The groove is packed with repair mortar over a bond coat, so it is sound and level.
4. 4
   Reinforce over the joint

   Bed a band of fibre mesh into the first coat of lining along the joint. At the wall to base joint, form the cove first.Why: A band of mesh in the lining carries it across a joint that may still move.
5. 5
   Close the tie-rod holes

   Clean out each hole and pack it with non-shrink grout made with RAPIDSTRONG GROUT ADMIX.Why: Each hole runs through the wall, so it is packed with non-shrink grout.
6. 6
   Line over everything

   The lining then runs continuously over the treated joints.Why: The lining runs continuously over the treated joints, with no gap at them.

Water tank waterproofing coating: choose and apply the lining

The lining is the water tank waterproofing coating that goes over the repaired floor and walls. Choose it by the tank, then apply it in the same order every time.

1. 1
   Check the repairs are dry and sound

   No running water, no loose plaster, every sleeve grouted. Dampen the surface before a cementitious lining.Why: A lining over a damp or loose repair is pushed off, so the repairs are dry and sound first.
2. 2
   Form the cove at the base and wall joint

   Round off the junction of the floor and the walls with repair mortar so the lining does not turn a sharp corner.Why: The cove lets the lining turn the corner from the floor to the wall without cracking.
3. 3
   Apply the first coat

   For ROOFCOAT FLEXI 100, mix 1 litre with 2 kg of cement and 200 ml of water. ROOFBOND RFX PLUS 210 is a two-part pack: mix the 5.1 kg liquid with the 15 kg powder. For CONPROOF EP, mix Part 1, Part 2 and clean water 1:1:1. Coat the floor and walls by brush.Why: The first coat is the primer for the lining, and the mix and dose are those of the product used.
4. 4
   Bed fibre mesh at the joint

   Press fibre mesh into the wet first coat along the cove and over repaired cracks.Why: The mesh in the wet coat reinforces it along the cove and over repaired cracks.
5. 5
   Apply the second coat

   Allow 3 to 4 hours between coats of a cementitious lining, or 6 hours for CONPROOF EP, then coat the floor and walls again at right angles to the first coat.Why: The second coat closes the pinholes in the first, and the waiting time stops it smearing.
6. 6
   Cure fully

   Let the lining cure before any water goes in. Never refill early. CONPROOF EP reaches full chemical cure in 7 days.Why: A lining that meets water before it has cured is softened, so the tank is not refilled early.
7. 7
   Water-tightness test, flush and refill

   Fill the tank and record the water level daily. Once it holds, drain, flush and put the tank back into use.Why: The level is recorded daily, since a slow leak shows over days, and the tank is flushed before use.

Sump waterproofing: underground water tank leakage and outside water pressure

An underground sump is pushed from both sides. The stored water presses outward, and where the water table is high, groundwater presses inward. A surface film applied on the inside can be pushed off from behind, so sump waterproofing and underground water tank leakage repair use linings that work with pressure from either side.

1. 1
   Stop water coming in

   Plug holes with RAPIDSTRONG PLUG 31 and inject running cracks and joints with RAPIDSTRONG GROUT PU.Why: A sump takes the water pressure of the ground, so entry points are plugged and injected first.
2. 2
   Repair as for any tank

   Grout the pipe sleeves, rebuild honeycombs and fill the cracks.Why: Sleeves, honeycombs and cracks are the same leak paths as in any tank.
3. 3
   Apply the pressure-resistant lining

   Coat the floor and walls with the crystalline or flexible cementitious lining chosen above, with a cove at the base and wall joint.Why: A sump lining has to resist pressure from outside as well as from the water inside.
4. 4
   Cure, then test

   Fill and record the level daily, and check that the walls stay dry when the tank is empty.Why: The level is recorded, and the walls are checked dry with the tank empty.

Tank interior: cementitious coating with mesh

The inside of a water tank takes constant immersion and pressure from the water, so the coating is built in layers: a primer, a reinforcing mesh and a top coat, all cementitious so that it bonds to the wet concrete. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Prepare the surface

   Clean the surface of dirt, dust and old coatings. Where loose mortar or cement lumps are present, chip them off with an electric chipping machine, remove the debris and wire-brush the surface.Why: A coating inside a tank bonds only to sound concrete; every loose patch becomes a leak path.
2. 2
   Open and plug the shutter joints

   Chisel the shutter joints open to a groove about 10 mm wide and deep, clean it, and fill it with WONDERSEAL PLUG XL mixed with cement at 1:1.Why: Shutter joints are the cold joints where one pour met the next, and they are where tanks weep. The plugging compound sets fast and does not shrink.
3. 3
   Primer coat

   Apply one coat of ROOFCOAT FLEXI 100 mixed 1:2 with cement, with about 500 ml of water, as a primer.Why: The thinner mix soaks into the concrete and gives the later coats a firm key.
4. 4
   Mesh and second coat

   Fix 45 GSM polyester fibre mesh on the primed surface and apply a coat of ROOFCOAT FLEXI 100 over it, mixed 1:2 with cement and about 250 ml of water.Why: The mesh reinforces the coat across hairline cracks, so the film does not split when the tank fills and empties.
5. 5
   Top coat

   Apply a top coat of ROOFCOAT FLEXI 100 at the same mix.Why: The last coat completes the thickness and closes any pinholes left in the one beneath.On site: Grouting through drilled holes comes before the coating wherever the shell itself seeps.

