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Concrete roof decks

Coating a concrete roof deck starts at the cracks

On exposed concrete, three things decide the job: a slab the engineer calls sound, a surface cured and dry enough to coat, and whether water drains or stands. Under acrylic the order runs: repairs, primer, fabric at every crack and joint, base coat, finish coat; Silicone44 goes straight onto cured concrete. Every layer has its Sherwin-Williams part number, in stock, shipped in 1–3 business days.

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Gloved hands brushing white coating over a strip of reinforcing fabric, an open pail beside them

Candidate or not

Coat the deck only if the slab is sound

On a concrete deck the trouble starts at the details: cracks across the slab, joints whose sealant has let go, spalls, failed sealant at penetrations and water standing in low areas. A coating system closes those over a slab that is still sound; it does not repair the slab. This build-up is for exposed concrete, bare or coated before; either way there is no insulation on top for a thermal scan to read, so the deck is surveyed on foot. Where the concrete sits under insulation and a membrane, that roof decides the system and the survey.

  • Structurally sound: deep spalls, deflection or a deck that moves are the building's engineer's call
  • Cured, clean and dry: for Silicone44, Uniflex asks for at least 29 days of cure, moisture of 15% or less, and sealers and release compounds removed
  • Cracks, joints and small holes that can be repaired one by one before anything is coated
  • Penetrations, curbs and drains that can be resealed and detailed, counted at the survey, not mid-job
  • Water that leaves at the drains, or low areas that can be corrected; where it still stands, the finish changes

Can this roof be restored?

In section

The build-up, from the deck up

Read it from the bottom. This is the acrylic system. Patching cement closes the holes and cracks in the slab, and a primer made for concrete goes over the clean deck. Fabric reinforces every crack, joint and transition, and sealant closes the metal penetrations. The base coat covers the field and the details, and the finish coat is what the weather sees. Where water stands, Silicone44 goes straight onto the cured concrete instead, and the primer and the base coat drop out.

Concrete deck, acrylic system

Section

  • Finish coat White acrylic KST041300 where the deck drains
  • Base coat Gray acrylic KST041320 or KST041520
  • Fabric and sealant KST20385A at cracks and joints; KST058300 at pipes
  • Concrete primer Acrylic KST036500
  • Repairs Patching cement KST041220 in holes and cracks
  • Concrete deck Sound, cured, clean and dry
Order of application, read from the deck up. Illustrative diagram — proportions are not to scale.

Step by step

The order of work, deck to finish

Part numbers are Sherwin-Williams Uniflex. Each product's data sheet names the preparation, the rate and the cure, and a written specification wins over this list.

  1. Survey the deck and lay a test patch

    Walk the deck and mark every crack, joint, spall, penetration and low area on a plan of the roof; that plan becomes the material count. Lay an adhesion test patch of the system before the full order. Anything structural goes to the building's engineer.

  2. Clean and repair

    Laitance, curing compounds, oil and loose material come off, and the deck dries as the data sheet requires. Close small holes, splits and cracks with acrylic patching cement KST041220. A deep spall, or a crack that keeps moving, goes back to the engineer before anything is coated.

  3. Prime the concrete

    Under the acrylic system, prime with acrylic concrete primer KST036500 where the data sheet calls for it; it is listed for concrete and for surfaces with a pH of 6 to 13. Concrete parapets and curbs take the same primer as the field. Silicone44 takes no primer on concrete.

  4. Detail cracks, joints and penetrations

    Reinforce cracks, joints and transitions three-course: a coat, polyester fabric KST20385A, a coat. Seal pipes, vents and other metal penetrations with MS hybrid sealant KST058300, which is listed for metal; its data sheet says what the concrete side of the bead needs.

  5. Base coat

    Lay the base coat over the field and the detailed areas: gray acrylic elastomeric KST041320, or the commercial grade KST041520. Both are listed as a base coat on concrete.

  6. Finish coat

    Where the deck drains, finish in white acrylic elastomeric KST041300. White finish coats reflect sunlight and keep the roof surface cooler. Where water still stands after the low areas are corrected, the system changes to Silicone44, with no primer and no base coat.

The adhesion test patch

By part number

The materials for this deck

Genuine Sherwin-Williams Uniflex under its own part numbers, in the maker's sealed packaging, in stock and shipped in 1–3 business days. Silicone44 KST044300 (white) goes straight onto cured concrete where water stands. Everything is priced per project.

