External Wall Waterproofing: Coatings and Systems to Stop Rain Seepage

Wall seepage in the rainy season: check the wall first

Damp patches inside after rain, peeling or blistering paint, cracks in the outside plaster and green or black algae on the walls are the usual signs that rain is getting through an external wall. External wall waterproofing keeps rain out of a sound wall. It does not cure damp that rises from the ground or a leak from a pipe or bathroom on the other side, so the first job is to find which wall you have. Five checks decide the treatment.

Find the right external wall waterproofing method

Match your wall to one route. Each route tells you what is done and takes you to its section.

Treat the outside, where the water comes in. Waterproof putty or paint on the inside face does not stop rain that is entering from outside.

Exterior wall waterproof coating finder

Armstrong makes acrylic elastomeric, fibre-reinforced, solar reflective, rubberised and polyurethane waterproof coatings for external walls, along with clear treatments for stone and brick. Every one is a sound choice. They differ in chemistry and finish, so the pick depends on the wall and what you want from it. Answer three questions for a starting recommendation, then confirm it with an Armstrong expert.

External wall crack repair: seal cracks, joints and window frames

Most rain enters an external wall through cracks, joints and the gaps around openings, not through the open plaster. These are sealed before any primer or coating. A rigid filler in a crack that moves cracks again, so the filler is matched to the crack.

1. 1
   Clean and mark

   Remove loose paint, dust, algae and fungus by wire brushing and washing. Let the wall dry, then mark every crack, joint and gap.Why: Sealant and coating bond to a clean, dry wall. Marking every crack first means none is coated over unseen.
2. 2
   Open the cracks

   Cut each crack wider than a hairline to a V-groove and brush out the dust. A sealant needs depth to grip and to stretch.Why: A sealant needs depth to grip and to stretch. A hairline filled flush has nothing to hold on to.
3. 3
   Seal cracks and joints

   Fill moving cracks and joints with ROOFEXSEAL PU 212. Fill fine static cracks with RAPIDSTRONG CRACK SEALER 85 or RAPIDSTRONG CRACK SEALER 75.Why: Moving cracks need a flexible sealant, while fine static cracks can take a filler. Using the wrong one reopens the crack.
4. 4
   Seal around frames

   Run WONDERSEAL XL into the gap between each window or door frame and the wall. It can be painted over once it has skinned, in 15 to 45 minutes.Why: The gap between frame and wall is where driving rain gets in first, and a flexible sealant takes the movement of the frame.
5. 5
   Check sills and chajjas

   A sill or chajja should slope away from the wall and have a drip groove underneath. Seal the junction where it meets the wall.Why: A sill that slopes toward the wall, or has no drip groove, sheds water onto the wall instead of away from it.
6. 6
   Let the sealant cure, then coat

   Once the sealants have cured, the wall is primed and coated.Why: A coat over uncured sealant traps solvent and bonds badly.

How to waterproof an external wall: prepare, prime and coat

This is the route for most walls. The elastomeric waterproof coating is the layer that keeps the rain out. It is thicker than paint and stretches with the wall, so it bridges hairline cracks that a paint film follows.

1. 1
   Strip and clean

   Remove loose and flaking paint, dust, oil, fungus and algae. Wire brush the wall, wash it down and let it dry fully.Why: Coatings bond to sound, clean walls. Algae, dust and flaking paint are release layers.
2. 2
   Seal cracks, joints and frames

   As in the crack repair section. Nothing is coated until the cracks are sealed.Why: Nothing is coated until the cracks are sealed, because a coat over an open crack cracks along it.
3. 3
   Treat salts where they show

   Where there are white salt deposits, brush them off and apply RAPIDSTRONG SALT FLUSHER in two coats. Leave it 24 hours before the next layer.Why: Salts under a coating push it off. Brushing them off and sealing stops that.
4. 4
   Prime

   Apply RAINCARE WP Exterior Primer diluted with 20% water and allow at least 4 to 6 hours. Very porous walls may need two coats.Why: The primer soaks into the wall and gives the coating something to bond to, and a very porous wall may need more than one coat.
5. 5
   Apply the waterproof coating in two coats

   Apply the elastomeric coating of your choice in two coats. For RAINSEAL 200, allow 3 to 4 hours between coats, and thin it with up to 20% water where needed. Coat the full wall, including sills, chajja tops and the parapet.Why: Two coats close the pinholes a single coat leaves, and give the stretch across hairline cracks.
6. 6
   Finish with an exterior emulsion, if you want colour

   White elastomeric coatings can be left as the finish or tinted. For a decorative colour and sheen, apply SHINESHIELD Exterior Emulsion, SHINESHIELD PREMIUM Exterior Emulsion or SHINESHIELD ULTIMATE Exterior over the coating, with 4 to 6 hours between coats. These emulsions are made to go over RAINSEAL 200 or RAINPROOF PU 400 as the base coat.Why: The emulsion gives the colour and protects the elastomeric coat from the sun.

