Design Expansion Joint

Every control layer in an enclosure is asked to be continuous. A movement joint is the place where continuity has to survive a gap that changes width every day for sixty years. A layer that is continuous but cannot move will tear, and it will tear where nobody is looking.

Workflow

Step 1. Take the movements and what has already been decided

Ask, in one message:

1. What structural joints already exist, where, and why? Those are fixed and the enclosure has to follow them.
2. What is the cladding, and what moves it: temperature, moisture, curing, creep, or load? Name each mechanism you know about.
3. What is the geometry: overall lengths, re-entrant corners, changes in height, wings meeting a core, an addition meeting an existing building?
4. What is the temperature range this facade will actually see, in the sun and in winter, and from what source?
5. Are two structures involved, with different foundations or different ages?
6. What is already fixed: a facade module, a panel size, a joint pattern already drawn?

Movement sources get listed separately, not merged into one allowance. Thermal movement reverses; moisture expansion in fired clay does not; concrete shrinkage happens once and never comes back. A single number that covers all of them covers none of them.

Step 2. Sort the joints into a hierarchy and place them

Not every joint is the same joint. Present the hierarchy as a table, with the placement reasoning in full sentences:

Joint type	What it separates	What moves across it	Where it has to be

Work down the hierarchy:

• Building separation joint. Two structures, moving independently. The enclosure has no choice about where this is, and every control layer has to cross the largest gap in the building here.
• Structural expansion joint. One structure, relieved. Again fixed by others; the enclosure follows.
• Enclosure movement joint. The facade system relieving its own movement, which may or may not line up with the structure's. Say plainly when it does not, because a facade joint half a bay from a structural joint is a common and expensive mistake.
• Cladding control joint. Within one material, relieving that material's own behaviour. These are the ones that get placed by pattern and should be placed by accumulation.

Then say where movement accumulates: at the ends of long runs, at re-entrant corners, at changes of height, at openings that interrupt the field. Placement follows accumulation, not a spacing rule.

Step 3. Work each crossing, and read the gaps

For each joint in the hierarchy, one table:

Control layer	What crosses the gap	Movement it must take	Value needed from you

• Water. What sheds water across a moving gap, and where water that gets in is collected and discharged. A joint that has no discharge is a funnel.
• Air. The air seal across the gap, its expected movement range and whether it is reachable for replacement. This is the layer most often left as a sealant bead doing a structural job.
• Vapor. Whether the vapor strategy survives the interruption, or whether the joint creates a path the rest of the assembly was designed to prevent.
• Thermal. Whether insulation is continuous across the gap, or whether the joint is a thermal break in the wrong sense: a straight line of missing insulation the full height of the building.

Then read between the rows:

• Is any single material being asked to do two of these jobs across a moving gap?
• Is the joint accessible for inspection and replacement, and if not, what is the design life of the least durable component in it?
• Does the joint change direction, and does the crossing material survive that corner?

Step 4. Work the ends, corners and intersections

Joints fail where they stop and where they meet. Name every one:

• Where each joint terminates: at the roof, at grade, at a parapet, at a foundation.
• Where a vertical joint meets a horizontal one.
• Where a joint turns a corner, inside or outside.
• Where a joint crosses another assembly: a window, a louvre, a soffit, a balcony.
• Where the joint passes the roof and the below-grade waterproofing, both of which have their own movement provision.

Step 5. Name what the joint cannot take

Report the limits rather than a drying direction:

• What movement was assumed, and what happens if the real movement exceeds it.
• Which component in the joint has the shortest life, and what replacing it involves.
• What the joint does under a seismic event, if seismic separation was named in Step 1, and whether that is a different requirement from thermal movement.
• Whether any part of the joint depends on adhesion, and what that adhesion is to.

Step 6. Report, and hand off

The hierarchy table, the crossing tables, the ends and intersections, then the limits. Close with the hand-offs, every time:

• The movement range has not been calculated here and has to be, against material properties and a stated temperature range.
• Structural and seismic separation requirements are open and belong to the engineer.
• Air barrier continuity across every joint belongs to air-barrier-trace, and joints are where it most often fails.
• Sealant and gasket selection against the calculated movement is open.

Count the open values on the face of the output. Then ask whether to write the result to a file and where, and write it directly, tables included.

Rules

• Never supply a number. Not a spacing, not a width, not a coefficient, not a movement capability, not as a placeholder.
• Never merge movement sources into one allowance. List them separately with their mechanisms; reversible and irreversible movement are different problems.
• Take structural joints as fixed and say so. The enclosure follows them.
• Place joints by accumulation, never by a remembered spacing rule.
• Every crossing names a physical material and the movement it has to take.
• Name every termination and intersection, even when the answer is short.
• Say on every run that the movement range has not been calculated here.

Anti-patterns

• Recalling a joint spacing for a cladding material because the figure feels standard. Spacing depends on the material, the fixing, the exposure and the climate.
• Placing a facade joint near but not at a structural joint.
• Drawing the air barrier as continuous across the gap with no material named, or with a sealant bead expected to take movement it was never rated for.
• Treating thermal, moisture and shrinkage movement as one number.
• Quoting a code or a manufacturer's movement capability found by web search.
• Calling the joint resolved without saying where it terminates at the top and the bottom.
• Leaving a joint with no discharge path for water that gets into it.
• Running air-barrier-trace from inside this skill. It is a hand-off.

Resources

None. This skill is one file. Output is written directly at the path you choose.