Seeping concrete: grout through drilled holes before any lining goes on

When water leaks, seeps or sweats through porous, honeycombed or cracked concrete, it has to be stopped inside the concrete first. This is the standard negative-side method for tanks, pools, basements, lift pits and retaining walls. A lining laid over concrete that is still seeping is pushed off by the water behind it: it blisters, lifts and fails. Grouting through drilled holes fills the internal voids and capillaries along the path of the water, so the surface is dry and sound before the coating or membrane goes on. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Drill the injection points

   Drill 12 to 14 mm holes with an electric drill into the leaking or honeycombed zone, to the depth the member needs (14 to 16 mm on thick retaining walls).Why: The holes intercept the pathways the water is travelling along, so the grout reaches the voids instead of just sitting on the surface.
2. 2
   Fix the nozzles

   Fix a rubber injection nozzle in each hole with a quick-setting putty of cement mixed with the set accelerator RAPIDSTRONG QS 113. For active flow, the single-component plugging compound RAPIDSTRONG PLUG can be used to fix the nozzle instead.Why: The nozzle must be locked into the hole before pumping starts. If it is not, the pressure blows it out and the grout runs back out of the hole.On site: The datasheet gives RAPIDSTRONG QS 113 at 500 ml to 2 L per 50 kg of cement and asks for a site trial, so trial the dose on the job before fixing every nozzle.
3. 3
   Seeping or sweating concrete: cement slurry

   Mix a cement slurry with a non-shrink grout additive and pump it into each nozzle with a grout pump at about 140 psi, until that point refuses to take any more. Any of three additives can be used: RAPIDSTRONG GROUT ADMIX at 250 g per 50 kg of cement, RAPIDSTRONG GROUT PB 66 (a polymer grout additive, dosed as on its datasheet) or ROOFEX GP (a crystalline grout compound, dosed at 250 g per 50 kg of cement).Why: The slurry fills the internal voids and capillaries. The additive keeps it from shrinking away from the concrete as it sets, which an ordinary cement slurry does, and the crystalline option also seals the capillaries.On site: Work from the lowest nozzle upward on a wall, so the slurry pushes the water and air out ahead of it.
4. 4
   Running or gushing water: polyurethane resin

   Where water is flowing or gushing, cement slurry would simply wash away before it sets. First stop the surface flow with RAPIDSTRONG PLUG in a V-groove, then inject RAPIDSTRONG GROUT PU, the single-component polyurethane grout, with air-compressed injection equipment at about 500 g per grout point.Why: The polyurethane reacts with the water itself, expanding and stiffening to block the flow, so it works where cement cannot.On site: A hard-working point can take 0.3 to 0.75 kg. Polyurethane grouting alone only stops the flow; it is not a guaranteed waterproofing system, so follow it with the lining or coating the structure needs.
5. 5
   Remove the nozzles and seal

   Once grouting is complete, remove the nozzles and patch every hole flush with a putty of ROOFCRETE AR and cement.Why: This leaves a smooth, sound, watertight surface ready for the final lining.
6. 6
   Check before lining

   Look at the face again after the grout has set. Any point still wet gets another nozzle beside it before the coating, membrane or plaster goes on.Why: A single wet patch left behind becomes the first blister under the new lining.

Overhead tank and roof waterproofing: the standard sequence

This is the standard sequence for this area, in the order it is carried out. Each step prepares the surface for the next, so a step skipped or done out of order shows up later as a leak, a blister or a hollow tile. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Prepare the surface

   Clean the surface of dirt, dust and old coatings. Chip off loose mortar and cement lumps with an electric chipping machine, remove the debris and wire-brush the surface.Why: Every membrane in this sequence is only as good as its grip. It bonds to clean, sound material, not to dust or loose screed.
2. 2
   Groove and seal cracks and joints

   Open every crack and joint to a V-groove about 6 mm wide and deep, clean it out, and fill it with ROOFEXSEAL PU 212, the single-component polyurethane sealant.Why: A hairline crack moves with temperature. A rigid coating over it cracks along the same line, but a flexible sealant in the groove moves with it.
3. 3
   Stop the leaks

   Where water comes through, grout it through drilled holes as in the section above: 12 to 14 mm holes, rubber nozzles fixed with cement and RAPIDSTRONG QS 113, cement slurry made with RAPIDSTRONG GROUT ADMIX (250 g per 50 kg of cement), RAPIDSTRONG GROUT PB 66 or ROOFEX GP pumped at about 140 psi until refusal, then seal the holes with ROOFCRETE AR and cement.Why: The membrane goes on a dry, stable surface. Water pushing from behind lifts any coating laid over it.
4. 4
   Two coats of the cementitious membrane

