Metal Sheet Roof Waterproofing: Seal the Laps and Bolts, Then Coat

GI sheet roof leakage: check where the water gets in

A leaking tin shed, water dripping along a purlin, an AC sheet roof that leaks in every heavy rain, a factory floor that is wet under the same line of bolts: these are the usual signs of metal sheet roof leakage. The sheet itself rarely lets water through. The laps, the J-bolts and fasteners, the ridge, the gutter line and the rusted or cracked patches do. Six checks find the source. Do them from below and from a safe position, and never walk on AC sheets without crawl boards.

Find the right metal sheet roof waterproofing method

Match what you found to one route. Most leaking sheet roofs are repaired and coated, not replaced. Each route tells you what is done and takes you to its section.

Seal first, coat second. A coating bridges a hairline. It does not bridge an open lap, a loose bolt or a hole. Every leak point is closed, and all loose rust, old sealant and flaking paint removed, before the roof is coated.

Metal and AC sheet roof coating finder

Armstrong lists several coatings for metal and AC sheets. Every one is a sound choice. Answer three questions for a starting point, then confirm it with an Armstrong expert.

Metal sheet joint leak: how to seal laps, J-bolts and ridges

Routes A and C. This is the work that stops the leak. It is done on every sheet roof whether or not the whole roof is coated afterwards. RAINSEAL BXL is a ready-to-use acrylic blended bituminous paste that Armstrong recommends for sealing cracks and joints of AC and GI sheets. It needs no mixing and is pressed in with a trowel or putty knife.

1. 1
   Clean each leak point

   Remove dust, old cracked sealant, loose rust and flaking paint from laps, fastener heads, the ridge and the flashings. Wire brush metal. The surface must be dry.Why: Sealant bonds to clean, sound surfaces. Rust, old sealant and dust stop it.
2. 2
   Tighten or replace fasteners

   Tighten loose J-bolts and screws and replace missing or perished washers. A fastener that moves will break any seal put over it.Why: A loose fastener lets water run down the shank into the building, and sealant alone will not hold it.
3. 3
   Seal the laps

   Press RAINSEAL BXL firmly into the open edge of each leaking end lap and side lap and level it flush. Where a lap gapes, it is first closed with extra fasteners.Why: Laps open and close with temperature. A flexible sealant pressed into the open edge keeps the lap sealed.
4. 4
   Seal the fastener heads

   Cover each fastener head and washer with RAINSEAL BXL so that water cannot track down the shank. Nut and bolt heads are filled with the bitumen-acrylic sealant before the geo-fabric patches go on.Why: Water tracks down the shank of every fastener, so the head and the washer are sealed over.
5. 5
   Seal the ridge and flashings

   Fill the gap under the ridge cap and at the edges of flashings with RAINSEAL BXL. Where a metal flashing meets a masonry wall and the joint moves, use ROOFEXSEAL PU 212, which bonds without a primer to metal, aluminium, concrete and masonry.Why: The ridge cap and flashing edges are where the roof planes meet and where wind-driven rain gets in.
6. 6
   Fill cracks and small holes

   Cut a crack to a V shape, clean it and fill it with RAINSEAL BXL. Small holes are patched.Why: A crack cut to a V holds filler, while a flush fill has nothing to bond to.
7. 7
   Reinforce with mesh, if coating

   Where the roof is to be coated, a strip of 45 GSM fibre mesh or geo-fabric is bedded over each sealed lap and line of fasteners after the first coat, as layer 2. Geo-fabric patches of about 4 by 4 inches are fixed over each nut and bolt; over the full sheet the fabric is laid in 1 m widths with a 50 mm overlap.Why: The mesh strip carries the coating across the lap, so movement does not split the coating on the line it moves at.

Rusted metal roof repair: treat the rust before coating

Route C. A coating applied over loose rust comes off with it, and rust keeps growing under a film that has been painted across it. Armstrong makes three products for this stage. Which one is used depends on how much rust there is and how clean the metal can be made. For structural steel, trusses and purlins, see the steel rust protection page.

ALSTRONG RC is applied between 15°C and 25°C according to its data. On a sheet roof that means early morning work in summer.

AC sheet roof leakage: waterproofing asbestos cement sheets

Route D. Asbestos cement sheets do not rust. They leak through cracks, through porous weathered patches, at laps and at the bolts. Old sheets are brittle and will not carry a person between purlins. Work from crawl boards laid over the purlins.