Waterproof plaster for external walls: new walls and replastering

A coating bonds to the plaster under it, so loose or hollow plaster is replaced first. On a new wall, water resistance is built into the plaster before any coating goes on.

1. 1
   Hack off failed plaster

   Remove all loose, hollow and crumbling plaster back to sound material and clean the masonry.Why: New plaster bonds only to sound material. Hollow plaster behind new plaster is a delayed failure.
2. 2
   Bond coat

   Apply a bond coat of ROOFBOND PB 210 mixed 1:1 with cement so the new plaster grips the old wall.Why: The bond coat ties the new plaster to the old wall, which would otherwise shrink away from it.
3. 3
   Plaster with an integral 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. RAPIDSTRONG CP is a ready-mix cement plaster that needs only water on site.Why: The compound makes the plaster itself resist water, so less rain gets through to the wall.
4. 4
   Cure for at least 7 days

   Cure new plaster with water for at least 7 days and let it dry before priming.Why: Cement gains strength only while it is kept moist. Plaster that dries early cracks.
5. 5
   Prime

   New plaster is alkaline. Prime with RAINCARE CP Primer on new plaster, or RAINCARE WP Exterior Primer before an elastomeric coating.Why: New plaster is alkaline and attacks coatings, so it is primed.
6. 6
   Coat

   Apply the waterproof coating in two coats, as for any external wall.Why: The waterproof coat protects the plaster from rain and finishes the wall.

Clear waterproofing for stone, brick and cladding

Natural stone, exposed brick, cladding and heritage surfaces should not be painted. They are protected with a treatment you cannot see. A repellent soaks in and leaves no film, so the surface looks unchanged. A clear coating forms a film and gives more protection, with some gloss.

Seal open joints in stone and cladding with ROOFEXSEAL PU 212 before the clear treatment. Exposed concrete has one more enemy besides rain: carbonation, which lets the steel inside rust. ALSTRONG AC is listed for anti-carbonation protection of exterior concrete.

External wall inspection: wall types, rain exposure and where the water enters

Two walls on one building can need different treatment. Check what each wall is made of, which faces take the most rain and where the water gets in before you choose a coating. Rain enters only where water, an opening and a driving force meet, so sealing the openings and coating the surface stops it.

The face that meets the monsoon wind takes the most water, and top floors take more than lower ones, so inspect those first. Shaded faces stay damp and grow algae. Chalk-mark every crack, and read the stains below openings: they trace the path of the water.

Window sills, beam junctions, chajjas, parapets and pipe holes: where rain gets in

The open wall is rarely the problem. Rain finds the six places marked in the drawing, and each needs its own detail before the coating goes on.

ROOFEXSEAL PU 212 quantity: width in mm times depth in mm times length in metres, divided by 1,000, gives litres. A 600 ml sausage fills 20 metres of a 5 mm by 6 mm joint, and a 10 metre crack opened to 10 mm by 10 mm needs 1 litre, about 2 sausages.

Spalled concrete with exposed steel. Armstrong's sequence is ROOFBOND EP primer, ALSTRONG ZP 777 on the steel, then ROOFCRETE EP as the repair mortar. For shallow patches, ROOFCRETE AR 576 is listed for chajja and balcony restoration from 6 to 100 mm, on a tacky bond coat. Repair comes before rebuilding, and a structural engineer judges any lost steel. See structural concrete repair.

External wall: joints, gaps and two coats

External walls leak at their joints before they leak through the plaster itself: at cracks, at the gap between brick and beam, and around iron members. The sequence seals those first and then protects the whole face with two coats. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Seal the joints

   Clean the surface, cut a V-groove about 6 mm wide and deep along cracks and joints, and fill it with ROOFEXSEAL PU 212.Why: Brick and concrete expand and shrink at different rates, so the joint between them always moves. The flexible sealant takes that movement.
2. 2
   Fill wide brick gaps

   Where gaps between brick walls and beams or iron members are wide, fill them with the polymer mortar ROOFCRETE AR 576, about 6 inches wide and 2 inches deep, then seal the edge with ROOFEXSEAL PU 212 at 6 by 6 mm.Why: A wide gap needs a mortar with body; the sealant at the edge then keeps the movement joint flexible.
3. 3
   Two coats of coating

   Apply two coats of RAINSEAL 200, diluted with about 20% water, with a colour stainer added where a coloured finish is wanted.Why: Two coats give a continuous, breathable, weatherproof film that also reflects heat.

