Storm season: how to read an impact approval

Every impact window sold in South Florida is sold on the strength of an approval, and almost nobody reads one. Storm season is a good moment to fix that, because the difference between two quotes is usually hiding in the document rather than in the brochure.

An approval is product-specific, not company-specific. A Notice of Acceptance covers one system, in one range of sizes, with one glass make-up, at one design pressure. A manufacturer can hold an excellent approval and still ship you a unit outside it — larger than the tested size, or glazed differently. The approval is only a promise about the configuration it describes.

Design pressure is the wind load, in pounds per square foot, that the unit was tested to hold in both directions. It is assigned to your building by the engineer of record from the exposure, height and zone. A higher number is not better in the abstract; it is only better than the number your building requires.

Missile level is the debris test. Large Missile Impact — a nine-pound timber at roughly fifty feet per second, followed by cyclic pressure loading — is what the High Velocity Hurricane Zone requires below thirty feet. Small Missile applies higher up the elevation. Both are pass or fail; there is no partial credit.

Tested size is the maximum width and height the approval covers. Ask for the tested size next to your largest opening, not the catalogue maximum. Where an opening exceeds it, the honest answer is a mullion or a different system — not a quiet upsize.

Four questions get you most of the way: Which approval number covers this exact unit? What is its tested maximum size against my largest opening? What glass make-up is priced, and is it the one that was tested? When does the approval expire relative to my permit date?

Fortwell publishes the approval number, design pressure, tested size and expiry on every system page, and issues jurisdiction-specific approval packages on request. If a supplier cannot produce the same four figures for the unit they are quoting, that is the answer to the question.