Cool Roof Coating: Heat Reflective Paint That Waterproofs the Roof

Top floor too hot in summer: check the roof first

A top floor that stays hot long after sunset, a ceiling that is warm to the touch and an air conditioner that never switches off all point to the roof. A terrace in the sun absorbs heat all day and passes it down through the slab into the rooms below. A cool roof coating, also called heat reflective roof paint or roof cooling paint, turns most of that sunlight back before it becomes heat. Five checks decide which coating and how many coats. They need no instruments.

Find the right cool roof coating method for your roof

Match what you found to one route. Armstrong makes several heat reflective roof coatings and every one of them also waterproofs. They differ in reflectivity, chemistry and finish, so the choice depends on the roof and on what you want from it.

Repair first, then cool. A cool roof coating reflects heat only while it stays bonded, clean and whole. Loose, old and flaking material is removed, cracks are sealed and the roof is made to drain before the first coat goes on.

Cool roof coating finder

Armstrong makes several heat reflective roof coatings and a six-layer heat reflective system. All of them cool and waterproof. Answer three questions for a starting point, then confirm it with an Armstrong expert.

How to apply cool roof coating on a terrace

This is Route A: a sound RCC terrace that is hot but does not leak. The work is cleaning, sealing the few cracks there are, and two to three coats. SUMMERSHIELD COOL 666 and SUMMERPROOF COOL 644 are the coatings made for the highest reflectivity. RAINSEAL 200, RAINSEAL FRC 623 and RAINSEAL CFR 613 are heat reflective waterproof coatings that suit the same roof. Pick the one that fits the roof, or ask an Armstrong expert.

1. 1
   Remove everything loose

   Take off dust, moss, algae, oil and every bit of loose, old or flaking coating by wire brushing and washing. Armstrong's direction for SUMMERSHIELD COOL 666 is to clean the surface of dirt, dust and other contamination and wash it. Let the roof dry.Why: A reflective coat only reflects as well as it bonds. Algae, oil and flaking coating are release layers, and the new coat peels off them in the first monsoon.
2. 2
   Seal the cracks

   Open each roof crack to a 4 to 5 mm V-groove and seal it with ROOFEXSEAL PU 212. Roof cracks take PU 212 only. Expansion joints take ROOFEX-SEAL PS.Why: Roof cracks open and close with heat. A flexible polyurethane sealant moves with them, while a rigid filler cracks again and lets water through the new coat.
3. 3
   Make the roof drain

   Fill low spots that hold water and patch any broken screed with a polymer repair mortar made with ROOFCRETE AR 576 over a bond coat of ROOFBOND SBR 401. Standing water leaves dirt on a white roof and lowers its reflectivity.Why: Standing water leaves dirt rings on a white roof, and dirt cuts reflectivity. A roof that drains stays white and keeps its cooling effect.
4. 4
   Check the coving and the parapet

   The coating is carried over the coving and up the parapet. Where the corner is a sharp angle or the skirting is cracked, make it good first. The terrace waterproofing page gives the coving and skirting details.Why: A sharp corner or cracked skirting is where a coating tears first. Making it good lets the coat run round the corner as one skin.
5. 5
   Apply the first coat

   Dilute SUMMERSHIELD COOL 666 with 10% water and apply by brush or sprayer. SUMMERPROOF COOL 644 is also thinned with 10% water, and works as its own primer when 1 litre is diluted with 0.5 litre of water. RAINSEAL 200, RAINSEAL FRC 623 and RAINSEAL CFR 613 take up to 20% water.Why: The thinned first coat soaks into the surface and gives the later coats something to grip. A first coat applied thick sits on top and peels.
6. 6
   Apply the second and third coats

   Recoat SUMMERSHIELD COOL 666 after 3 to 4 hours at 30°C. Two to three coats are recommended, and three coats are enough to meet the stated SRI. RAINSEAL 200 takes two coats, 3 to 4 hours apart. Apply each coat at right angles to the one before.Why: Each coat adds film thickness, and film thickness is what gives the stated reflectivity and life. Two coats close the pinholes a single coat leaves.
7. 7
   Coat the parapet walls too

   Armstrong states that, for full heat resistance, all parapet walls are covered with the coating along with the terrace or roof. Carry it up the inner face and over the top.Why: The parapet takes sun on both faces and radiates heat down into the room. Leaving it uncoated leaves a hot path around the cooled slab.
8. 8
   Let it cure

   SUMMERSHIELD COOL 666 cures fully in 4 to 5 days at 30°C. SUMMERPROOF COOL 644 cures in 2 to 3 days. RAINSEAL 200 needs 3 days before foot traffic. Keep people and stored goods off the roof until then.Why: A reflective film that is walked on or rained on before it cures marks, softens and loses thickness.

Terrace heat and leakage: a reinforced cool roof waterproofing build-up

This is Route B. Many hot top floors also have a roof that cracks or leaks. One build-up handles both: the leak repair comes first, a reinforced membrane goes down with fibre mesh as layer 2, and the cool roof coats form the top layers. This page covers the heat side. The full leak diagnosis, the five checks and the repair sequence are on the terrace waterproofing page, and the two pages are meant to be used together.

