Gap 90: Fixing an Oversized Fire Door Gap
NGP’s GAP90 kit and other listed gap kits — when they are the right fix, and when they are not

The gap that a gasket can’t fix

If you walk fire doors for long enough, you meet this door. It closes, it latches, the label is there, and the gauge still slides into the gap at the latch edge with room to spare. Somebody will tell you to stick a gasket on it. Somebody else will say the whole door has to come out.

There is a middle answer, and in the field it often goes by one name: “Gap 90.” That is National Guard Products’ GAP90, part of its GapGuard line — a kit tested and certified to bring an oversized head or jamb gap back under control. This article explains what it is, what it covers, where it does not belong, and what to check when you find one already on a door.

Measure first

NFPA 80 sets the clearance at the head, the jambs and the meeting edges of a pair: 1/8 in. ± 1/16 in. for steel doors, so 3/16 in. is the most you can have, and no more than 1/8 in. for wood doors (6.3.1.7.1 in the 2013 edition). Measure from the pull side (6.3.1.7.2). Under the door the limit is 3/4 in. (4.8.4). Measure the whole length of every edge — a door that sags is tight at the top of the latch edge and wide at the bottom.

Latch edge of a wood fire door. Wood allows no more than 1/8 in. Field photo.

Meeting edges of a steel pair. Steel allows 1/8 in. ± 1/16 in. Field photo.

Write down the largest gap, where it is, the door material, the rating on the label and whether a closer arm sits at the head. Those five things decide everything below. Our Gap Axe gauge has the steel and wood limits marked on the blade.

What over the limit looks like: you can see the floor through the meeting edges of this pair. Field photo.

The Gap Axe’s 3/8 in. step sits in that gap with room to spare. No door may have more than 3/16 in. here, and GAP90-ME stops at 3/8 in., so a gap this wide may be past a kit altogether. Field photo; the gauge was moved slightly left in editing, to where it should have been placed against the left door.

Ask why the gap opened

A kit closes a gap. It does not stop the door from moving. Before you order anything, find the cause:

• Loose or stripped hinge screws. The most common one. The door drops at the latch side and opens a wedge. Tighten or replace the screws.
• Worn hinges. Replace with hinges of the size the door needs.
• The door sits off-center. NFPA 80 lets you shim hinges with steel shims when that is what it takes to meet the clearances (6.4.3.4 in the 2013 edition). Often that alone brings a door back in.
• The frame has moved — racked, bent, pulled loose from the wall. That is a frame repair, and a kit on a moving frame will be out of range again soon.
• The door was never right — replaced with the wrong size, or the floor finish was taken up under it. Those gaps are steady, and they are where a listed kit earns its place.

That last list item is the honest test: is the gap steady? If yes, a kit can be the fix. If the door is still moving, fix the movement first and measure again.

What GAP90 is

GAP90 goes on the frame, not the door. It has three parts, and NGP’s cut sheet says all three must be installed:

• 9945 steel stop extender — a 16-gauge steel piece, gray primer, screwed to the frame with #10 × 1 in. TEK screws and embossed with the UL mark. It narrows the opening.
• 9400 intumescent seal — a self-adhesive strip (9400GR gray or 9400C charcoal) that swells in a fire and closes what is left.
• 5050C silicone bulb seal — the smoke seal the door closes against.

NGP’s current cut sheet (May 2026) certifies it for head and jamb gaps over 1/8 in. and up to 9/16 in., on hollow metal doors rated up to 180 minutes and wood doors rated up to 90 minutes, tested under UL 10C (positive pressure). It comes in 36, 48, 84 and 96 in. lengths and is non-handed. A strike shim may also be needed so the latchbolt still engages properly.

The rest of the family

From NGP’s current GapGuard catalog and the May 2026 GAP90 cut sheet. GAP90 is one brand; other makers sell listed gap fillers and listed edges too. The same checks apply to any of them.

Door-mounted pieces on real doors

These are from a recent walkthrough. They show the door-mounted side of the family, and their shape matches NGP’s drawings for its 9990 (door top) and 9590 series (door bottom). No maker’s mark is readable in these shots, so that match comes from the shape, not from a label. To be sure, read the UL label on the piece: NGP’s GapGuard products carry file number R25568. We don’t yet have a field photo of the frame-mounted GAP90 stop extender.

Door top: a stainless piece screwed across the top edge of a wood door that also carries a closer. NGP points to its 9990 for doors with parallel-arm closers.

The same piece from its end. NGP’s 9990 drawing shows this build: an L-shaped cap over the top with a flat plate on the other face. It also shows end caps, and this end has none, so check any installed piece against its own sheet.

Door bottom: a stainless piece along the bottom edge, for a door that sits too high off the floor. Stainless also runs up the latch edge.

The bottom corner up close, with the same build as NGP’s 9590 drawing: a J-shaped shoe on one face and a plate holding a black sweep on the other. This end has no end cap either. The door must still latch, bottom latch included if it has one.

What NFPA 80 says, by edition

The clearance limits have stayed the same for years. What changed is how plainly the standard says an add-on product may fix a gap that is too big. That matters, because the edition your inspector reads depends on who they are.

What this means in practice:

• Under the 2025 edition a listed accessory for excess clearance is permitted in so many words (6.3.1.8.3 for the head, edges and meeting stiles; 4.8.4.3 for the bottom).
• Under older editions — including the 2010 edition CMS surveys hospitals against — no sentence names head and jamb gap products. A few years ago some door experts called these fixes “not authorized by NFPA 80.” The argument for using one is that it is a tested, certified accessory installed exactly per its instructions, not a change to the door. Get the AHJ to accept it, in writing if you can, and keep the product’s instruction sheet with the door records.
• Gasketing alone is never the fix. Gasketing on a fire door has to follow a listing (6.4.8 in the 2013 edition), and ordinary seals, sweeps and door shoes are not tested to make an oversized gap safe.

When a kit is the right fix — and when it isn’t

Try it: which fix fits this door?

Installing it right

From NGP’s 2023 GAP90 installation instruction. Follow the sheet that comes in the box; the details can change between versions.

• Cut the steel extender to length. Where a parallel-arm closer shoe or a rim or surface vertical-rod exit device strike is in the way, cut it and leave it out at that spot.
• Screw it to the frame stop with TEK screws in every mounting hole.
• Clean the surface (above 50 °F) before the self-adhesive seals go on. Press them in place and do not peel them off to move them.
• Cut the seals around the frame’s fire label — don’t cover the label.
• Run the seals the full length, except at the latchbolt.
• Make sure the seals don’t stop the door closing and latching. Add a strike shim if the latchbolt no longer engages properly.

Then test the door: open it fully, let it go, and watch it close and latch on its own. Write it down — NFPA 80 wants a record of the work and the retest.

Finding one on your round

A kit that is already on a door is part of the assembly now. Check it like the rest of the door:

• All three parts are there: the steel extender with the UL mark, the intumescent strip and the bulb seal. A seal on its own is not the kit.
• Measure the gap now. It must still be inside the kit’s range — if the door kept sagging, it may not be.
• The seals are intact, still stuck down, not torn, and not painted over.
• The frame label is still readable.
• The door closes and latches from fully open. On a pair, the coordinator still works.
• There is a record: which product, which instruction sheet, who installed it, and the AHJ’s acceptance if your edition needs it.

Writing it up