Roof expansion joint installation for low-slope commercial roofs

Roof expansion joints, and why the building moves

A roof expansion joint is the detail that carries a building's structural expansion joint up through the roof and keeps it watertight while the two sides move independently. The building moves. It expands and contracts with temperature, the structure flexes under load, and on some sites it has to take seismic movement, so the designer splits the structure with a deliberate gap and lets each part move on its own. The roof sits on top of that gap, and if you run plain membrane across it, the cycling tears the sheet. The expansion joint is how the roof moves with the building instead of fighting it.

The detail that works is a raised one. Two curbs, one on each side of the structural gap, with the roof membrane flashed up each curb as ordinary base flashing and a flexible cover spanning between them. The movement happens in the cover, up out of the water, and the roof flashing on each side never sees it. The companion flashing guide covers the base-flashing logic that each side of this joint relies on, and it treats the bellows cover in brief; this guide is the full build.

Get the joint right and it disappears into the roof for thirty years. Get it wrong, run it flat, and it becomes the one line on the roof that leaks no matter how many times someone caulks it, because the building is going to keep moving and the flat membrane is going to keep tearing. The fix for a bad expansion joint is rebuilding it raised, not sealing it again.

What is the difference between an expansion joint and an area divider?

A roof expansion joint sits over a structural expansion joint and allows movement. An area divider sits on a continuous deck where there is no structural joint, and it does not allow movement; it relieves the stress that thermal cycling builds up in a large field of membrane by breaking that field into smaller sections. They look alike on the roof, two raised members close together, but they are doing different jobs, and confusing them on a submittal is a tell that the person drawing it has not built one.

You can tell a structural expansion joint by where it goes. It runs through the entire building, starting at the foundation and continuing up the walls, through every floor, and finally through the roof. The structural engineer decides where it lands and how wide the gap opens, because it is a structural decision, not a roofing one. The roof has to land on top of it and accommodate whatever movement the engineer designed for. You do not get to move it or close it.

An area divider is the roofer's tool for a problem the structure did not solve. On a long run of adhered or attached membrane, thermal expansion and contraction load the sheet, and past a certain length that stress concentrates and works the seams. The area divider is a raised double curb, like an expansion joint to look at, but the two members are joined with no opening because there is nothing to span. It breaks the roof into manageable rectangles so no single field gets long enough to tear itself. Both get the same flashing and metal-cover care, but only one of them moves.

Why does a roof expansion joint need a raised curb?

A roof expansion joint needs a raised curb because the joint is the one spot on the roof that has to move and stay dry at the same time, and you cannot do both flat. Build it on two raised curbs, flash the membrane up each one, and put the moving cover above the water line, and the joint sheds water to both sides while the movement happens up in the cover. Run it flat across the deck and you have put a moving, flexing seam in the lowest, wettest part of the detail. It gets walked on, it ponds, and it tears. That is the chronic-leak version, and it is the most common way the joint is done wrong.

The curbs do three things at once. They lift the moving cover up out of any standing water, so the bellows is never sitting in a puddle that works into it. They give the membrane a curb to flash up, the same as a wall or an equipment curb, so each side terminates as ordinary base flashing with a top edge that is protected. And they make the joint a high point on the roof, which is what keeps water from running into it in the first place. The industry detail puts those curbs up high enough to act as base flashing, commonly at least 8 in above the finished roof, the same minimum the companion flashing guide covers for any base flashing.

The blunt version: a flat membrane-only expansion joint is a leak you scheduled. The membrane has to flex there for the life of the building, flat membrane flexed in standing water fails, and once it does the only real repair is to tear it out and build the raised curbs that should have been there. Spend the money on the curbs the first time.

