Waterproofing for New House Construction: What to Do at Each Stage

Waterproofing for new house construction: check where the building stands

Waterproofing for new house construction is not one job at the end. It is a short task at each stage: the foundation before it is backfilled, the concrete as it is mixed, the bathrooms before tiling, the terrace before the screed, the walls before paint. Each stage is covered by the next one, so each has to be done and checked at its own time. This page lists the waterproofing steps for a new building, the Armstrong product used at each stage, and the detailed page for each area. Six checks tell you where to start.

Waterproofing during construction is far simpler than repair later

Engineers describe the effect of waiting with the Law of Fives (De Sitter, 1984). One unit spent on doing the job properly during construction saves about five units of maintenance once problems begin, 25 units of repair once damage spreads, and 125 units of major rehabilitation.

Find the waterproofing steps for your stage of construction

Match the building to one route. Each route tells you what is done at that stage and takes you to its section.

Waterproof each stage before the next one covers it. A membrane that is tested and then covered works for the life of the finish above it. One that is skipped can only be reached by breaking that finish.

New construction waterproofing finder

Armstrong makes integral compounds, cementitious and polyurethane membranes, bituminous, crystalline and elastomeric coatings, and every one has its place in a new building. Answer three questions for a starting point, then confirm it with an Armstrong expert.

Foundation waterproofing and the damp-proof course at plinth level

Below ground, concrete and masonry sit against damp soil for the life of the building. Three things keep that water out of a new house: dense concrete in the footings, a coating on the faces that will be buried, and a damp-proof course that stops ground moisture climbing into the walls. All three are done before backfilling and before the walls go up.

1. 1
   Put an integral compound in the footing and plinth concrete

   Add CONPROOF LW 200 at 125 to 200 ml per 50 kg bag of cement, CONPROOF No. 1 at 100 ml per bag or CONPROOF 500 at 500 ml per bag. For ready-mix, CONPROOF RMC ADMIX 88 is poured into the transit mixer at site. CONPROOF LW 200 is listed for foundations and basements. Doses and plasticizers by grade are on the waterproof concrete page.Why: The compound makes the footing concrete dense and water-resisting from the start. It cannot be added later.
2. 2
   Clean the faces that will be buried

   Once the shuttering is off, remove loose material, laitance, dust and form-release oil. Fill honeycombs and tie-rod holes before any coating goes on. In Armstrong's retaining wall procedure, tie-rod holes, shutter joints and pipe penetrations are sealed flush with a putty of ROOFBOND SBR 401, cement and graded sand at 1:1:1.Why: Coatings bond only to clean, sound concrete. Honeycombs and tie-rod holes left open become leaks straight into the building.
3. 3
   Coat the footings and the earth face of the walls

   RAINSEAL BMW 250 is a water-based acrylic-bituminous coating with 500% elongation, used on foundation footings and recommended for external retaining walls. Apply two coats by brush or sprayer, diluted with up to 20% water, at 1 litre per 30 sq.ft for the two coats. Each coat dries in 40 to 50 minutes.Why: Below ground, concrete sits in damp soil for the life of the building. A flexible coat keeps the ground water and soil salts out.
4. 4
   Lay the damp-proof course at plinth level

   Lay a damp-proof course across the full thickness of every wall, at least 150 mm above the finished outside ground. A cement mortar course dosed with CONPROOF LW 200 or CONPROOF No. 1 is the usual form. The damp-proof course page covers new and existing walls.Why: The DPC stops ground moisture climbing up the walls. It is laid before the walls go up, when it is easy to do properly.
5. 5
   Keep the damp-proof course clear

   Do not carry plaster down over the damp-proof course line, and keep outside paving and soil below it. A bridged damp-proof course behaves like a missing one.Why: Plaster, paving and soil built over the DPC bridge it, and the damp finds its way round.
6. 6
   Protect the coating, then backfill

   Cover the coated faces before the soil goes back so stones do not cut the coating. Armstrong specifies a mortar screed or protection board over its retaining wall treatment.Why: Backfilled stones cut and puncture a coating. A protection layer keeps it whole for the life of the building.

Basement, retaining wall, sump and lift pit in a new building

These parts hold water or stand in it, so they are waterproofed on the face the water touches. That is positive-side waterproofing, and it comes first. Each has its own page.