1. 1
   Clean gently and let it dry

   Wash off moss, algae and dirt and remove loose and flaking paint. Let the sheets dry completely. The coatings below are applied on a dry surface.Why: AC sheets are brittle, so they are cleaned gently, and a wet sheet blocks the coating.
2. 2
   Fill the cracks

   Cut each crack to a V shape, remove loose particles and fill it with RAINSEAL BXL. Armstrong recommends it for sealing the cracks and joints of AC sheets.Why: A cracked AC sheet leaks through the crack, and filling it stops that before the coating bridges it.
3. 3
   Seal laps and bolts

   Treat the laps, the bolt heads and the ridge as in the sealing section, with RAINSEAL BXL.Why: These are the usual leak points, and the coating alone does not close them.
4. 4
   Prime

   Many of the coatings are primed with the same product thinned with water: RAINSEAL FRC 623 and RAINPROOF PRG 909 with 50% water added. RAINCARE WP Exterior Primer is a primer listed for asbestos sheet surfaces under polyurethane and polymer coatings, diluted with 20% water. Site orders prime AC sheets with RAINSEAL RWC 902 diluted with about 50% water.Why: The primer soaks into the porous sheet and gives the coats above something to hold.
5. 5
   First coat

   Apply the first coat of the chosen coating over the whole roof.Why: The first coat carries the waterproof film and must cover the whole roof, edges included.
6. 6
   Fibre mesh as layer 2

   Bed fibre mesh or fibreglass tissue in the wet first coat, over cracks, laps and bolt lines, or over the whole roof where the sheets are weathered throughout. Fibre mesh is always layer 2.Why: Bedded in the wet coat, the mesh reinforces the film over cracks, laps and bolt lines, where movement is greatest.
7. 7
   Second coat, and a third if needed

   Apply the next coat over the mesh so it is buried, and a further coat where the mesh still shows.Why: The second coat buries the mesh, so the reinforcement is part of the film, and builds the film thickness.
8. 8
   Cure

   RAINSEAL RWC 902, RAINSEAL FRC 623 and RAINPROOF PRG 909 are cured for 3 days before movement on the roof or testing with water. RAINPROOF FPU 404 is given a full 7 days before heavy rain or heat loading.Why: The film is soft until it has cured, and foot traffic or rain before then marks or washes it.

Coatings Armstrong lists for AC sheet roofing: RAINSEAL RWC 902, RAINPROOF PRG 909, RAINSEAL FRC 623, RAINSEAL 200, RAINPROOF FPU 404 and, where heat is the complaint, SUMMERSHIELD COOL 666 or SUMMERPROOF COOL 644. RAINCARE PU CLEAR is a transparent coating listed for asbestos sheets. Any of these suits the job. Pick by colour and heat, or ask an Armstrong expert.

AC sheet roof: five layers with geo-fabric

An asbestos-cement or metal sheet roof leaks at its overlaps and at every bolt. The coating is built up in layers with a reinforcing geo-fabric so it holds across the ridges, laps and bolt heads. Doses, thicknesses and coats are the standard figures; the site and the exposure decide where they move.

1. 1
   Strip and clean

   Remove the old tar felt and clean off dirt, dust and other contamination.Why: New coatings only hold on a clean surface.
2. 2
   Seal overlaps and bolts

   Wherever required, fill the overlap joints and the nut and bolt heads with the bitumen-acrylic sealant RAINSEAL BXL (written WONDERSEAL BXL in some site orders).Why: Laps and bolts are the leak points. A fabric laid over an open gap does not seal it.
3. 3
   Patch the bolts

   Fix 60 GSM geo-fabric, cut about 4 by 4 inches, over each nut and bolt with one coat of RAINSEAL BMW 250 diluted with about 20% water.Why: The patch reinforces each bolt head, which is the point of highest movement on a sheet.
4. 4
   Primer

   Apply one coat of RAINSEAL RWC 902 diluted with about 50% water as a primer.Why: The primer penetrates the sheet and gives the coats above something to bond to.
5. 5
   Fabric and second coat

   Lay 60 GSM geo-fabric over the whole sheet, in 1 m widths with a 50 mm overlap, and apply RAINSEAL RWC 902 over it, diluted with about 20% water.Why: The full-width fabric ties the whole roof into one reinforced membrane.
6. 6
   Two top coats

   Apply two coats of RAINSEAL 200, diluted with about 20% water.Why: The top coats protect the membrane from sun and rain and reflect heat off the sheet.

Metal roof waterproof coating: build-ups for GI, tin and AC sheet roofs

Routes B and E. Once the leak points are sealed and the rust treated, a flexible coating over the whole roof closes the hairlines, protects the sealed details from the sun and slows further rusting. A sheet roof moves a great deal between a cold night and a hot afternoon, so the coating has to stretch and recover.