   Apply two coats of ROOFBOND RFX PLUS 210 as a slurry, about 1.5 kg per sq.m in total, over the slab and up the vertical surfaces to 300 mm above finished floor level. Put the second coat on once the first has set enough to take it.Why: Two coats close the pinholes a single coat leaves, and the 300 mm upturn means water on the floor cannot run behind the membrane at the wall.
5. 5
   Pond to test

   Pond water on the floor for 7 days and check the ceiling below for any damp patch before anything is laid over the membrane. Some specifications call for a ponding test without fixing a period, so the test period is set by the job.Why: This is the only moment a leak can be fixed easily. Once the screed and tiles are on, finding and repairing the same leak means breaking all of it out.
6. 6
   Seal around every pipe

   Seal the periphery of each pipe with polymer concrete made with ROOFCRETE AR 576. Fix a socket around the pipe, grout the socket with the same polymer concrete, and prove it with a ponding test.Why: A pipe passing through a slab leaves a gap that ordinary mortar shrinks away from. This is one of the commonest places for a leak to come back.

Overhead water tank waterproofing: inside, outside and roof

An overhead tank leaks from the inside out, and it also takes sun and rain on the outside. Daily heating and cooling cracks the walls and the junction between the tank roof and the walls. Overhead water tank waterproofing therefore has three parts: positive-side lining inside, negative-side protection outside, and the tank roof.

Damp streaks or algae on the columns below the tank, and a damp ceiling in the room under a terrace tank, usually trace back to the base and wall joint or a pipe sleeve. Check those first.

Waterproofing a new water tank or sump during construction

A new tank is made watertight in the concrete first, with the lining as the second line of defence. Newly built tanks that leak usually do so at pipe sleeves, the base and wall joint or honeycombed concrete, and often because the concrete was not cured long enough.

1. 1
   Add an integral waterproofing compound to the concrete

   Use CONPROOF LW 200 at 200 ml per 50 kg bag of cement, CONPROOF 500 or CONPROOF No. 1. For ready-mix concrete, CONPROOF RMC ADMIX 88 is added in the transit mixer at site.Why: The compound makes the concrete dense from the start, and a lining after it has less to hold back.
2. 2
   Compact well and avoid honeycombs

   Most new-tank leaks start in poorly compacted concrete low on the walls.Why: Most new-tank leaks start in poorly compacted concrete low on the walls.
3. 3
   Cure the concrete

   Cure for the full period. ROOFEXCURE WB is a spray-applied curing compound where water curing is difficult.Why: Concrete that dries early stays porous, and a curing compound reaches faces that are hard to water.
4. 4
   Grout the pipe sleeves and tie-rod holes

   Use a non-shrink grout made with RAPIDSTRONG GROUT ADMIX.Why: Every sleeve and tie-rod hole is a path through the wall, and non-shrink grout closes it.
5. 5
   Line the tank before the first fill

   Form the cove, then apply the lining chosen for the tank in two coats.Why: The lining goes on before the first fill, while the whole tank is clear.
6. 6
   Water-tightness test

   Fill, record the level daily, and only then hand the tank over.Why: A tank is handed over only after it has held water, since a leak is easier to fix before use.

Water tank waterproofing chemicals and products by role

How much waterproofing material for a 10,000 litre sump

Approximate quantities for a sump 2.5 m long, 2 m wide and 2 m deep. The floor and four walls come to 23 sq.m, or about 250 sq.ft. Figures are worked out from each product's stated coverage. An Armstrong expert confirms the final quantity after seeing the site.

Standards and the water-tightness test for water tanks

• IS 3370 is the Indian code for concrete structures that store liquids. Its test: fill the tank, let it stand, then record the level every 24 hours for 7 days.
• An underground tank passes when the drop stays within the limit in the code. An overhead tank passes when its outer faces stay dry.
• ROOFCOAT FLEXI 100 is tested to ASTM D412, ASTM C836 and ASTM D4541.
• CONPROOF EP is tested to ASTM C-881 and EN 13813.
• CONPROOF LW 200 and CONPROOF 500 conform to IS 2645:2003.
• Drinking water: Armstrong tank linings are tested for contact with drinking water, and the report is available on request.

Water tank waterproofing material calculator

Enter the inside size of your tank or sump and choose a lining to see how much waterproofing material it needs. The calculator gives quantities only. Use the button at the end to talk to an Armstrong expert about your quantity.

Seven mistakes that make a water tank leak again

1. Using a roof or wall coating inside. A drinking-water tank needs a lining tested for water contact.
2. Coating over a running leak. Water pressure lifts the lining at that spot. Seal the leak first.
3. Ignoring the base and wall joint. It is the highest-stress point and needs a cove and mesh.
4. Leaving pipe sleeves ungrouted. Ordinary cement slurry shrinks and leaves a gap around the pipe.
5. Lining over loose plaster or algae. The lining bonds to whatever is under it. Remove everything loose first.
6. Using a soft coating that cleaning wears away. The lining has to resist a broom and a scrubber for years.
7. Refilling before the lining cures. A lining that has not cured cannot hold constant immersion.

Water tank waterproofing questions and answers