Elastomeric Roof Coatings: Types, Coverage and How to Apply

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An elastomeric roof coating is a liquid-applied coating that cures into a flexible, rubber-like membrane over an existing roof. “Elastomeric” means it stretches and returns to shape, so it can move with the roof as it heats up, cools down, and shifts without cracking. Most are applied with a roller or sprayer at a set thickness, and they are used to stop leaks, seal seams and fasteners, and extend the life of metal, flat, and low-slope roofs.

This guide explains the main types of elastomeric coating, where each one fits, how much you need, how to apply it, and how SANI-TRED compares.

What an elastomeric coating does

Roofs move. A metal roof can grow and shrink noticeably between a cold night and a hot afternoon, and seams, fasteners, and flashings take the stress. A rigid coating cracks at those points. An elastomeric coating stretches across them.

A good elastomeric coating does four jobs:

  • Seals small cracks, seams, and fastener holes with one continuous layer
  • Wraps around pipes, vents, drains, and rooftop units without cutting and lapping material
  • Protects the roof surface from sun and weather
  • Reflects sunlight, when the coating is light colored, which helps keep the building cooler

Elongation is the number to look for. It is the percentage a cured coating can stretch before it tears. Many roof coatings list it on the data sheet, and higher numbers mean more room for movement.

Types of elastomeric roof coating and where each fits

Acrylic

Acrylic coatings are water based, easy to clean up, and among the least expensive. They are usually white and reflect a lot of sunlight. Acrylics work best on sloped roofs that shed water quickly. On flat roofs where water ponds, many acrylics soften and lose their bond over time. They are also a poor choice for metal roofs that already have rust, because a water-based film can hold moisture against the steel.

Silicone

Silicone holds up well to UV and standing water, which makes it common on flat commercial roofs and over spray foam. The drawback shows up later. Very little bonds to cured silicone, so future repairs and recoats usually have to be silicone too. Silicone also tends to collect dirt, which lowers its reflectivity over time.

Polyurethane

Polyurethane coatings are tough and resist impact and foot traffic, so they are often used on roofs that people walk on. Some systems use an aromatic polyurethane base coat with an aliphatic (UV-stable) topcoat. Many polyurethanes are solvent based and have a strong smell during application, so check the product’s VOC content.

Butyl and other rubber coatings

Butyl rubber coatings are known for low vapor permeability and good water resistance, and they are used on flat and low-slope roofs and over single-ply membranes. Other liquid rubber coatings fall in this broad group too. SANI-TRED PermaFlex is a liquid polyurethane that cures to a flexible rubber and is solvent free and low VOC.

Quick guide by roof type

  • Sloped metal roof, no ponding: acrylic, silicone, polyurethane, or liquid rubber all work. Rust and loose fasteners must be dealt with first.
  • Flat roof with ponding water: silicone or a liquid rubber rated for standing water.
  • Roof deck with foot traffic: polyurethane or liquid rubber.
  • Spray foam roof: silicone or acrylic, as specified by the foam maker.
  • Plywood or concrete roof deck: a coating that bonds into the substrate. See how to waterproof plywood decks and roofs.

How much coating do you need?

Coating thickness is measured in mils (thousandths of an inch). The math is simple: one U.S. gallon of any liquid spread over 1,604 square feet is 1 mil thick (wet).

  • Square feet per gallon = 1,604 ÷ desired mil thickness
  • Mil thickness = 1,604 ÷ square feet per gallon

So a coating applied at 240 square feet per gallon goes on about 6.7 mils thick. At 80 square feet per gallon it is about 20 mils. These figures assume a flat, smooth surface. Rough concrete, corrugated metal, and porous surfaces use more. Water-based coatings also lose thickness as they dry, so a product’s dry mil spec can be well below its wet mil spec. Check the data sheet for both.

Use the mil thickness calculator to work out how many gallons a roof needs at the thickness you want.

How to apply elastomeric roof coating

1. Inspect the roof

Walk the whole roof and mark blisters, open seams, loose or missing fasteners, cracks, and soft spots. If the deck is rotten or insulation is wet, repair that first. A coating seals a sound roof. It does not fix a failing one.

