Roof Waterproofing Guide: How to Waterproof a Terrace or Roof So It Stays Dry

What a waterproofed roof is made of

Roof waterproofing is not one product. It is a set of layers that each do a different job. The concrete slab should be dense. The screed above it should slope so water runs to the drain. The coating on top should be flexible enough to stretch when the roof moves in the heat, and it should turn up the parapet wall so water cannot get behind it.

Why roofs leak

Water rarely comes through the middle of a sound slab. It finds the weak points. Before choosing a product, walk the roof and look for these six.

1. Cracks in the slab or screed. Heat makes the roof expand by day and shrink by night. Rigid surfaces crack, and water follows the crack.
2. The parapet junction. The corner where the floor meets the wall moves the most. A coating that stops at the floor leaves this corner open.
3. Expansion and construction joints. Joints are built to move. They need a sealant that moves with them, not mortar.
4. Pipes and fixtures through the slab. Every pipe, railing post and tank leg is a hole in the waterproofing unless it is sealed around.
5. Ponding water. A roof without enough slope holds water for days. Standing water finds every pinhole.
6. The drain outlet. A blocked or badly detailed outlet sends water under the coating at the lowest point of the roof.

If the ceiling below shows damp patches or white salt deposits and you are not sure of the source, start with our guide to water leakage, dampness and efflorescence.

Why it pays to waterproof early

A roof leak is never only a wet ceiling. Water that gets into the slab reaches the steel inside it. The steel rusts and swells, the concrete around it cracks and falls away, and what started as a coating job becomes a structural repair.

Engineers describe the cost of waiting with the Law of Fives, set out by De Sitter in 1984. Every unit of money spent on doing the job properly at the start saves five units of maintenance once problems begin, 25 units of repair once damage spreads, and 125 units of major rehabilitation or renewal.

Figure 3. The Law of Fives (De Sitter, 1984): each stage of delay multiplies the cost by about five.

What early waterproofing saves you

• The structure. A dry slab keeps its reinforcement steel free of rust, which is what keeps the roof strong.
• The rooms below. No damp ceilings, peeling paint, damaged plaster or water near electrical fittings.
• Repeat spending. One properly prepared system avoids patch repairs every monsoon.
• Time. Coating a sound roof takes days. Repairing a damaged one means breaking out concrete, treating steel and rebuilding before any coating can start.
• Comfort. A reflective waterproof coating also lowers the heat coming through the roof.

Choose by your roof

The right system depends on what the roof is made of and what state it is in. Find your situation below.

Clay tile roofs are a separate job. See the Mangalore tile joint filling and waterproof coating procedure.

The seven types of roof waterproofing compared

Each type works in a different way. Most good roofs combine two or three of them.

How far each coating stretches

Elongation is how much a cured film can stretch before it breaks. A roof that cracks or moves needs a coating that can stretch across the gap. These are the stated figures for Armstrong roof coatings.

Figures are each product's stated elongation at break. Several are stated as minimums ("more than" or "plus"). More stretch is not always better: cementitious coatings stretch less but bond strongly and can take a screed or tiles on top. For a closer look at that trade-off, read cementitious vs acrylic elastomeric coatings, or the full guide to types of waterproofing chemicals.

Step-by-step roof waterproofing procedure

The coating is the last step, not the first. Most failed roofs were coated well over a surface that was prepared badly.

1. Inspect and clean

   Remove loose material, dust, moss, oil and any old coating that is flaking or blistered. Mark every crack, joint, pipe and low spot. The surface should be sound and clean before anything goes on it.

2. Repair the cracks

   Open each crack into a small V-groove, clean it and fill it. Use RAPIDSTRONG CRACK SEALER 85 for fine and hairline cracks and RAPIDSTRONG CRACK SEALER 75 for fine to medium cracks. A crack that is still moving needs a flexible sealant such as ROOFEXSEAL PU 212, which takes ±25% joint movement. For cracks that run through the slab, follow the concrete crack repair guide.

3. Seal the joints and junctions

   Form a cove where the roof meets the parapet so the coating turns the corner in a smooth curve. Use RAINSEAL FLEX TAPE at roof-to-wall junctions. Seal expansion joints with ROOFEX-SEAL PS, which covers joints 6–25 mm wide in a single application, over ROOFEXSEAL PRIMER PS / NPS. Seal around every pipe and fixture.

4. Correct the slope

   Water must run to the drain. Where it ponds, lay a screed to slope before coating. ROOFCRETE AR 576 is listed for terrace waterproofing screeds, and ROOFCRETE URP for waterproofing mortars and sloping RCC roofs. Use ROOFBOND SBR 401 as the bond coat between old concrete and new screed. See screed over the RCC mother slab for the detail.

5. Prime

   A primer seals the pores and gives the coating something to grip. ROOFBOND BCS PRIME 501 is a penetrative water-barrier primer that covers 4–5 sq.m per litre and can be recoated in 3–4 hours. CONPROOF EP is used as a primer coat on roof terraces and before PU coatings. SHINESHIELD WP 305 is the primer before RAINPROOF PWR 690.