Which waterproofing chemical to mix in concrete and plaster

The chemical mixed into concrete for waterproofing is an integral waterproofing compound. It is a liquid measured per bag of cement and added to the mixing water. It makes the concrete or plaster denser and lines its fine pores so less water passes through. Armstrong makes four, and all of them suit a new house. Pick by how the concrete is made and what the specification asks for.

1. 1
   Choose the compound before the first pour

   Use the same compound through the job so the dose is the same at every mix. Where a specification asks for an ISI-marked compound, CONPROOF No. 1 is the one Armstrong lists as BIS certified.Why: Using the same compound with the same dose through the job keeps the quality even from pour to pour.
2. 2
   Measure it by the bag

   Count the cement bags in each batch and measure the compound for that number. A dose poured by eye is the usual reason the result varies from slab to slab.Why: The compound works on the cement, so a dose poured by eye gives concrete that is dense in one batch and porous in the next.
3. 3
   Do not add extra water

   Use only enough water for a workable mix. Excess water bleeds out and leaves connected channels through the concrete. For higher grades and pumped concrete a plasticizer gives the workability. ROOFPLAST PC 455 is the PCE superplasticizer used for structural concrete, and the full list by grade is on the waterproof concrete page.Why: Excess water bleeds out and leaves connected channels through the concrete that no compound can close.
4. 4
   Compact every pour

   Honeycombs low on walls and around congested steel are where new buildings leak first.Why: New buildings leak first at honeycombs low on walls and round congested steel. Full compaction closes them.
5. 5
   Cure for at least seven days

   The first seven days decide how dense the concrete becomes. Keep it wet, or spray ROOFEXCURE WB once the surface water has gone, typically 1 to 2 hours after placing, at 4 to 6 sq.m per kg. On shuttered concrete it is sprayed as soon as the formwork is removed.Why: The first week decides how dense the concrete becomes. Concrete that dries out early stays porous.
6. 6
   Use a compound in the plaster too

   Outside walls, bathroom walls and parapets are plastered with CONPROOF 500 at 500 ml per bag or CONPROOF No. 1 at 100 ml per bag in the mix. Cure the plaster for at least 7 days.Why: Plaster takes the rain on outside walls and the water in bathrooms. A compound in it cuts the water paths through the plaster.

Bathroom waterproofing before tiling, the water tank and tile fixing in a new house

Wet areas are waterproofed on the bare slab, after the plumbing is fixed and tested and before any tile is laid. The order matters more than the product: sleeves first, then joints, then the cove, then the membrane, then a water test. This is the Armstrong sequence in brief. The bathroom waterproofing page gives every step in full.

1. 1
   Clean the slab and test the plumbing

   Remove all loose material, dust and mortar droppings. Have every supply and waste line pressure-tested, because no membrane fixes a leaking pipe.Why: A bathroom is tested before waterproofing so any plumbing leak is found while the slab is still open.
2. 2
   Grout the pipe sleeves

   Pack around every pipe through the slab with a non-shrink grout made with RAPIDSTRONG GROUT ADMIX, dosed at 250 g per 50 kg of cement. Ordinary cement slurry shrinks and leaves a gap.Why: Pipes leave a gap where they pass through the slab. Non-shrink grout fills it so water cannot follow the pipe.
3. 3
   Seal construction joints and cracks

   Cut a V-groove along each one, clean it and fill it with ROOFEXSEAL PU 212.Why: Joints and cracks move, so a flexible sealant in a groove takes the movement that a rigid fill would not.
4. 4
   Form the cove

   Apply a bond coat of ROOFBOND SBR 401 and form a rounded fillet at the wall and floor junction in mortar dosed with CONPROOF LW 200.Why: A rounded fillet lets the membrane run round the corner as one skin, instead of tearing at a sharp angle.
5. 5
   Apply the membrane in two coats

   Coat the floor and take it up the walls: 2,000 mm on the shower walls, 1,000 mm behind the basin and taps, and at least 300 mm on every other wall. ROOFCOAT FLEXI 100 and ROOFBOND RFX PLUS 210 are the flexible cementitious coatings. ROOFCOAT PU 2K is the polyurethane-cement membrane for under-tile wet areas, premium. All three suit a new bathroom.Why: The membrane is the waterproof layer. Taking it up the walls to the stated heights stops water running behind it at the edges.
6. 6
   Pond the floor with water for 7 days