1. 1
   Repair the roof first

   Remove loose, old and flaking material. Seal cracks with ROOFEXSEAL PU 212 in a V-groove, cut out hollow or crumbling patches and rebuild them with a polymer repair mortar made with ROOFCRETE AR 576 over ROOFBOND SBR 401, and make good the coving and skirting. Follow the repair route on the terrace waterproofing page.Why: A cool roof coat cools the roof; it does not stop a leak. The leak is repaired first so the new coats are not laid over wet or moving material.
2. 2
   Apply the first coat

   For a build-up in SUMMERSHIELD COOL 666 alone, the first coat is a primer coat of the same product diluted with 20% water. Where a separate waterproof base is wanted, the base coat is RAINSEAL BMW 250, RAINSEAL RWC 902 or RAINPROOF PU 400. RAINSEAL RWC 902 and RAINPROOF PU 400 are stated for use with fibre mesh or geo-fabric, and RAINSEAL BMW 250 with glass fibre tissue or geotextile. RAINPROOF PU 400 starts with a primer coat of the same product diluted 2:1 with water. A cementitious base of ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210 also takes fibre mesh and a cool roof coat on top.Why: The first coat is the primer for everything that follows. The choice of base decides whether the build-up is cool-roof-only or waterproof and cool.
3. 3
   Lay the fibre mesh as layer 2

   After the first coat, lay fibre mesh or geo-fabric cloth over the whole roof, across the coving and up the parapet. Fibre mesh is always layer 2. Armstrong build-ups with RAINSEAL 200 use a 45 GSM fibre mesh. Overlap the strips. Overlap the strips by about 50 mm.Why: The mesh reinforces the film across hairline cracks, so a small movement in the slab does not split the whole coating.
4. 4
   Bury the mesh

   Apply the next coat over the mesh so that it is completely covered. With SUMMERSHIELD COOL 666 this coat is mixed with 10% water.Why: A mesh that is not completely covered wicks water and lets the film peel. Burying it makes the reinforcement part of the film.
5. 5
   Finish with two cool roof coats

   After that coat dries, apply two more coats of the cool roof coating, diluted with 10% water. Over RAINSEAL 200, RAINSEAL RWC 902, RAINSEAL BMW 250, RAINPROOF PU 400, ROOFCOAT FLEXI 100 or ROOFBOND RFX PLUS 210, Armstrong generally recommends two coats of SUMMERSHIELD COOL 666.Why: The top coats carry the reflective pigment and take the sun, weather and wear. They are applied last and applied twice.
6. 6
   Cure, then test

   Let the build-up cure, then flood the roof for 24 to 48 hours and check the ceiling below, as on the terrace waterproofing page. RAINSEAL RWC 902 is cured for 3 days at 30°C before water testing, and SUMMERSHIELD COOL 666 cures fully in 4 to 5 days at 30°C.Why: The flood test finds a leak while it can still be fixed in a few hours. The same leak found after the finishes go on means breaking out the finishes.

These are figures from Armstrong's own laboratory testing under controlled conditions. The result on a real roof depends on the slab, the sun on the walls and windows, ventilation and how clean the coating is kept.

Heat reflective coating over old waterproofing, bitumen and sheet roofs

Routes C and D. A roof that is already waterproof does not need to be stripped to be made cool. A reflective coat goes over the sound layer that is there. What matters is that the old layer is firmly bonded, and that anything loose comes off first.

Recoating, step by step

1. 1
   Test the old layer

   Scrape and pull at the old coating in several places. Whatever lifts, flakes or blisters is removed back to a firm edge. A new coat bonds to whatever is under it, so a coat applied over loose material comes off with it.Why: A new coat is only as sound as what is under it. Testing in several places shows whether the old layer can be kept or has to come off.
2. 2
   Wash off dirt and algae

   Wire brush and wash the roof. Dust and algae sit between the old layer and the new one and weaken the bond.Why: Dust and algae sit between the old layer and the new coat and weaken the bond, so they are washed off first.
3. 3
   Repair what the cleaning exposed

   Seal cracks with ROOFEXSEAL PU 212. Patch bare or broken areas and bring them up to the level of the surrounding coating.Why: Cleaning exposes cracks and bare patches that were hidden. They are repaired now, before the coat goes over them.
4. 4
   Prime where needed

   RAINSEAL CFR 613 is primed with the same product diluted 2:1 with water. SUMMERPROOF COOL 644 works as its own primer at 1 litre to 0.5 litre of water.Why: Some coatings need a primer coat of themselves to bond to an old surface. Skipping it is the commonest cause of peeling over old roofs.
5. 5
   Apply two coats

   Apply two coats of the chosen coating, the second at right angles to the first, over the roof and the parapet.Why: Crossing the second coat at right angles to the first closes the brush marks and pinholes in the first.
6. 6
   Let it cure

   RAINSEAL CFR 613 needs 7 days of full cure before water exposure. SUMMERSHIELD COOL 666 cures in 4 to 5 days at 30°C.Why: Some coatings need up to a week before water exposure. Rain on an uncured film washes out the binder.

