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Video · Maintenance & ponding

Cleaning a Flat Roof Before You Coat It

From KARNAK Roof Coatings on YouTube · Our notes added

Video: KARNAK Roof Coatings on YouTube

Watch “How to Clean and Prep Roof using Karnak 799 Cleaner” on YouTube

  • Mod bit
  • Built-up
  • Metal
  • Spray foam
  • TPO
  • EPDM
  • Acrylic-coated
  • Silicone-coated

What this video shows

KARNAK, a long-time roof coatings maker, shows its cleaning and prep step using its 799 cleaner. Prep is the part of a coating job most DIYers rush, and it decides whether the coating stays bonded for years or peels off in sheets.

Key points

  1. Sweep and blow off loose debris before any washing.
  2. Use a roof-specific cleaner to cut dirt, chalk and mildew, then rinse completely, because residue blocks adhesion.
  3. Pressure-wash with a wide fan tip and moderate pressure. Don't blast granules off mod bit or gouge foam.
  4. Let the roof dry completely before priming or coating, including seams and low spots.
  5. Make repairs after cleaning and before coating.

Products in this video

The products used or shown, or the closest match we could verify. Typical price ranges, not live prices.

Roof cleaner / coating prep · $25–$50

Rust-OleumRust-Oleum One-Step Roof Prep Cleaner, 1 gal

Size
1 gal
  • Mod bit
  • Built-up
  • Rolled
  • Metal
  • Spray foam
  • TPO
  • EPDM
  • Acrylic-coated
  • Silicone-coated

A roof-specific cleaner for getting dirt, chalk and mildew off before a coating goes on. Most coating failures trace back to prep, and this is the cheap part of the job.

Read the fine print: Rinse completely and let the roof dry, because cleaner residue blocks adhesion. Follow the label for dilution and protecting plants, and check that your coating maker accepts it as prep.

Best for: Washing a roof before priming or coating

Check price on Amazon

Affiliate link — we may earn a commission at no extra cost to you. Coverage and specs are the manufacturer's stated figures. Disclosure

Gas pressure washer · $330–$430

SimpsonSimpson MegaShot 3100 PSI Gas Pressure Washer (MS61222-S)

  • Mod bit
  • Built-up
  • Rolled
  • TPO
  • PVC
  • EPDM
  • Spray foam
  • Metal
  • Silicone-coated
  • Acrylic-coated

Enough flow to wash a whole roof before coating in a reasonable time, with no outlet needed on the roof. Pair it with a roof cleaner and a wide fan tip.

Read the fine print: Full pressure up close strips granules off mod bit and gouges foam and soft membranes. Use a 25- or 40-degree tip, keep the wand moving, and stay well back from the edge, because the kickback plus a wet surface is a fall risk.

Best for: Washing a flat roof before priming or coating

Check price on Amazon

Affiliate link — we may earn a commission at no extra cost to you. Coverage and specs are the manufacturer's stated figures. Disclosure

Stiff push broom · $25–$45

LibmanLibman 825 Rough-Surface Heavy-Duty Push Broom, 24 in.

Size
24 in.
  • Mod bit
  • Built-up
  • Rolled
  • TPO
  • PVC
  • EPDM
  • Spray foam
  • Metal
  • Silicone-coated
  • Acrylic-coated

Stiff bristles clear loose granules, grit and leaves off a flat roof before you wash, inspect or coat.

Read the fine print: Go easy on old mod bit. Scrubbing hard strips granules you want to keep.

Best for: Sweeping debris and loose granules before inspecting or coating

Check price on Amazon

Affiliate link — we may earn a commission at no extra cost to you. Coverage and specs are the manufacturer's stated figures. Disclosure

Cordless leaf blower · $120–$180

DEWALTDEWALT 20V MAX XR Cordless Leaf Blower (DCBL722B, tool only)

  • Mod bit
  • Built-up
  • Rolled
  • TPO
  • PVC
  • EPDM
  • Spray foam
  • Metal
  • Silicone-coated
  • Acrylic-coated

Cordless, so there's no cord dragging across the roof. Use it to clear drains and scuppers, and to blow off dust right before primer or coating.

Read the fine print: Tool only: the battery and charger are sold separately. Watch your footing, and don't walk backward while you blow.

Best for: Clearing leaves from drains and the final dust-off before coating

Check price on Amazon

Affiliate link — we may earn a commission at no extra cost to you. Coverage and specs are the manufacturer's stated figures. Disclosure

Read the full guide

Coatings How to coat a flat roof Coating a flat roof is mostly prep: confirm the roof is dry inside, wash it, make repairs, reinforce seams and flashings with fabric, and prime where the label says, then roll or spray at the manufacturer's spread rate and check thickness with a wet-mil gauge. One gallon spread over 100 sq ft is about 16 wet mils, and the dry film is that times the coating's volume solids. Pick a window with no rain or dew for the label's cure time: often 24 hours or more for acrylic, as little as 15 minutes to a few hours for silicone.

Video by KARNAK Roof Coatings, embedded from YouTube. The summary, key points and cautions on this page are our own; KARNAK Roof Coatings doesn't endorse TheFlatRoofPro.com or the products listed. Roofing work for hire generally requires a licensed roofing contractor.