6. Apply the coating in coats

   Apply the first coat evenly and take it up the parapet. Bed a reinforcing mesh into the wet coat at junctions, cracks and joints, or across the whole roof for a reinforced system. Apply the second coat at right angles to the first once the first has dried. Recoat times differ by product: RAINSEAL 200 takes a second coat after 3–4 hours, while ROOFCOAT FLEXI 100 needs 24 hours between coats. Our guide to fibreglass mesh vs geo-fabric explains which reinforcement to use.

7. Cure, then test with water

   Let the coating cure fully before anyone walks on it. RAINSEAL 200 needs 3 days at 30°C. Then block the outlets, flood the roof with a shallow depth of water and leave it for 24 to 48 hours. Check the ceiling below for any damp. Drain the roof, fix anything the test shows, and only then lay tiles, a protective screed or a cool roof topcoat.

Multi-layer waterproofing systems

A multi-layer system is a fixed build-up in which every layer is specified. The mesh in the middle gives the membrane strength across cracks and joints. The topcoat protects it from the sun.

Cool roof coatings: waterproofing that also cuts heat

A dark or bare concrete roof absorbs the sun's heat and passes it into the rooms below. A white solar reflective coating sends most of that heat back. It is also a waterproof coating, so one product does both jobs.

These coatings can go over existing waterproof coatings, terraces, sloped roofs and metal roofs. Coat the parapet walls along with the roof so heat does not enter from the sides. For the full method, read the SUMMERSHIELD COOL 666 guide.

When to waterproof a roof

The dry season is the best time. The surface is dry, coats dry on schedule and the coating cures without interruption. If you can plan ahead, finish the work before your monsoon starts.

You do not have to wait for the dry season if the roof is leaking now. Waterproofing is done in the rainy season too. Pick a dry spell with no rain in the forecast for the days the work and the curing will take, let the surface dry out first, and plan the job in stages so each coat has time to dry. Near the coast, humidity slows drying, so allow more time between coats.

Check the cure time of the product you are using. RAINSEAL 200, for example, needs 3 days at 30°C before it is walked on or tested with water.

What decides the cost of roof waterproofing

Armstrong does not publish prices, because no two roofs cost the same. Five things decide the figure on your quote.

• The condition of the roof. A cracked, ponding roof needs repair and a new screed before any coating. A sound roof does not.
• The area and the length of the edges. Parapets, joints and pipes take more work per metre than open floor.
• The system chosen. A two-coat acrylic, a PU membrane and a five-layer reinforced system use different amounts of material and labour.
• The number of coats and the thickness. Coverage per litre falls as thickness rises.
• Access. Height, water supply and how material reaches the roof all affect labour.

How much material for a 1,000 sq.ft roof

These quantities are worked out from each product's stated coverage for a 1,000 sq.ft (93 sq.m) roof. Rough or porous surfaces use more. Parapet upturns are extra.

For a quantity and quote matched to your roof, request a quote and an Armstrong distributor will assess the site.

Seven common mistakes

1. Coating over a dirty or damp surface. The coating bonds to the dust, not the roof. Some coatings, such as RAINCARE PU CLEAR, need a completely dry surface.
2. Skipping crack and joint repair. A coating bridges fine cracks. It does not repair an open joint.
3. Ignoring the slope. No coating is meant to sit under standing water for weeks.
4. Stopping at the floor. The coating must turn up the parapet. Most edge leaks start where it did not.
5. One thick coat instead of two. Two thinner coats at right angles cover the pinholes that one coat leaves.
6. Coating just before rain. A coating that has not cured can wash off or blister.
7. Skipping the water test. It is far easier to fix a leak before the tiles go down than after.

Why Armstrong for roofs

Most roof problems are not solved by one tin of coating. They need the right admixture in the concrete, the right filler in the crack, the right sealant in the joint, the right primer, and then the right membrane for that roof. Armstrong makes all of them, so every layer of the roof comes from one manufacturer and is made to work with the next.

• An answer for every roof. New slabs, leaking terraces, old bitumen, metal and AC sheets, podiums, balconies and industrial roofs each have a product listed for them.
• A choice within every type. Six acrylic coatings, four polyurethane membranes, three polyureas, four crystalline systems and eight multi-layer systems, so the system fits the roof and not the other way round.
• Made in India for Indian roofs. Armstrong formulates and manufactures in Bengaluru for Indian site conditions: strong sun, heavy monsoon rain and wide swings in temperature. The coatings state the figures that matter here, such as 2000 hours of UV resistance, heat reflectivity up to 87% and service temperatures from -20°C to +80°C.
• Quality you can check. Armstrong Chemicals is an ISO 9001:2015 certified company, and each product page gives its tested figures and the standards it follows.

All Armstrong roof waterproofing products

Every Armstrong product used on roofs and terraces, grouped by the stage of work. Select a product for its full data, coverage and application steps.

Questions and answers