Fire-Rated Glazing Typical Edge Clearance Reference

Frequently asked questions

Is this edge clearance value calculated from the glass and frame properties?

No. Fire-rated glazing edge clearance is not governed by a calculable thermal-expansion formula — it is fixed by the specific assembly's UL 9/UL 10B/UL 10C listing and NFPA 80/257 requirements. The figures here are commonly-cited typical values for reference only, not a substitute for the tested listing.

Can I use this value directly for installation?

No — always verify the exact bite/clearance required against your specific product's UL listing and the manufacturer's installation instructions before installing. Using a different clearance than what was tested and listed voids the fire rating of the assembly.

What happens if my exact glazing/frame combination isn't in the table?

This lookup defaults to the hollow-metal frame value for the selected glazing type when a specific combination isn't listed, but that default is only a rough reference point — the manufacturer's actual listed clearance for your specific frame type must still be confirmed.

Does the answer change with the fire rating — is a 20-minute lite treated the same as a 90-minute one?

It is treated the same, and that is the honest limit of this page. Two selections go in and nothing else, so a 20-minute door lite and a 90-minute window come back with the same figure whenever the glazing category and frame material match. Rating duration is what drives the things that actually differ between those two openings: the maximum exposed glass area the listing permits per lite, the glass make-up and thickness, and the frame profile it was tested in. The three glazing options also cut straight across a larger split — fire-protection rated glazing, tested as an opening protective and generally doing nothing about radiant heat coming through the glass, versus fire-resistance rated glazing tested as a wall. Multi-ply intumescent products commonly sit on the second side of that line and ceramics commonly on the first, and this page never asks which one you are holding. There is no design step here waiting to be finished by arithmetic: the figure you build to is printed in the listing for that product at that rating, and on an opening that gets inspected it is the label that gets read.

The frame options are hollow metal and aluminum — does picking aluminum mean an ordinary storefront frame will do?

No, and that is the assumption hidden inside the option. Fire-rated aluminum framing is a specific tested product family, and the listed systems are commonly built either as a steel sub-frame wearing aluminum cover caps or as aluminum extrusions filled with a fire-blocking insulating core, because bare aluminum has lost most of its strength long before a furnace test reaches its peak temperatures. A standard commercial storefront extrusion does not become rated by having rated glass set into it, and hollow metal gets no free pass either — the frame has to be a labelled fire-rated frame, not simply a steel one. All the selector does is move the typical figure one step: aluminum returns a smaller figure than hollow metal for the same glass. For two of the three glazing types that step lands on a figure the table already carries — ceramic in aluminum returns what wired glass in hollow metal returns (9.5 mm, restated as 0.37 in when the page is set to imperial), and multi-ply in aluminum returns what ceramic in hollow metal returns. Wired glass in aluminum is the exception: it drops below every other figure on the page rather than landing on one of them. Before any of it matters, confirm the frame carries its own label for the rating you need and that glass and frame are listed together, because a listing commonly pairs one glazing product with one framing system.

What actually goes wrong with this dimension on site?

Three things, roughly in this order. The wrong dimension gets used: edge clearance is the gap between the edge of the glass and the back of the glazing pocket, edge cover or bite is how far the stop laps the face of the glass, and face clearance is the gap between the glass face and the stop. Those are three different gaps, and the trade quotes them loosely — bite in particular gets written as though it were another word for edge clearance, which it is not. A listed detail gives them separately and this page hands back one number under one label, so work out which dimension your detail is naming before you compare anything to it. Then the glass gets ordered before the detail is read. Fire-rated ceramic and multi-ply lites are cut at the factory rather than trimmed on the bench, and on a multi-ply product the sealed edge is part of the assembly, so a lite that measures oversize is a re-order and a delay, not a morning's work. Last, the pocket gets filled with whatever is in the van: the setting blocks, glazing tape and compound are named in the listing alongside the glass, and swapping them is the same class of change as swapping the glass. Read the numbers as fractions rather than as precision, too — they are a quarter, three-eighths, a half and five-eighths of an inch converted, and then rounded a second time for display because the page rates its own confidence as low. A result reading 16 mm — or 0.63 in with the page set to imperial — is five-eighths of an inch either way, not a millimetre-accurate requirement.