The expansion joint cover: prefab vs field-built

The cover is what spans the gap between the two curbs and absorbs the movement, and the choice is between a prefabricated cover and a field-built one. The prefab cover is the right default. It is a factory assembly: a flexible bellows with a metal flange down each side, made to span a stated joint width and take a stated movement. The bellows is a flexible rubber membrane, EPDM at 60 mil being the common stock, supported on a closed-cell foam core that lets it compress and stretch, with the metal flanges bonded and mechanically locked to the bellows at the factory. You set it over the curbs, fasten and flash the flanges to each side, and the movement lives in the foam-backed center.

The metal flanges are galvanized steel as the standard build, with stainless and aluminum offered where the environment or the spec calls for them. Manufacturers make the cover in styles to match the geometry: curb-to-curb for a joint running across the field, and curb-to-wall where the joint meets a parapet or a higher building face. Picking the right style for the condition is half the job, because the curb-to-wall transition is where a field improviser gets into trouble.

Field-building the cover out of loose metal and membrane is the old way, and it is the way that leaks. A field-formed metal cap over a flat or low joint depends entirely on the hands that bent it and the sealant holding it, and it has no real movement capacity, so the first hard thermal cycle works it loose. Reach for the field-built cover only where no prefab unit fits the geometry, and even then the bellows principle still applies: the membrane has to flex somewhere up out of the water, not at the deck. The membrane type drives the flashing, not the cover. A single-ply roof welds or bonds the membrane to the cover flange and the curb; a built-up or modified-bitumen roof strips the flashing in with the cover's flange set into the plies. Confirm the cover, the flange detail, and the movement rating against the membrane manufacturer and the project spec, because the warranted assembly is theirs to specify.

The curbs and the structure underneath

The two curbs are the foundation of the whole detail, and each one is anchored to its own side of the structural gap. That is the point that gets missed. The curb on the left fastens to the structure on the left of the joint, the curb on the right fastens to the structure on the right, and the gap between them stays open so the two sides can move without dragging each other. Anchor a curb across the gap, or bridge the two with a single nailer, and you have stitched the joint shut, which defeats the structural engineer's entire reason for splitting the building.

Build the curbs like any roof curb. Wood nailers or a framed curb set on each side of the gap, fastened down to the deck or the structure, tall enough to act as base flashing at the common 8 in minimum or whatever the manufacturer's detail and the climate require. A cant strip at the base of each curb eases the membrane up off the deck and out of the inside corner, the same cant the companion flashing guide calls for at walls and curbs. The height has to clear whatever water and snow back up against the joint, so heavy-snow country pushes the curb taller, the same way it pushes any base flashing taller.

Coordinate the curb height with the tapered insulation. On a tapered roof the deck under the joint is not flat, and the curbs have to land on a known height so the cover spans cleanly and the flashing height comes out right on both sides. A curb set without checking the stack-up is the one that ends up buried on the high side or short on the low side, with the cover sitting cockeyed over a joint that is no longer square.

Flashing the membrane up each curb

Each side of an expansion joint is an ordinary base flashing, and that is the good news: the detail you already know how to build is what makes the joint watertight. The roof membrane turns up the outboard face of each curb, runs up to the top, and terminates there, fastened with a termination bar and protected at the top edge by the cover's flange acting as the counterflashing. The companion flashing guide covers the base-flashing height, the termination bar, the lap sealant, and the counterflashing logic in full; the expansion joint uses every bit of it, twice, once on each curb.

The cover flange is the counterflashing. When the prefab cover sets over the curbs, its lower flange welds or bonds to the field membrane alongside the joint, the termination bar and anchors lock the flange and the underlying membrane to the curb, and the upper flange laps down over the bar so water sheds over the face instead of getting behind the membrane edge. That layered flange is the double-level detail the prefab covers are built around, and it is why the prefab unit beats a field cap: the counterflashing is engineered into it.

Compatibility decides whether the flange and the membrane become one watertight surface or just two materials touching. The cover's bondable flange has to match the membrane, TPO flange to a TPO roof, PVC to a PVC roof, so it welds into the field. On EPDM it splices with primer and tape; on built-up and mod-bit it strips in with the plies and flashing cement. Mix an incompatible flange onto the membrane and the joint between them is the leak. Match the cover to the membrane, the same matched-set rule the flashing guide lays out for every accessory.

