Basement Waterproofing Chemicals: Soil Side Before Backfill, Inside After

Basement leakage and cellar seepage: check these six things first

Damp patches low on the cellar walls, white salt on the plaster, paint that blisters every monsoon, water creeping along the wall and floor joint after heavy rain, and puddles in the parking bays are the usual signs of basement leakage. A basement sits in the ground, so the soil around it holds water against the walls and the raft after every spell of rain. Basement waterproofing is the work of keeping that water out. One question shapes the whole job: can the soil side of the wall still be reached, or only the inside? Six checks decide the cellar leakage solution and the basement waterproofing chemical to use.

Find the right basement waterproofing method

Match your cellar to one route. Each route tells you what is done and takes you to its section. Several Armstrong products suit most routes, and an Armstrong expert helps choose the one that fits the site.

Stop the water, repair the path, then coat. A coating over a running leak or an open joint fails at that spot, whichever product is used.

Basement waterproofing system finder

Armstrong makes coatings for the soil side, crystalline coatings for the inside and injection grouts for running leaks. Each is a sound choice for its job. Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

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.

Basement water leakage solution: stop running water, then repair joints and cracks

In an existing cellar the first job is to stop water that is flowing, and the second is to close every path it used. Both are done from inside, on bare concrete. All loose plaster, old paint, putty and flaking coating is removed first, back to sound concrete, so the entry points can be seen and the repair bonds.

How to stop water seepage in a basement wall, step by step

1. 1
   Pump out and clean

   Remove standing water and silt. Strip all loose, old and flaking material from the walls and the wall and floor joint, back to sound concrete.Why: Leaks can only be traced on a dry, clean surface, and silt and loose material block every repair.
2. 2
   Mark every entry point

   Mark running leaks in one colour and damp joints, cracks, honeycombs, tie-rod holes and pipe entries in another. This map decides the quantities.Why: Marking running leaks and damp joints in different colours gives a plan of the work before anything is cut.
3. 3
   Plug the point leaks

   Cut the hole back to sound material so the plug has a chase to sit in. Mix RAPIDSTRONG PLUG 31 at 1 kg to about 0.25 litre of water for 15 seconds, press it in and hold it firmly until it sets. Mix only small quantities at a time.Why: A plug sits in a cut-back chase and sets in seconds against running water, and nothing else holds in moving water.
4. 4
   Inject the running cracks and joints

   Fix packers along the crack or joint and inject RAPIDSTRONG GROUT PU or RAPIDSTRONG GROUT PFC 79. Both are single component grouts placed with a low-pressure hand pump, or a high-pressure pump with packers where the site needs it. Where there is too little water in the crack for the reaction, water is injected as well. The full method is on the injection grouting page. Armstrong grouting is done with 12 to 14 mm holes (14 to 16 mm on thick retaining walls) fitted with pressure packers about 75 mm long, fixed with RAPIDSTRONG PLUG. The PU grout is injected with an air-compressed unit at about 500 g per grout point, and the holes are sealed with ROOFCRETE AR and cement after the packers are removed.Why: Polyurethane injected through packers reacts with the water and seals the whole crack from inside.
5. 5
   Treat the construction joints

   Open the joint to a groove along its full length, inject it with RAPIDSTRONG GROUT VM 99 or ROOFEX GP, then fill the groove with ROOFCRETE AR over a bond coat of ROOFBOND SBR 401 applied wet on wet.Why: A joint is a plane of weakness along its whole length, so it is opened, injected and sealed along that length.
6. 6
   Rebuild honeycombed concrete

   Cut back to sound concrete. Patch with ROOFCRETE AR over ROOFBOND SBR 401. The bond coat is 1 part ROOFBOND SBR 401 to 1 part cement and covers 40 to 65 sq.ft per litre per coat. ROOFCRETE AR is mixed at 2 litres of its liquid part and 4.5 to 5.5 litres of water per 25 kg bag of powder, and is placed 6 to 100 mm thick while the bond coat is still tacky. Where the defect is deep or large, fix formwork and pour or pump ROOFCRETE MC 240, a non-shrink micro concrete with aggregate up to 6 mm that reaches more than 50 MPa at 28 days.Why: A honeycomb is a path through the wall, and repair mortar over a bond coat restores a dense, sound section.
7. 7
   Close tie-rod holes and pipe entries

   Clean out each hole and pack it with a putty of ROOFBOND SBR 401, cement and graded sand at 1:1:1, or with a non-shrink grout made with RAPIDSTRONG GROUT ADMIX at 250 g per 50 kg of cement. Mixed grout is used within 30 minutes.Why: Each hole runs through the wall, so it is cleaned and packed solid with a bonded putty.
8. 8
   Seal movement joints with sealant

   An expansion joint is sealed with ROOFEX-SEAL PS polysulphide sealant, never filled rigidly with mortar. Gun grade is used in vertical joints and pour grade in horizontal ones. It takes joints 6 to 25 mm wide in one pass, at a width to depth ratio of 2:1, and cures fully in 7 days. Concrete joint faces are primed first with the ROOFEX-SEAL PS primer for porous surfaces. See expansion joint sealing.Why: A movement joint is sealed with a flexible sealant, because any rigid fill cracks as the joint moves.
9. 9
   Form a fillet at the wall and floor joint

   Round off the junction of the wall and the raft with repair mortar so the coating does not turn a sharp corner.Why: The wall and floor joint is rounded so the lining does not turn a sharp corner and crack.