   Flood the coated floor and hold the water. Do not lay the base plaster or tiles until it has passed.Why: A 7-day flood test finds any leak before tiles hide it. Finding it later means breaking out the tiles.
7. 7
   Lay the waterproof base plaster, then tile

   The protective plaster over the membrane is dosed with CONPROOF 500. Tiles are fixed with TILEX 351 PLUS or TILEX 350 and the wet zones are grouted with TILEX EPOXY STAIN FREE 100. See tile fixing and grouting.Why: The protective plaster protects the membrane from the tiling work, and the tiles bond to it, not to the membrane.

The water tank, balconies and other wet areas

Terrace waterproofing and external wall waterproofing for a new building

The terrace and the outside walls take the sun and the rain. On a new terrace the membrane goes on the roof slab, a sloped screed protects it, and the weather coats go on top. On new walls the waterproofing starts inside the plaster and finishes with an elastomeric coating.

1. 1
   Cast and cure the roof slab

   Integral compound in the concrete, full compaction, and curing for at least seven days with water or ROOFEXCURE WB.Why: A dense, well-cured slab is the first layer of waterproofing. Everything above it works better on a sound slab.
2. 2
   Treat cracks, joints and pipe sleeves

   Open any crack to a 4 to 5 mm V-groove and seal it with ROOFEXSEAL PU 212. Expansion joints take ROOFEX-SEAL PS. Grout around every pipe and outlet with a non-shrink grout made with RAPIDSTRONG GROUT ADMIX.Why: Cracks and joints open and close with heat, so they are sealed with flexible sealant before the membrane covers them.
3. 3
   Form the cove at the parapet

   Build a rounded fillet where the slab meets the parapet, in mortar gauged with ROOFBOND SBR 401 and CONPROOF LW 200.Why: The corner at the parapet is where roof membranes tear first. A fillet carries the coating round it.
4. 4
   Apply the membrane on the slab

   Two coats of ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210, 3 to 4 hours apart, over the cove and up the parapet. FLEXI 100 is mixed 1 litre to 2 kg of cement and 200 ml of water. RFX PLUS 210 comes as a 20.1 kg unit of 5.1 kg liquid and 15 kg powder. Both suit a new terrace, so pick the one that fits the site.Why: The membrane goes on the slab itself, under the screed, so it is protected and stays flexible.
5. 5
   Pond test, then lay the screed

   Block the outlets and flood the roof before anything covers the membrane. Then lay the screed to slope with CONPROOF LW 200 at 125 to 200 ml per 50 kg bag of cement, and cure it.Why: Water finds a leak while it can still be fixed easily, before the screed covers the membrane.
6. 6
   Finish with the top coats

   Two coats of an elastomeric coating such as RAINSEAL 200, RAINSEAL FRC 623 or RAINSEAL CFR 613, or a cool roof coating, SUMMERSHIELD COOL 666 or SUMMERPROOF COOL 644, with 87% heat reflectivity. RAINSEAL 200 takes its two coats 3 to 4 hours apart and is air cured for 3 days before foot traffic. Carry every coat over the skirting. The terrace waterproofing and cool roof coating pages compare them.Why: The elastomeric top coats take the sun and weather and protect the membrane below them.

External wall waterproofing on a new house

1. 1
   Plaster with a waterproofing compound

   Add CONPROOF 500 at 500 ml per 50 kg bag of cement, or CONPROOF No. 1 at 100 ml per bag, to the plaster mix. Cure for at least 7 days and let it dry.Why: The plaster carries the rain on external walls. A compound in the mix reduces the water that gets through it.
2. 2
   Seal cracks, joints and frames

   Fill cracks and joints with ROOFEXSEAL PU 212 and the gap around door and window frames with WONDERSEAL XL.Why: Water gets into walls at cracks and at the gaps round frames long before it passes through the plaster.
3. 3
   Prime

   New plaster is alkaline. Prime with RAINCARE WP Exterior Primer, diluted with 20% water, before an elastomeric coating.Why: New plaster is highly alkaline, which attacks coatings. The primer protects the coating and gives it grip.
4. 4
   Apply an elastomeric coating in two coats