Ponding and low areas

Where water stands, the finish changes

Acrylic is the finish for a deck that drains: under prolonged ponding it can lose adhesion and peel. Correct the low areas first where you can. Even the silicone's own data sheet says to remove ponding, because standing water adds load to the deck, and whether this slab carries it is the building's engineer's question. Where water will still stand, Silicone44 goes directly onto clean, cured concrete, with no primer and no base coat, once an adhesion test passes.

  • Silicone44 KST044300 (white) or KST044320 (gray), straight onto the concrete: KST036500 and the acrylic base coats stay out of this build-up
  • Uniflex asks for concrete cured at least 29 days, moisture of 15% or less, and sealers and release compounds removed
  • Wider cracks are reinforced with polyester fabric KST20385A; Uniflex specifies its own sealant for the cracks, which we do not stock, so ask us
  • At curbs and penetrations, Uniflex's silicone specifications use its OneFlash sealant, which we do not stock either
  • Silicone44 does not fix drainage: Uniflex's silicone specifications still expect at least 1/4 in per ft of slope and working drains
  • The solvent-based pair, KST042200 and KST042100, does not list concrete, so it stays out of this build-up

Ponding water and roof coatings

Questions

Before you coat the deck

Can a drone thermal scan survey a concrete roof deck?
Not a bare one. A thermal scan looks for wet insulation under a membrane: it holds the day's heat longer than dry insulation and shows warmer after sunset. A bare or directly coated deck has no insulation on top to read, so it is surveyed on foot, with a test patch. Where the concrete is the deck under an insulated single-ply, modified bitumen or smooth built-up roof, that roof decides, and our drone moisture scan suits it unless it is ballasted, gravel-covered or already coated white. Lightweight insulating concrete holds moisture of its own and makes a thermal survey difficult.
Can new concrete be coated?
Once it has cured. For Silicone44, which goes directly onto the concrete, Uniflex asks for concrete cured at least 29 days, moisture of 15% or less, and sealers and release compounds removed. Under the acrylic system, the concrete primer KST036500 is listed for surfaces with a pH of 6 to 13: concrete is alkaline, and how alkaline decides whether a coating grips. Test the deck's dryness and pH as well as counting its age, and run an adhesion test before the order.
Can I coat over an old coating on the deck?
If the old coating is sound and still bonded, the adhesion test patch decides. Clean it, lay the system on a small area, let it cure as the data sheet requires, then pull it. Silicone44 goes directly over a sound, aged acrylic, silicone or urethane roof coating once it is washed, loose coating is scraped back and the adhesion tests pass. If nobody knows what the old coating is, test before you order: contractors commonly work to the rule that little sticks to cured silicone except more silicone.
How much material does a concrete deck take?
It depends on the deck area, the rate the data sheet names for each coat, and the detail lengths. The material estimator turns area and rate into gallons, pails and drums. Fabric is counted from the linear feet of cracks, joints and transitions, and a roll of KST20385A runs 324 ft; sealant is counted from the number of penetrations. Send the numbers through the quote form and we reply within one business day.
How long does a coated concrete deck last, and is there a warranty?
Applied over a sound, dry roof, a restoration system adds 10 to 15 years of service life. On concrete, sound means a slab the building's engineer is satisfied with, and every crack and joint closed and reinforced before the field is coated. Sherwin-Williams offers UNIGUARD system warranties of 10 or 15 years when one of its authorized contractors applies the system. The warranty is the manufacturer's, issued under its own terms, and the finished roof is inspected before one is issued; we supply the materials.
Who applies the system?
A roofing contractor; we never do the roofing work ourselves. If you have one, we supply the materials to them. If you do not, we work with a network of installers, roofing contractors who apply these systems, and can introduce you to one anywhere in the US, who looks at the roof and says whether it is a good candidate for restoration. Roofing and coating contractors who apply these systems can ask to join the network through the quote form; a person answers.

Will a thermal scan work on your roof?

Get the concrete deck counted and priced

Send the deck area, the linear feet of cracks and joints, a count of penetrations and whether water stands anywhere. The quote comes back with the price, the freight and a ship date. If you have no contractor yet, say so, and we can introduce you to an installer anywhere in the US.

We reply within one business day.

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