Cool roof for commercial buildings, new roofs and cool roof rules

Route E. Offices, hospitals, schools, factories and warehouses have large roofs, long cooling hours and often an engineer who writes the specification. On these roofs the coating is chosen by its tested figures, and on new buildings it is planned as the last stage of the roof and not added later.

A new roof: plan the cool roof as the final stage

1. 1
   Build the waterproofing first

   On a new terrace the membrane goes on the mother slab, the screed is laid to slope and the edges are closed with coving and skirting. That sequence is on the terrace waterproofing page and on the new construction waterproofing page.Why: On a new roof the membrane belongs under the screed on the mother slab. A cool roof coat on a roof that leaks only cools a leaking roof.
2. 2
   Let the screed cure and dry

   The cool roof coating goes on a cured, dry and clean surface.Why: A coating on a damp or uncured screed traps moisture, which blows blisters in the film in the sun.
3. 3
   Apply the cool roof coats last

   Two to three coats over the finished roof and the parapet, after other trades have left the roof. Scaffolding, tanks and solar frames fixed afterwards damage the coating and are best in place first.Why: Other trades damage a finished coating. Scaffolding, tanks and solar frames are fixed first and the coats go on after.

Best cool roof paint for a terrace: Armstrong heat reflective coatings compared

There is no single best cool roof paint. Each coating below reflects heat and waterproofs in the same coat, and each is right for a different roof. The table gives the stated figures so you can compare them directly.

How a cool roof coating reduces terrace heat

Two properties decide how cool a roof stays. Solar reflectance is the share of sunlight the surface turns back. Thermal emittance is how readily the surface gives up the heat it does absorb. The Solar Reflectance Index (SRI) combines the two into one number: a standard black surface scores 0 and a standard white surface 100, and a high-performance coating can score above 100.

• SUMMERSHIELD COOL 666 reflects 90% of solar infrared rays and 85% of ultraviolet rays, with 87% ± 2 heat reflectivity in a white finish.
• In air-conditioned buildings Armstrong states energy use up to 30% lower with SUMMERSHIELD COOL 666 and up to 25% lower with SUMMERPROOF COOL 644.
• White gives the highest reflectivity. Tinting a coating to a darker colour lowers it.
• The coating has to stay clean to keep working, which is why slope, drainage and washing matter.
• Every figure here is for the roof. Heat through walls, windows and poor ventilation is separate.

Cool roof coating chemicals and products by stage of work

How much cool roof paint for a 1,000 sq.ft terrace

Approximate quantities from each product’s stated coverage, for 1,000 sq.ft (93 sq.m) of plain surface. Rough surfaces use more, and the parapet faces are extra. An Armstrong expert confirms the final quantity after seeing the site.

Standards and tests for solar reflective roof coatings

• ASTM E 903 is the test for solar reflectance. SUMMERSHIELD COOL 666 is stated at 87% ± 2 and RAINSEAL FRC 623 at 80% ± 2 in a white finish.
• ASTM E 1980 is the method for calculating the Solar Reflectance Index. SUMMERSHIELD COOL 666 is stated at SRI 113.
• ASTM C 836 covers liquid-applied elastomeric waterproofing membranes, including crack bridging. SUMMERSHIELD COOL 666 shows no cracks up to 3 mm width.
• ASTM G 154 is the accelerated UV exposure test. SUMMERSHIELD COOL 666 and RAINSEAL FRC 623 show no deterioration and no colour fade after 2,000 hours.
• Water permeability: no leak at 7 bar for SUMMERSHIELD COOL 666, RAINSEAL FRC 623 and RAINSEAL CFR 613.
• The Energy Conservation Building Code sets a minimum solar reflectance and thermal emittance for a cool roof.
• IS 3067 is the Indian code of practice for damp-proofing and waterproofing of buildings, and gives general design details such as the fillet at junctions that the coating is carried over.

Cool roof paint quantity calculator

Enter the roof size and the parapet to see the litres needed. Quantities only. Figures are for a plain surface; rough surfaces use more.

Seven mistakes that make a cool roof coating fail

1. Coating over dust, algae or flaking paint. The coating bonds to the loose layer and peels with it. Everything loose comes off first.
2. Painting over open cracks. Cracks are opened and sealed with ROOFEXSEAL PU 212 before any coat. A coating is not a crack filler.
3. One thick coat. A single heavy coat leaves pinholes and dries unevenly. Two to three coats at the stated coverage do the job.
4. Leaving the parapet bare. Armstrong states that the parapet walls are coated along with the roof for full heat resistance.
5. Coating a roof that holds water. Ponding leaves dirt and algae on the white surface. Low spots are corrected first.
6. Tinting it dark. A dark shade absorbs the heat a white coat reflects. Choose white or the lightest shade that suits.
7. Using it as a floor finish. A cool roof coating is made for light foot traffic. Crowds, furniture and machinery need a trafficable build-up.

Cool roof coating questions and answers