Basement waterproofing from inside: negative side crystalline coating

Once the running water is stopped and the joints are repaired, the walls and the floor are treated from inside. This is negative side waterproofing: the water is still in the soil and still pressing on the back of the wall. A paint or a thin film on the inside face is pushed off by that pressure, which is why waterproof paint peels in a basement. A crystalline coating works differently. Its active chemicals travel into the damp concrete and grow crystals in the pores and hairline cracks, so the barrier sits inside the wall where water cannot lift it.

Which crystalline waterproofing for a basement wall

All of these Armstrong products suit a basement treated from inside. Pick the one that fits the wall and the water pressure, or ask an Armstrong expert to choose with you.

How to apply crystalline waterproofing on cellar walls

1. 1
   Expose the concrete

   Remove paint, putty, plaster and every loose or flaking layer. A crystalline coating must touch the concrete itself. It does nothing on top of old paint.Why: A crystalline coating has to touch bare concrete to grow crystals in it, and paint or plaster blocks it.
2. 2
   Finish the leaks and repairs

   No running water, every joint and honeycomb rebuilt, the fillet formed at the wall and floor joint.Why: Water running behind a coating washes it away, so there is no running water before coating.
3. 3
   Dampen the surface

   Wet the concrete well so the pores are full, with no water standing on the surface. Crystalline products need moisture in the concrete to work.Why: The crystals form in water, and a dry face pulls the water out of the slurry.
4. 4
   Apply the first coat

   Mix ROOFEX COP or ROOFEX with water to a slurry and brush or spray it on. ROOFEX COP covers 0.75 to 1 sq.m per kg per coat. ROOFEX goes on at 0.75 kg per sq.m per coat. Work it into the surface, the joints and the fillet.Why: The slurry carries the active chemicals into the concrete, and the stated coverage gives the stated protection.
5. 5
   Apply the second coat

   Brush the second coat at right angles to the first, at the same rate. With ROOFEX COP the second coat goes on within 48 hours, while the first is still green.Why: Crossing the first coat closes the gaps in it, and applied while it is still green the two coats bond.
6. 6
   Moist cure

   Mist the coating with a fine spray 3 to 4 times a day for 2 to 3 days so the crystals keep growing. Do not let it dry out early.Why: The crystals need moisture for days to grow, and a membrane curing compound would block the surface.
7. 7
   Protect the coating where needed

   Where the wall will be knocked or the level must be made good, apply a protective plaster of ROOFCRETE AR or a sand and cement plaster modified with ROOFBOND SBR 401 once curing is complete.Why: Where the wall will be knocked or levelled, a protective layer keeps the coating whole.
8. 8
   Watch it through the next heavy rain

   Check the marked points again after rain. Any point that shows damp is re-injected, which is why re-injectable resins are used at joints.Why: Rain shows any point that still leaks, and it is re-injected before the work is accepted.

New basement waterproofing: positive side coating before backfilling

A new basement gets its waterproofing in two layers. The concrete is made dense and watertight first. Then the soil side of the walls is coated while the excavation is still open. After backfilling, that face cannot be reached again without digging, so this is the one stage that cannot be put off.

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 at 500 ml per bag, or CONPROOF No. 1 at 100 ml per bag. CONPROOF LW 200 is added with the mixing water, never straight onto dry cement. For ready-mix concrete, CONPROOF RMC ADMIX 88 is added in the transit mixer at site. See waterproof concrete.Why: The compound makes the basement concrete dense and water-resisting from the start.
2. 2
   Treat every construction joint as it is made

   Clean the hardened face of laitance and loose stone before the next pour and apply a bond coat of ROOFBOND SBR 401 or ROOFBOND EP. The fresh concrete is placed while the bond coat is still tacky. ROOFEX is also applied at construction joints, at 1.5 kg per sq.m as a slurry or as dry powder.Why: Each joint is the weakest plane in the wall, so it is cleaned and treated as it is made.
3. 3
   Use ROOFEX on the raft

   On fresh raft concrete, ROOFEX is spread as a dry shake at 1.6 kg per sq.m before final trowelling.Why: The dry shake works into the fresh raft surface, so the raft is waterproof through its top layer.
4. 4
   Compact and cure

   Most leaks in new basements start in honeycombed concrete low on the walls. Vibrate well and cure for the full period. ROOFEXCURE WB is a spray-applied curing compound where water curing is difficult. It is sprayed 1 to 2 hours after the concrete is placed, or as soon as the formwork is struck, at 4 to 6 sq.m per kg. Curing compounds are not used on concrete treated with ROOFEX or ROOFEX COP, which are cured with water.Why: Most new basements leak from honeycombs low on the walls, and compaction and curing give a dense wall.
5. 5
   Repair the outside face