The bellows and the flexible center

The bellows is the part that moves, and it is the whole reason the detail works. It is a flexible membrane, the EPDM or neoprene cover, draped over a closed-cell foam core into a raised loop or fold that sits between the two flanges. When the building expands and the gap narrows, the loop compresses. When the building contracts and the gap widens, the loop stretches open. All of that motion happens in the foam-backed center, well above the curbs, so the base flashing on each side stays still. The bellows takes the cycling that would otherwise tear the membrane.

The reason a flat membrane joint fails is exactly the reason the bellows succeeds. Plain membrane stretched across a moving gap has no slack and no shape to give, so every cycle pulls directly on the sheet and the seams, and reinforced field membrane is built to resist stretch, not to flex back and forth forever. The bellows has the slack built in as a loop and the foam to spring it back, so it accommodates the movement instead of resisting it. The movement rating on the cover is real: it states how far the joint can open and close before the bellows is overworked, and you size the cover to the engineer's designed movement plus margin.

Do not stretch the bellows installing it. Setting a cover sized for a narrow joint over a wider gap pre-tensions the bellows, so it starts its life already near the end of its travel and tears early. Set the cover at the joint width it was made for, leave the loop relaxed, and let it work. A bellows installed flat, pulled tight, or pinched under a flange has been defeated before the first thermal cycle.

Where do you put area dividers on a large roof?

Area dividers go on a large roof at a spacing that keeps any single field of membrane short enough that thermal movement does not over-stress it, with the common figure for attached or adhered systems landing around every 150 to 200 ft between structural expansion joints. That is a starting rule, not a law. The exact spacing depends on the membrane, the attachment method, the climate's temperature swing, and the manufacturer's published guidance, so confirm it against the system you are installing rather than treating 150 to 200 ft as gospel.

The divider is a raised double curb, built and flashed like an expansion joint but with the two members joined and no open gap, because there is nothing structural to span. Its job is purely to break the run. You lay the dividers to split the roof into rectangles, working with the drain layout and the building geometry so a divider does not dam water in a flat run. A divider sitting across the drainage path becomes a wall water ponds against, so it gets the same drainage care as any raised obstruction, with the field sloped to carry water past it.

The membrane system tells you whether you need dividers at all. Fully adhered and mechanically attached systems carry their thermal stress into the membrane and the seams, so they get the dividers. A loose-laid, ballasted system floats more freely and is far less prone to the same stress buildup, so the spacing question changes. When the roof is large enough that the question comes up, get the manufacturer's area-divider recommendation in writing, because it ties to the warranty.

Carrying the joint up a wall or parapet

An expansion joint rarely stays in the field of the roof. It runs to an edge, turns up a parapet, or meets a higher wall on a stepped building, and the transition is where the leaks concentrate. The joint has to be carried up the vertical face continuously, so the movement is accommodated through the corner and up the wall, not stopped dead at the base of the parapet where the geometry gets hard.

The three-way intersection is the worst point on the whole detail. Where the horizontal roof joint, the vertical wall joint, and the curb all meet, you have base flashing turning a corner, a bellows changing direction, and a cover transitioning from a curb-to-curb style to a curb-to-wall style, all at one spot that still has to move and stay watertight. That corner is the chronic failure on expansion joints, the same way the four corners are the chronic failure on equipment curbs in the companion flashing guide. Flat sheet and a field-bent cap will not survive there.

Specify the factory transition. Manufacturers make the curb-to-wall cover and the molded or prefabricated corner and intersection pieces exactly because the field-built version at a three-way joint is so unreliable. Use the factory corner, tie the wall joint and the roof joint into it per the manufacturer's detail, and treat that intersection as the highest-risk point to probe on the QA walk. The transition you fabricate by hand at a moving three-way corner is the one that opens up a season later.