2. Clean it

Sweep or blow off debris. Pressure wash to remove dirt, chalking, and loose material, then let the roof dry completely. On metal, remove loose rust down to sound steel. Remove peeling coatings, because a new coat is only as strong as the old layer under it.

3. Repair seams, fasteners, and penetrations

Seal open seams, tighten or replace loose screws, and build up flashing around pipes and vents. Most coating makers sell a matching mastic or a reinforcing fabric for this. With SANI-TRED, you mix two parts LRB (Liquid Rubber Base) with one part TAV (Thickening Activator) into a rubber patching compound and run a 3/4 inch bead on open seams and a 1 inch bead at wall junctions and around fixtures.

4. Prime if the product calls for it

Some coatings need a primer on metal, old asphalt, or single-ply membranes. SANI-TRED uses PermaFlex as the primer at about 240 square feet per gallon.

5. Apply the coating

Roll or spray in even passes and keep a wet edge. Start at the far side and work toward your exit so you never step in wet coating. Brush around pipes and fixtures. Apply a second coat at right angles to the first if the product calls for two coats. Check thickness as you go with a wet mil gauge.

6. Let it cure

Stay off the roof until the coating has cured. Cure times range from hours to days depending on the product, temperature, and humidity. Water-based acrylics need dry weather and temperatures above the maker’s minimum to cure properly.

Tips for a coating that lasts

  • Pick a dry day with no rain in the forecast for the product’s rain-safe window.
  • Avoid coating a hot metal roof in full afternoon sun, which can make some products skin over too fast.
  • Build extra thickness in low spots and at seams.
  • Keep drains and scuppers clear so water does not sit longer than it has to.
  • Inspect the roof each year and touch up damage early.

How SANI-TRED compares

SANI-TRED PermaFlex is a liquid polyurethane that cures to a tough, flexible rubber. It penetrates into concrete and wood and bonds to properly prepared concrete, wood, aluminum, steel, and most other building materials. The published specs are 590 percent elongation and 2,030 psi tensile strength, and the cured rubber stays flexible and gets stronger with age.

A few differences from typical roof coatings:

  • Standing water: SANI-TRED is waterproof, so ponding water does not soften it the way it can soften water-based acrylics.
  • Metal roofs: it seals screw and nail heads and seams and acts as a rust inhibitor in one coat. See the metal roof sealing comparison for photos of acrylic failures on steel.
  • Repairs: SANI-TRED uses what it calls molecular welding, so a damaged area can be repaired on its own without redoing the whole roof.
  • Safety: solvent free, non-flammable, and low VOC.
  • Warranty: the roof system carries a lifetime warranty.

The SANI-TRED process is prep, prime with PermaFlex, patch with LRB/TAV, and topcoat with PermaFlex at about 240 square feet per gallon. It can be brushed, rolled, or sprayed. The full process is in the roof sealing guide.

Frequently asked questions

How long does elastomeric roof coating last?

It depends on the type, the thickness, and the prep. Thin acrylic coats may need recoating in several years, while thicker silicone, polyurethane, and rubber systems last longer. Many makers recommend a recoat schedule, so check the warranty terms.

Can you put elastomeric coating on a flat roof?

Yes, if the coating is rated for ponding water. Many acrylics are not. Silicone and waterproof rubber coatings are better choices for flat roofs.

Is elastomeric roof coating waterproof?

Most are water resistant once cured, but water-based coatings can soften under standing water. A coating that is fully waterproof, such as a liquid rubber, holds up better where water sits.

How thick should elastomeric roof coating be?

Follow the maker’s spec. Many roof coatings call for 20 mils or more dry in total. Divide 1,604 by the target wet mils to get square feet per gallon.

Can elastomeric coating be applied over an old coating?

Only if the old coating is well bonded and compatible. Peeling material must come off. Silicone is the hardest to recoat with anything but silicone.

Can I apply elastomeric roof coating myself?

Yes. Most homeowners roll it on with a thick-nap roller. Wear fall protection, work on a dry day, and follow the coverage rate on the label.

Coat your roof with SANI-TRED

If you want a roof coating that stretches with the roof, bonds to metal, wood, and concrete, and does not break down under standing water, SANI-TRED is built for that job. It is DIY friendly, and our techs will help you plan the project.

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