   Clean the wall of laitance, dirt, release agent, curing compound and loose particles. Pack honeycombs with polymer repair mortar. Seal tie-rod holes, shutter joints and pipe entries flush with a putty of ROOFBOND SBR 401, cement and graded sand at 1:1:1.Why: The coating bonds to a sound outer face, so holes and voids are packed before it goes on.
6. 6
   Apply one coat of ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210

   Both are flexible cementitious coatings, and Armstrong lists basements and retaining walls among the uses of each. ROOFCOAT FLEXI 100 is mixed at 1 litre to 2 kg of cement and 200 ml of water. ROOFBOND RFX PLUS 210 comes as a 5.1 kg liquid part and a 15 kg powder part that are mixed together. Apply one continuous, even coat of either over the whole wall.Why: The flexible cementitious coat is the first waterproof layer against the concrete.
7. 7
   Apply two coats of RAINSEAL BMW 250

   Dilute RAINSEAL BMW 250 with 20% clean water and apply two coats, 6 to 8 hours apart. It is an acrylic and bitumen blended coating with 500% elongation that resists dilute acids, dilute alkalis and salt solutions in the soil. Each coat dries in 40 to 50 minutes, depending on the weather.Why: The bituminous coats add a second membrane and resist the salts in the soil.
8. 8
   Protect the coating

   Cover the finished coating with a mortar screed or a protection board before any soil goes back. Where a screed is used, sprinkle fine dry sand on the last coat while it is wet so the mortar bonds.Why: Backfill scrapes a coating, and a mortar screed or board keeps it whole.
9. 9
   Backfill with care

   Backfill in layers and keep sharp rubble away from the wall. Drainage behind the wall is covered on the retaining wall waterproofing page.Why: Layered backfill with no sharp rubble keeps the coating from being punctured.

When cellar leakage keeps coming back: re-injection, drainage and the last resort

A cellar that leaks again after treatment has usually not failed everywhere. Water has found one path that was missed, or the pressure outside is higher than the repair was planned for. Work up the ladder below one rung at a time. Digging down to the outside of the wall is the extreme case and comes last.

Positive side vs negative side basement waterproofing: compare the Armstrong options

Every product here is a sound choice for the job it is made for. They differ in chemistry, in which face of the wall they go on and in the water pressure they are made for. Figures are from Armstrong product data.

How dry does a basement need to be? Parking, storage and living space

Waterproofing is chosen for the use of the space. The British code BS 8102 sets out grades of dryness for below-ground rooms, and the same idea applies to any cellar.

Basement waterproofing chemicals and products by stage

How much waterproofing material for a 1,000 sq.ft cellar

Approximate quantities for a cellar 40 ft long and 25 ft wide with walls 10 ft high. The floor is 1,000 sq.ft and the four walls come to 1,300 sq.ft, or about 121 sq.m. The figures below are for the walls only and are worked out from each product's stated coverage. Injection grout, plugging compound and repair mortar depend on the number and size of the leaks, so they are measured at site. An Armstrong expert confirms the final quantity after seeing the site.

Standards and tests behind basement waterproofing products

• ROOFCOAT FLEXI 100 is tested to ASTM D 412, ASTM C 836 and ASTM D 4541.
• ROOFBOND RFX PLUS 210 is tested for water penetration to BS EN 12390, crack bridging to ASTM C 836 and EN 1062-7, and bond strength to ASTM D 4541.
• ROOFEX 2K 711 is tested for hydrostatic resistance to ASTM D 5385 and adhesion to ASTM D 4541.
• ROOFEX COP complies with BS 1881 Part 116:1983 and ASTM C 1107.
• RAPIDSTRONG EC 211 has a compressive strength above 30 MPa, tested to BS 1881 Part 116.
• CONPROOF LW 200 and CONPROOF 500 conform to IS 2645:2003, the Indian standard for integral waterproofing compounds.
• One metre of water standing against a wall presses on it with about 0.1 bar. A 3 m deep cellar with saturated soil to ground level sees about 0.3 bar at the base of the wall.

Basement waterproofing material calculator

Enter the size of the cellar to see the coating needed. Quantities only. Injection grout, plugging compound and repair mortar depend on the leaks found and are measured at site.

Seven mistakes that make a basement leak again

1. Painting the inside with waterproof paint. Water pressure from behind pushes a film off the wall. Use a coating that works inside the concrete.
2. Coating over running water. Nothing bonds to a flowing leak. Plug or inject first.
3. Treating joints like plain wall. Joints are where the water enters. They are opened, injected and rebuilt, and are never left to a brush coat alone.
4. Coating over plaster, putty or old paint. A crystalline coating must touch concrete. Remove every loose and old layer first.
5. Backfilling before the soil side is coated and protected. That face cannot be reached again. Sharp rubble against an unprotected coating punctures it.
6. Letting the crystalline coat dry out. It needs moisture to grow crystals. Mist it for 2 to 3 days.
7. Relying on the pump. A pump manages water that has entered. It does not waterproof the cellar.

Basement waterproofing questions and answers