How to read a metal roof finish warranty

Two documents come with a metal roof: the workmanship warranty from the installer, and the manufacturer's warranty on the material. The material warranty usually splits again into the steel and the paint. The paint document is the one almost nobody reads, and it is the one that decides what the roof looks like in year twenty. Here is how to read it.

The three numbers

A finish warranty usually gives you three separate figures, and they are not interchangeable.

Film integrity — how long the coating is warranted not to peel, crack, check or blister. This is the headline number. On a good PVDF coating it is often 40 years.

Chalk — how long before the resin is allowed to break down at the surface and leave a white powder. Measured against a number on a scale (ASTM D 4214). A coating is warranted not to chalk worse than, say, a rating of 8 for 30 years.

Fade — how much color shift is allowed, in ΔE units, over a set period (ASTM D 2244). Five units over 30 years is the kind of figure you see on a 70% PVDF finish; larger numbers over shorter periods on polyester.

A "40-year warranty" that turns out to be 40 years of film integrity with 10 years of chalk and fade is a very different promise from one with 30 years on all three. Read all three.

PVDF versus SMP versus polyester

This is where the numbers come from. A 70% PVDF fluoropolymer holds color and gloss the longest and chalks the least; its warranty numbers are the longest. A silicone-modified polyester — WeatherXL from Sherwin-Williams, Ceram-A-Star 1050 from AkzoNobel, the two we buy — is harder and cheaper and gives shorter chalk-and-fade terms, especially on dark colors. A plain polyester is shorter still.

The tradeoff is real: PVDF is a softer film — it can scratch in handling — and it costs more. SMP has closed most of that gap, which is why it is what we put on most homes; PVDF is what the interlocking shingles carry, and it is available on 24-gauge standing seam steel if you want it.

What the tests mean

When a data sheet lists ASTM standards, this is what they are checking. Color change (D 2244). Chalk resistance (D 4214). Gloss retention (D 523). Dew-cycle weathering (D 3361) and humidity (D 2247) — does it blister in wet conditions. Salt spray (B 117) — corrosion. T-bend (D 4145) — can it be formed without cracking. Impact (D 2794), abrasion (D 968), pencil hardness (D 3363) and cross-hatch adhesion (D 3359). A coating that has been run through all of these and published the results is telling you something; one that just says "premium finish" is not.

The exclusions worth finding

• Cosmetic hail denting — almost always excluded, and often by your insurer too.
• Damage from other trades, pressure washing, or chemicals.
• Within a few miles of salt water, or in industrial fallout.
• Unpainted cut edges and scratches through the coating, which can creep.
• Pro-rating: some finish warranties pay less every year. Some asphalt warranties pay less every month. Find the word.

What this changes

When two quotes are a few thousand dollars apart, look up the coating named on each. If one is PVDF and one is SMP, you are not comparing the same roof. This is why system choice is only half the decision.