Glass in fire doors: sizes and labels
A two-page glazing handout from Anemostat that Donald keeps coming back to. How big the glass can be, how a light kit is sized, and how to read the mark etched in the corner.
Here is an old handout from Anemostat that I still pull out every now and then. It is only two pages, but it covers most of the questions we get about glass in fire doors: how big it can be, how a light kit is sized, and what all those letters on the glass label mean.
Glass is usually the weak spot of a rated door. If the glass or the kit isn’t right, the rest of the door doesn’t matter much.
Glass in a fire door must carry its own label, be rated for the door it is in, be safety glass too, and sit in a labeled light kit (or the door’s listing). An opening is only as good as its lowest-rated part.
What the code says
Three kinds of glazing
The handout sorts glass into fire-rated only (walls, transoms, borrowed lights), safety/impact-rated only (non-rated doors and other hazardous locations), and fire AND safety rated. Glass in a fire door has to be the third kind: NFPA 80 (2013, 4.4) allows only labeled fire protection- or fire resistance-rated glazing, where the door listing permits it, and it must also meet the impact safety rules.
The opening takes the lowest rating
An assembly is rated by its weakest part (NFPA 80, 2013, A.6.3.1.1; and 6.3.2 for frames that hold glass). Put 20-minute glass in a 90-minute door and you do not have a 90-minute opening any more. In a wall that needs 90 minutes, that is a deficiency, not a smaller rating.
How big the glass can be (NFPA 80, 2013, Table 4.4.5)
| Door rating | Glazing limit per leaf |
|---|---|
| 20 min to 1½ hours | The largest area the manufacturer tested |
| 3 hours | 100 sq in. (0.065 m²) |
| Any single exposed light | 1296 sq in. (0.84 m²), no side over 54 in., unless tested larger |
A footnote to the table bars fire protection-rated glass over 100 sq in. in temperature-rise doors (typically stair doors). The handout’s “100 sq in. on 60/90 minute doors” comes from that rule and the old wired-glass days; today 1- and 1½-hour doors go by the tested area, so check the door and glass listings.
Cutout, glass size, visible lite
Listings limit the VISIBLE lite — the glass you can see inside the kit — by area and by maximum width and height. Anemostat’s rule of thumb for its low-profile kit: order the door cutout, the glass is about 1 in. smaller, and the visible lite about 2 in. smaller. Other kits differ, so size from the kit maker’s sheet, not this rule.
Reading the glass label
Every pane carries a permanent mark you can see after it is installed (NFPA 80, 2013, 4.4). The IBC marking key (Table 716.1(1)): D = meets fire door criteria, H = passed the hose stream, T = meets the 450 °F temperature-rise limit for 30 minutes, OH = fire window/sidelight/transom criteria including hose stream, W = wall assembly criteria, and the number = minutes. NT on a label means it is NOT temperature-rise rated. The safety marks are separate: 16 CFR 1201 Cat II or ANSI Z97.1. So “D-H-NT-90” = door glass, hose stream tested, not temperature rise, 90 minutes.
The light kit has a label too
The frame that holds the glass is listed on its own (an embossed label on the kit). Vision panels go in labeled light kits or per the door listing, installed to the maker’s instructions (NFPA 80, 2013, 4.4). Broken glass is replaced with labeled glazing installed per its listing (5.5).
Practice drill: check it on your next round
- Find the etched or printed mark in a corner of every pane. No mark = finding, even if it looks like wired glass.
- Does the minutes on the glass match (or beat) the door label?
- Stair door with a big lite? Check it is temperature-rise rated or fire resistance glass, not plain fire protection glass over 100 sq in.
- Light kit frame labeled, beads tight, nothing loose or missing (one of the 13 annual items, NFPA 80 5.2.3.5.2).
- Replaced glass after a break? It must be labeled fire-rated glazing, not hardware-store safety glass.
Sources and further reading
- Anemostat — “Teach me about Glazing” and “Labeling” handout (shared by Donald) ↗· shared by Donald
- NFPA 80 (2013) — 4.4, Table 4.4.5 and 5.5, decoded
- IBC (2024) — 716.1, glazing marks
Section numbers are from NFPA 80 (2013) and NFPA 101 (2012), the editions CMS and The Joint Commission enforce for hospitals. Check the edition your authority having jurisdiction has adopted.