A low-slope section over a porch, a bay or an addition often gets a liquid coating instead of a membrane, and the spec for that coating usually gets written as one line: roof coating, two coats. That line does not tell a St. Louis homeowner what went on the roof, how thick it is, or what happens at the seams and edges, which is where these roofs leak first. If you are comparing bids on a flat or nearly flat section, the spec needs the product name, the dry film thickness and the detail work written out.
A coating is a surface system, not a paint job
A roof coating sits on a substrate such as a plywood or OSB deck, an older membrane, or an earlier coating. It bonds to that surface and forms a continuous film over it. The film can only be as sound as the layer underneath. A soft deck, a lifted seam or a flashing that was never lapped correctly will keep leaking under a fresh coat, and the new coat may hide the leak for a season before it shows up on the ceiling.
That is why the prep work belongs in the spec too. The surface has to be dry, clean and free of loose granules, dust and old failed coating before any material goes down. Ask the roofer whether the old surface will be pressure washed, scraped, or primed, and whether the manufacturer requires a primer for that particular substrate. A primer that the manufacturer does not require can still be a line item, so read the bid for it.
Chemistry is a category, the product is the spec
Coatings come in several chemistries, and each behaves differently in the heat and cold of a St. Louis year. Acrylic coatings are water-based and are generally easier to apply in mild weather. They are not built to hold standing water for long periods, so a low spot that stays wet after every rain will wear them faster than the field around it. Silicone coatings shed water well and tolerate ponding better, but they resist adhesion from other products, so a later repair or a different recoat can be difficult. Urethane and polyurea systems appear on some larger commercial work and are applied with equipment, which makes the crew and the rig part of the spec.
No chemistry is the answer by itself. The name that matters is the manufacturer's product line, because that product has a data sheet with its own substrate list, its own minimum thickness, and its own cure times. When a bid says silicone and stops there, you cannot check it against anything. When a bid names the product and the line, you can read the data sheet yourself and ask the roofer why the crew is following it or departing from it.
What the spec should name
- The manufacturer and the exact product line, not only the chemistry.
- The substrate the coating goes over, and the primer if the manufacturer requires one for that substrate.
- The minimum dry film thickness in mils, or the gallons per square the crew will apply, with the number of coats stated.
- The reinforcing fabric or mat used at seams, laps and penetrations, and the places it runs.
- The cure time before rain is allowed to touch the surface and before any foot traffic or a second trade goes up.
- The product warranty, in writing, from the party that stands behind the product, along with the conditions the warranty depends on.
Dry film thickness is the number that matters
A crew can say they applied two coats and still lay down a thin film. Coverage depends on how much material goes on each square and how evenly it is rolled or sprayed, and the eye is a poor gauge of either. Coating manufacturers publish a minimum dry film thickness for their systems, and that number is the one to put in the spec. Ask the roofer to state the minimum, and ask how it will be confirmed, whether by wet film gauge readings during application, by counting material against the square footage, or by both.
Thickness is uneven by nature. A coating that looks uniform from the ladder can run thin at the low corners where water collects, and it can run thick in the pools that formed while it was still wet. Ask the roofer where the crew will check thickness, and whether the check points include the low areas and the edges, not only the middle of each section.
Seams, edges and penetrations decide the leak
The field of a coated roof is usually the simplest part of the job. Leaks start where the film changes thickness or direction: the edge at the gutter, the seam where two sections meet, the curb around a vent or a drain, and the wall flashing where the roof runs into siding or brick. Each of these needs its own detail in the spec, because the crew treats them differently from the field.
Ask whether reinforcing fabric is embedded in wet coating at those locations, how far the fabric extends past the edge, and whether the edge is finished with metal or a termination bar. A spec that says the roof will be coated and the seams detailed does not tell the crew which detail to use or where it stops. Write the locations down on the roof plan, and make sure the roofer's scope names the same locations.
Drains and scuppers deserve their own line. A coating that ends a quarter inch above a drain outlet will hold water at the outlet, and water at the outlet is where the sealant fails first. If the section drains toward a gutter, look at where the gutter outlet sits relative to the coated edge, and whether the bid includes the work to make that transition watertight.
How St. Louis weather works on a coating
The freeze and thaw cycle is hard on any low-slope surface. Water that gets into a seam or under a loose edge freezes, expands, and lifts the film, and the next thaw leaves the gap open a little wider. The first winter often shows the problem, so the edges are the first place to check after a cold snap. A spec that treats the edge like the field does not give the crew any reason to build it differently.
Summer is the other half of the problem. A dark or bare low-slope surface can sit under strong sun for hours, and the surface can run well above the air temperature on a still afternoon. Light colors reduce that heat load. The color name alone is not enough, so if the manufacturer publishes a solar reflectance figure for the color, put that figure in the spec too. Colors can also shift in the sun over several years, so the spec should name the exact shade and the product it comes in.
Hail can dent a metal flashing or split a thin film, and a coating that has been struck by hail may look intact from the ground. After a storm, walk the coated section the same way you would walk the shingles, and photograph the seams, the edges and the drain area first. Keep the spec and the photo file together, so that anyone who looks at the roof later can match what was installed with what is there now.
Foot traffic, rain and the cure window
Many coating problems are scheduling problems. The product needs a set time to cure before rain can touch it, and that time depends on temperature and humidity on the day of application. A crew that runs ahead of a forecast front can wash out a fresh film. Ask the roofer how the cure window is handled, who decides whether the crew stops for weather, and how the cure time shows up in the schedule you signed.
Foot traffic is the other half of that window. Once the coating is down, anyone walking across it, including a second trade for a gutter or a vent, can mark or damage a film that has not set. The spec should say when the surface may be walked, who is allowed on it, and what protection the crew puts down for the trades that come after.
What to ask before you sign
- Which product, by manufacturer and product line, and where is its data sheet?
- What is the minimum dry film thickness for this system on this substrate, and how will the crew confirm it?
- Where does the reinforcing fabric go, and does it cover every seam, curb, drain and edge on the plan?
- How long is the cure window before rain, foot traffic or a second trade on the roof?
- Who holds the product warranty, and what conditions must be met to keep it in force?
- Who walks the coated section after it cures, and when will the walk happen?
Keys Roofing can walk a low-slope section with you before the coating is ordered, then put the product, thickness and detail lines into a written scope with photos and measurements. The office number is (314) 220-2333.