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Thermal movement, interstory drift and structural deflection added together as one number.
Add the three sources rather than taking the largest: a slab edge that deflects 3 mm under live load, a frame that drifts 4 mm between floors under wind, and a mullion that grows 5 mm on a hot afternoon can all be asking the same joint to move at once. A typical curtain wall head joint sees 6 mm (about 1/4 in); a joint at a movement break or a seismic separation sees far more.
The ASTM C920 class printed on the sealant's data sheet.
Class 25 is the workhorse polyurethane and the default here. Silicones qualified at Class 50 halve the joint width the same movement demands, which is often the difference between a joint that fits behind the cover and one that does not. Asymmetric classes such as 100/50 extend further than they compress and are not offered here, because a single symmetric number cannot describe them honestly.
How far the gap as built may differ from the gap as drawn.
This is the erection tolerance of the primary structure plus the setting tolerance of the wall, and it is added to the movement-driven width rather than shared with it. A cast-in-place slab edge is the usual culprit: 6 mm (1/4 in) is a normal allowance and a slab edge poured against a bowed form can want twice that.
Minimum designed joint width
1.25 in
- Width demanded by movement alone
- 1 in
- Construction tolerance added
- 0.25 in
- Backer rod diameter
- 1.56 in
- Sealant bead depth at mid-joint
- 0.5 in
- Movement the finished joint can absorb
- 0.31 in
They open the calculator with your figures already in it
Curtain Wall Perimeter Joint Width Calculator: 1.25 in — 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)
- ASTM C1193, Standard Guide for Use of Joint Sealants — a butt joint's width is sized so the anticipated movement stays inside the sealant's rated movement capability, with the construction tolerance added on top rather than absorbed by it
- ASTM C920, Standard Specification for Elastomeric Joint Sealants — the movement classes offered here (12½, 25, 35, 50) are that specification's own, and the class number is the percentage of installed joint width the sealant is qualified to move
- Backer rod diameter of 1.25 x the joint width is a RULE OF THUMB, stated as one: the rod must compress to stay put without being crushed, and the 25% oversize is the figure the trade works to rather than a value published in a standard
- Bead depth follows the 2:1 width-to-depth rule with a 6 mm floor and a 12 mm ceiling — again a rule of thumb, and again the sealant manufacturer's own data sheet governs where the two disagree
Inputs used
- Total Anticipated Movement
- 0.25 in
- Sealant Movement Capability Class
- Class 25 (±25%)
- Construction Tolerance Allowance
- 0.25 in
Intermediate steps
- Width demanded by movement alone
- 1 in
- Construction tolerance added
- 0.25 in
- Backer rod diameter
- 1.56 in
- Sealant bead depth at mid-joint
- 0.5 in
- Movement the finished joint can absorb
- 0.31 in
What this calculation does not cover
- Sizes a butt joint working in extension and compression. A joint also racking in shear — a head joint at a drifting floor edge — needs the shear component resolved into the movement figure before it is entered.
- Says nothing about adhesion. Every substrate pairing in a perimeter joint (aluminium, sealed concrete, air barrier membrane) needs its own primer and its own adhesion test.
Add the equipment this sizes
This result is a specification — 1.25 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
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 citations4
- ASTM C1193, Standard Guide for Use of Joint Sealants — a butt joint's width is sized so the anticipated movement stays inside the sealant's rated movement capability, with the construction tolerance added on top rather than absorbed by it
- ASTM C920, Standard Specification for Elastomeric Joint Sealants — the movement classes offered here (12½, 25, 35, 50) are that specification's own, and the class number is the percentage of installed joint width the sealant is qualified to move
- Backer rod diameter of 1.25 x the joint width is a RULE OF THUMB, stated as one: the rod must compress to stay put without being crushed, and the 25% oversize is the figure the trade works to rather than a value published in a standard
- Bead depth follows the 2:1 width-to-depth rule with a 6 mm floor and a 12 mm ceiling — again a rule of thumb, and again the sealant manufacturer's own data sheet governs where the two disagree
Which documents these citations point at
Standards referenced: ASTM C1193, ASTM C920 (ASTM International, United States).
Cite this page
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