Cement-based waterproof coating for external walls: ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210

ROOFCOAT FLEXI 100 and ROOFBOND RFX PLUS 210 are flexible polymer-modified cementitious coatings, mixed on site and brushed on in two coats. Either suits an external wall. Choose by the finish wanted, the mixing and curing routine, and the exposure of the wall, or ask an Armstrong expert.

ROOFCOAT FLEXI 100 takes regular cement inside and white cement on exposed outside faces, with universal stainers for colour. Weigh the cement, use one lot per wall face, and judge colour on a dry trial patch. Pot life is 40 to 60 minutes for ROOFCOAT FLEXI 100 and within 2 hours at 20°C for ROOFBOND RFX PLUS 210. Fibre mesh, where used, goes in as layer 2, after the first coat.

Scaffolding, weather window and repainting: planning external wall waterproofing

Work from a scaffold or cradle, top to bottom. Mask glass and aluminium, and fill every scaffold hole flush with polymer putty before coating.

Work the faces that are out of direct sun and leave a hot face until the sun moves off it.

Repainting an already painted wall

Strip paint that peels, blisters or chalks to sound plaster by wire brushing and washing. Clean paint that is sound and well bonded, and repair the cracks. Chalky or porous plaster takes ROOFBOND BCS PRIME 501 or RAINCARE WP Exterior Primer. Never paint over a damp wall.

Algae and fungus on external walls

Algae feeds on damp, so fix the leak or overflow first, then wire brush and wash off all growth before any primer. Armstrong states no algae or fungus growth for RAINSEAL 200, anti-algae properties for SHINESHIELD ULTIMATE Exterior and anti-fungal properties for SHINESHIELD PREMIUM Exterior Emulsion. See wall dampness treatment.

Inspection checklist and what not to do

1. After repair: every crack, frame gap, pipe gap and scaffold hole is sealed.
2. After the first coat: no pinholes or thin edges at corners and sills.
3. Quantity check: wall area times coats, divided by the stated coverage, gives the litres. Compare with the empty containers.

Do not coat a damp wall or a hot wall in afternoon sun. Do not apply one coat where two are stated. Do not thin beyond the stated limit, which is up to 20% water for RAINSEAL 200. Do not stop the coat at the sill or leave the parapet top bare.

Best waterproof coating for exterior walls: compare the options

On an outside wall you choose two things: the waterproof layer and the visible finish. Some coatings do both. All eight coatings below waterproof an external wall. Pick by the wall and the finish, or ask an Armstrong expert.

Cement-based alternative: ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210.

Exterior emulsions for colour and finish

Waterproof paint or waterproof coating?

An exterior emulsion is a thin decorative film. It gives colour and sheds rain, and it follows cracks without bridging them. An elastomeric waterproof coating is a thicker, rubber-like film that stretches as the wall moves. On a wall that takes driving rain, the coating does the waterproofing and the emulsion goes over it as the finish.

Exterior wall waterproofing chemicals and products by stage of work

How much waterproofing material for 1,000 sq.ft of external wall

Approximate quantities worked out from each product's stated coverage. Rough or porous walls use more. An Armstrong expert confirms the final quantity after seeing the site.

Stated test figures for exterior wall coatings

• RAINSEAL 200: 450% elongation, crack bridging up to 2 mm, 2,000 hours of UV resistance, no algae or fungus growth.
• RAINSEAL FRC 623: no leak at 7 bar.
• ROOFEXSEAL PU 212: ±25% joint movement.
• CONPROOF 500 and CONPROOF No. 1 are integral waterproofing compounds for plaster.

External wall waterproofing material calculator

Enter the size of your outside walls and choose a coating to see how much primer and waterproof coating they need. The calculator gives quantities only. Use the button at the end to talk to an Armstrong expert about your quantity.

Seven mistakes that let wall seepage come back

1. Coating over unsealed cracks. The coating splits along the crack.
2. Filling a moving crack with a rigid filler. It cracks again. Use a flexible sealant.
3. Using emulsion alone on a wall that takes driving rain. A thin paint film cannot bridge cracks.
4. Coating over loose paint, algae or hollow plaster. The coating comes off with whatever is under it.
5. Skipping the primer on new plaster. Fresh plaster is alkaline and attacks the finish.
6. Ignoring window frames, sills and chajjas. These let in more water than the open wall.
7. Treating the inside face. The rain is coming in from outside.

External wall waterproofing questions and answers