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The joint width the listed system is installed at, before any movement.
The listing states a maximum nominal width and the system is qualified only up to it. Measure the gap as built, at the point it is widest along the run — a head-of-wall gap that varies with deck flute depth is not one width but a range.
The percentage of its nominal width the listed system is qualified to open and close.
Systems are commonly listed at 12.5, 25, 33 or 50 percent, and the class was demonstrated by cycling the joint through that range before the fire test. A joint that will move more than its class is outside its listing, and the listing is the only thing the rating rests on.
Total length of joint to be treated with this detail.
Head-of-wall joints run the full length of every rated partition, which is why the quantity surprises people: a modest floor plate can carry several hundred metres of it once every corridor and shaft wall is counted.
How deep the mineral wool packing sits in the joint.
The packing is the part that actually stops fire passing through the gap; the sealant over it is the smoke and gas seal and the surface that moves. The depth is stated in the listing and is not a site decision.
How far the packing is squeezed below its supplied width when it is packed in.
A listing normally calls for the mineral wool to be compressed by a stated percentage, which is what keeps it in place and closes the gap as the joint moves. Twenty-five percent means material a third wider than the joint goes into it — that surplus is real material to buy, and it is the line most take-offs miss.
The thickness of sealant over the packing, as the listing specifies it.
Thicker is not better. A firestop sealant bead has a specified depth because a bead too deep cannot accommodate the movement it was cycled through, and one too thin loses its smoke seal. Take the number from the tested system rather than from the tube.
Whether the listed system seals one side of the joint or both.
Two-sided systems are common where the rating is required from both directions, and they double the sealant without doubling the packing. Which applies is stated in the listing, and it is one of the details most often changed on site without anyone checking the drawing.
Firestop sealant for the run
0.988 gal
Sealant is quantified at the nominal width, which is what a run is installed at. The figure at the widest in-service width is given alongside because a joint that opens under thermal or seismic movement takes more material where it is topped up, and because it is the width the detail has to work at.
- Narrowest width in service
- 0.56 in
- Widest width in service
- 0.94 in
- Total movement the joint absorbs
- 0.37 in
- Backing material before compression
- 40.52 gal
- Uncompressed backing width required
- 1 in
- Sealant if the run were at its widest
- 1.23 gal
They open the calculator with your figures already in it
Firestop Joint Movement and Sealant Volume Calculator: 0.9877 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- UL 2079 Tests for Fire Resistance of Building Joint Systems — a joint system is qualified for a movement class stated as a percentage of its nominal width, and is cycled through that range before the fire and hose-stream exposure
- ASTM E1966 Standard Test Method for Fire-Resistive Joint Systems — the companion test method covering the same cycling and exposure, and the standard many listings are published against alongside UL 2079
- Volume is geometry, not a published rate: sealant = nominal joint width x listed fill depth x run length x number of treated faces; backing = the uncompressed width needed to reach the specified compression, x backing depth x run length. The fill depth and the compression percentage both come from the tested system's own listing
Inputs used
- Nominal Joint Width
- 0.75 in
- Movement Class (% of nominal width)
- 25
- Length of Joint Run
- 195 ft
- Backing Material Depth
- 4 in
- Backing Compression (%)
- 25
- Sealant Fill Depth
- 0.13 in
- Faces Treated
- One face — sealant to a single side of the joint
Intermediate steps
- Narrowest width in service
- 0.56 in
- Widest width in service
- 0.94 in
- Total movement the joint absorbs
- 0.37 in
- Backing material before compression
- 40.52 gal
- Uncompressed backing width required
- 1 in
- Sealant if the run were at its widest
- 1.23 gal
Confidence note: Sealant is quantified at the nominal width, which is what a run is installed at. The figure at the widest in-service width is given alongside because a joint that opens under thermal or seismic movement takes more material where it is topped up, and because it is the width the detail has to work at.
What this calculation does not cover
- Quantities only. Whether a system suits your joint is decided by its listing, not by arithmetic.
- The listed fill depth and compression are inputs taken from that listing; nothing here validates them.
- Deck flutes at a head-of-wall joint hold considerably more material than a flat soffit of the same run.
- Primer, backing pins and mechanical retention where a listing requires them are separate items.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- UL 2079 Tests for Fire Resistance of Building Joint Systems — a joint system is qualified for a movement class stated as a percentage of its nominal width, and is cycled through that range before the fire and hose-stream exposure
- ASTM E1966 Standard Test Method for Fire-Resistive Joint Systems — the companion test method covering the same cycling and exposure, and the standard many listings are published against alongside UL 2079
- Volume is geometry, not a published rate: sealant = nominal joint width x listed fill depth x run length x number of treated faces; backing = the uncompressed width needed to reach the specified compression, x backing depth x run length. The fill depth and the compression percentage both come from the tested system's own listing
Which documents these citations point at
Standards referenced: UL 2079 (UL Solutions, United States); ASTM E1966 (ASTM International, United States).
Cite this page
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At its listed class this joint works between 14.3 mm (0.56 in) and 23.8 mm (0.94 in) — a total swing of 9.5 mm (0.37 in) that the sealant has to survive without splitting or losing adhesion. The packing behind it has to be 33% wider than the gap before it is compressed into place, which is 153.4 L (40.5 gal) of mineral wool for this run and the line a take-off written off the joint width alone will be short by.