1. 1
   Prepare the whole roof

   Remove dust, grease, oil, loose rust and loose paint. The surface is to be absolutely dry. Factory oil on new GI sheets and chalky old paint both stop a coating bonding.Why: Coatings bond to dry, clean metal only. Grease, rust and loose paint stop them.
2. 2
   Seal and treat first

   Complete the lap, fastener, ridge and rust work in the sealing section.Why: The coating bridges small gaps, and the laps, fasteners and rust are where it would fail first.
3. 3
   First coat

   Apply the first coat by brush or airless spray. RAINSEAL RWC 902 and RAINPROOF PRG 909 take up to 20% water in the first coat. RAINPROOF FPU 404 takes up to 20% water. RAINSEAL 200 and RAINSEAL BMW 250 can also be mixed with up to 20% water.Why: The first coat is the primer for the rest of the build-up, and its dilution depends on the product.
4. 4
   Fibre mesh over the laps as layer 2

   Bed fibre mesh or geo-fabric in the wet first coat along every lap, ridge and fastener line. On a roof in poor condition the mesh covers the whole surface. For damaged GI and ACC sheets, RAINSEAL 200 is built up in five layers with a 45 GSM fibre mesh or geo-fabric as layer 2.Why: Laps and ridges move most, so the mesh goes there to hold the film together.
5. 5
   Second coat

   Apply the second coat over the first and over the mesh. RAINSEAL RWC 902 and RAINPROOF PRG 909 are recoated after 6 to 8 hours, RAINPROOF FPU 404 after a minimum of 4 to 6 hours, RAINSEAL 200 and RAINSEAL FRC 623 after 3 to 4 hours.Why: The second coat buries the mesh and builds the thickness that gives the stated life.
6. 6
   Add the heat reflective coats, if wanted

   For a hot shed, finish with two to three coats of a white cool roof coating over the waterproof coats, or use a white reflective coating for all the coats.Why: On a hot shed the white coats reflect the sun and lower the temperature under the sheets.
7. 7
   Cure before rain

   Allow 3 days of curing for RAINSEAL RWC 902, RAINPROOF PRG 909, RAINSEAL 200 and RAINSEAL FRC 623, and 7 days for RAINPROOF FPU 404. Plan the work for a dry spell.Why: Rain on an uncured coat washes out the binder and leaves it thin.

Shed roof heat reduction

A bare metal roof in the sun makes the space under it very hot. SUMMERSHIELD COOL 666 is recommended for ACC, iron and GI sheets and SUMMERPROOF COOL 644 for metal roofs and ACC and GI sheets, both at 87% heat reflectivity. Armstrong states a temperature reduction of up to 8 to 10°C for SUMMERSHIELD COOL 666, applied in 2 to 3 coats, and up to 6 to 10°C for SUMMERPROOF COOL 644. SUMMERPROOF COOL 644 is applied on metal roofs when the surface is between 5°C and 30°C, so it goes on in the cool part of the day. RAINSEAL 200 and RAINSEAL FRC 623 are white waterproof coatings with 80% heat reflectivity, stated to lower the temperature below by 6 to 8°C. The cool roof coating page explains how these work and how many coats are needed.

Does production have to stop?

The work is done from above, so the building under the roof stays in use. Sealing and coating are done bay by bay. Where a roof must be back in service at once, RAINPROOF PHB 660 is a fast-setting, two-component hybrid polyurea and polyurethane coating listed for metal surfaces. Its two parts are mixed 2:1 and applied by brush or sprayer.

Multi-layer membrane systems used on GI sheet roofs

Armstrong builds a sheet roof membrane in layers: a primer coat, a coat, geo-fabric or fibre mesh laid while that coat is wet, then the top coats. The fabric is buried by the next coat, so it holds the film together across the laps and bolt heads, where a sheet roof moves most. The layer pattern stays the same from roof to roof. The membrane in the coats can be any Armstrong membrane listed for metal sheets in the comparison further down, so confirm its dilution and recoat time on its data sheet. Three builds used on site:

Dilutions and amounts are as written in site orders. The data sheet of the product chosen decides them for your roof. Older site orders write RAINSEAL BMW 250 as RAINSEAL B 300 and RAINPROOF PRG 909 as RAINPROOF PU 370.

Named systems for industrial roofs

For large industrial and power house roofs an engineer may specify a named Armstrong system. RAINPROOF PUF 5 Layer System is a five-layer fibre reinforced polyurethane system listed for industrial roofs, and RAINCARE PRC 5 Layer System is a five-layer PU blended acrylic elastomeric system. In each the second layer is fibre mesh or geo-fabric. Both are listed for GI sheet roofs.