   The coating is the layer that keeps the rain out. RAINSEAL 200, RAINSEAL FRC 623, RAINSEAL CFR 613, SUMMERSHIELD COOL 666 and SUMMERPROOF COOL 644 all suit an outside wall. Coat the sills, chajja tops and the parapet as well.Why: The elastomeric coat is the layer that keeps the rain out, and it stretches across hairline cracks.
5. 5
   Add the colour coat if wanted

   SHINESHIELD Exterior Emulsion goes over the coating for colour. Inside the house, new plaster is primed with RAINCARE CP Primer before paint.Why: The colour coat gives the finish, and protects the coating from the sun.

The external wall waterproofing page compares all the wall coatings, and wall dampness treatment covers damp that shows up later.

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.

Builder skipped waterproofing? What can still be done after construction

Waterproofing can be added after construction, but not all of it and not in the same place. What was meant to go under a finish now has to go on the face that can still be reached. In every case, loose, old and flaking material is removed first, and the smallest repair that reaches the problem is tried before anything is broken open.

Where concrete has spalled and steel is showing, the repair is structural: see structural concrete repair. An Armstrong applicator or distributor can inspect the building and tell you which stages need attention now and which can wait.

Waterproofing steps for a new building: the stage-by-stage schedule

This is the list to hand to your contractor. Each row is one stage of building a house: what is done, the Armstrong products used for it, and what to check before the next stage covers it. Where several products are listed, each suits the job. Pick the one that fits the site, and an Armstrong expert helps you choose.

Water tests to witness before each stage is covered

Waterproofing chemicals for new house construction: stated figures

How much waterproofing material for a 1,200 sq.ft house

Approximate quantities for a single-storey house with a 1,200 sq.ft terrace, two bathrooms of 5 ft by 8 ft and 2,000 sq.ft of outside wall. Each bathroom is taken as 66 sq.ft: the floor plus a 300 mm upturn on every wall. The higher coating on shower and basin walls adds to this. Figures are worked out from each product's stated coverage or dose. An Armstrong expert confirms the final quantity after seeing the site.

Standards and tests for waterproofing during construction

• IS 2645:2003 is the Indian Standard for integral waterproofing compounds for cement mortar and concrete. CONPROOF LW 200 and CONPROOF 500 comply with it, and CONPROOF No. 1 is listed as BIS certified to it.
• ROOFEXCURE WB complies with ASTM C-309 for curing compounds, Type 1D Class B, and is listed to IS 15865:2018.
• IS 3067:1988 is the code of practice for general design details and preparatory work for damp-proofing and waterproofing of buildings.
• IS 13182:1991 calls for impervious bathroom walls to at least 1 m, 2 m where a shower is provided, and at least 300 mm in water closets.
• IS 3370 is the code for concrete structures that store liquids, and it sets the water-tightness test for tanks.
• ROOFCOAT FLEXI 100: elongation of 120%+, tensile strength above 1.5 N/mm² and adhesion to concrete above 1.0 MPa.
• ROOFBOND RFX PLUS 210: elongation of 120%+, no water penetration at a 50 m water head and crack bridging up to 3.5 mm.
• ROOFCOAT PU 2K: nil water penetration at 5 bar.

New house waterproofing material calculator

Enter the areas you will coat and the number of cement bags to be dosed. Leave out any stage you do not need. Quantities only.

Eight mistakes that make a new house leak

1. Leaving waterproofing to the end. By then the foundation is buried, the bathrooms are tiled and the terrace is screeded.
2. Relying on the compound in the concrete alone. It makes concrete denser. It does not seal joints, pipe sleeves or cracks.
3. Pouring the compound by eye. The dose is per bag of cement and is measured, every batch.
4. Adding water at site for workability. Extra water leaves connected channels through the concrete.
5. Cutting curing short. The first seven days decide how dense the concrete and plaster become.
6. Tiling a bathroom before the ponding test. A coating can look finished and still leak.
7. Laying the terrace screed straight on the slab. With no membrane under it, the screed holds water against the slab.
8. Carrying plaster or paving over the damp-proof course. Ground moisture then climbs round it.

New construction waterproofing questions and answers