When to replace roof sheets, and how to protect a new sheet roof

Route F is the extreme case and the last option. Wherever the sheet is still sound, Armstrong repairs it: the leak points are sealed, the rust is treated and the roof is coated. Replacement is kept for the sheet that has nothing left to bond to.

A new sheet roof: do it right from the start

1. 1
   Seal as the sheets are fixed

   Laps, ridge caps and flashings are sealed during fixing, when every joint can be reached.Why: Every joint is reachable while the sheets go on, and impossible to reach once they are on.
2. 2
   Detail the wall junctions

   Where the roof meets a wall or parapet, the flashing joint is sealed with ROOFEXSEAL PU 212. At the junction there are two ways to reinforce it. Either lay RAINSEAL FLEX TAPE under the coating, or build it from the coating itself: one coat over the junction, geo-fabric bedded into it while it is wet, then a second coat over the fabric.Why: The junction of roof and wall moves differently from the sheets and needs a flexible joint. Geo-fabric between two coats carries the membrane across the junction, the way it does across a lap.
3. 3
   Coat for heat and weather

   A white reflective coating on a new roof lowers the temperature in the building from the first summer and shields the sheet from weather. See the cool roof coating page.Why: A white coating on a new roof lowers the temperature from day one and protects the sheet from weather.
4. 4
   Inspect every year before the rains

   Look at fasteners, laps and gutters before each monsoon and reseal what has opened.Why: A fastener or lap that has opened before the monsoon leaks in the first rain, so it is found and resealed early.

Which coating sticks to metal and AC sheets: Armstrong products compared

Every coating below is listed in Armstrong data for metal sheets, AC sheets or both. None is a lesser product. They differ in chemistry, colour and heat performance, so pick the one that fits the roof, or ask an Armstrong expert to choose with you.

RAINPROOF FPU 404 colour is not stated in the data used for this page.

Sheet roof leakage chemicals and products by stage of work

How much coating for a 2,000 sq.ft metal sheet roof

Approximate quantities for 2,000 sq.ft (186 sq.m) of sheet surface, two coats, from each product’s stated coverage on a plain surface. A corrugated or trapezoidal sheet has more surface than its plan area, so measure along the profile or use the calculator, which adds an allowance. Sealing paste, mesh and rust treatment are extra. An Armstrong expert confirms the final quantity after seeing the site.

Standards and tests for metal roof waterproof coatings

• ASTM C 836 covers liquid-applied elastomeric waterproofing membranes. RAINSEAL RWC 902, RAINPROOF PRG 909 and RAINSEAL FRC 623 are stated as complying, with crack bridging of 2 mm.
• ASTM D 412 is the tensile and elongation test. RAINSEAL RWC 902 and RAINPROOF PRG 909 are stated at 700% elongation.
• ASTM G 154 is the accelerated UV test. RAINPROOF PRG 909 and RAINSEAL FRC 623 show no deterioration after 2,000 hours.
• Heat test: RAINSEAL RWC 902 and RAINPROOF PRG 909 show no shrinkage or brittleness after 24 hours at 120°C.
• ASTM B 117 is the salt spray test. ALSTRONG RC is stated at 340 hours at 25 microns.
• ASTM E 903 is the solar reflectance test used for the heat reflectivity figures of SUMMERSHIELD COOL 666 and RAINSEAL FRC 623.
• IS 277 covers galvanised steel sheets and IS 459 covers corrugated asbestos cement sheets.

Sheet roof coating quantity calculator

Enter the roof size to see the coating needed. Quantities only. Coverage figures are for a plain surface.

Eight mistakes that make a sheet roof leak again

1. Coating without sealing the laps. A coating does not bridge an open lap or a loose bolt. The leak points are sealed first.
2. Painting over loose rust. The film lifts with the rust and the metal keeps corroding under it. Loose rust is removed and the rest treated.
3. Sealing a loose fastener. The bolt keeps moving and breaks the seal. Tighten or replace it first.
4. Coating a damp or oily sheet. These coatings need an absolutely dry, clean surface to bond.
5. Leaving out the mesh at laps. Laps and fastener lines move the most. A strip of fibre mesh as layer 2 carries that movement.
6. Using a rigid filler. Cement mortar or hard putty on a sheet roof cracks in the first summer. The seal has to stay flexible.
7. Dry-grinding AC sheets. It releases fibres and damages the sheet. Clean them wet and gently.
8. Working just before rain. The coatings need days to cure. Rain on an uncured coat washes it out.

Metal and AC sheet roof waterproofing questions and answers