The seismic joint and larger movement

A seismic joint is an expansion joint sized for movement far larger than thermal cycling alone, because on a seismic site the two sides of the building can shift suddenly and by inches, not the fraction of an inch a temperature swing produces. The roof detail is the same family, raised curbs and a flexible cover, but everything is scaled up: a wider gap, a deeper bellows with more travel, and a cover rated for the multidirectional movement the structural engineer designed the joint to take.

The number that matters here is the movement rating, and it is the engineer's number, not the roofer's. The structural design states how far the joint can open, close, and shear sideways under the design event, and the cover has to be rated to take all of it, including the lateral component a thermal cover never sees. A standard thermal expansion joint cover dropped onto a seismic joint is undersized for the shear, and it shreds the first time the building actually moves the way the joint was built to let it.

Treat the seismic cover as engineered hardware, not a stock accessory off the truck. Match the cover's rated movement, in every direction, to the structural drawings, set it so the bellows is relaxed and free to take that movement, and confirm the selection with both the structural engineer and the cover manufacturer. The curbs, the flashing, and the drainage are the same as any expansion joint; the difference is how much the center has to move and survive.

Should water drain across a roof expansion joint?

No. Water should never drain across or pond against a roof expansion joint, and the detail is built specifically to keep it from doing so. The raised curbs make the joint a high point on the roof, so water sheds away from it to both sides and runs to the drains in the field, not into the moving cover. The day the joint becomes a low point or a dam is the day it starts leaking, because the one part of the roof that flexes is now the part sitting in water.

This is a drainage problem the flashing alone cannot fix, and it is solved with slope and crickets. The NRCA defines a cricket as a construction to divert water around or away from a chimney, wall, expansion joint, or other penetration, and the expansion joint is squarely on that list. Where the joint crosses the drainage path, a cricket on the uphill side splits the water and runs it around the joint to the drains, the tapered work the companion cricket guide covers in full. The cricket is sloped steeper than the field, commonly double, so its own diagonal valleys keep draining.

Lay the tapered package so the joint sits up and the water goes around it, not the other way. An expansion joint built correctly on its curbs but left in a flat dead zone, with the field sloping toward it instead of away, ponds along its base and tests the flashing constantly. The curbs lift the moving cover out of the water; the slope and the crickets keep the water from getting to the curbs in the first place. You need both.

Penetrations and equipment near the joint

Keep penetrations, curbs, and equipment off the expansion joint, with a clear margin to both sides. The membrane within reach of the joint is membrane that has to flex with the building, and a pipe boot, a drain, or an equipment curb set right next to the joint sits in that moving zone, so the building works the penetration flashing loose over time. A pipe set straddling or hard against the joint is the worst case, because the two sides move relative to the pipe and the boot has no way to keep up.

Coordinate the layout so nothing important lands in the movement zone. Push pipes, drains, and roof curbs out into the stable field on one side of the joint or the other, where the membrane is not cycling, and let each penetration get a clean flash on all sides per the companion flashing guide. A drain in particular belongs well off the joint, both because the joint should be a high point the drain is nowhere near, and because the moving membrane next to a clamped drain ring works the ring loose.

When a penetration genuinely cannot be moved, it gets its own detail and extra attention on the inspection, because it is being asked to stay watertight in a zone that flexes. That is a compromise, not a standard detail, and it goes on the maintenance list by name. The clean answer is the coordination meeting before the roof goes on, where the joint location and the equipment layout get reconciled while the pipe can still move.

Why do roof expansion joints leak?

Roof expansion joints leak because they get built flat instead of raised, and a flat joint puts a moving, flexing seam in the wettest part of the roof. The membrane there has to take the building's movement for thirty years, flat membrane flexed in standing water fails, and once the sheet or the seam opens the joint funnels water straight into the gap and down into the building. The single most common cause of an expansion joint leak is that it was never built up on curbs in the first place.

After the flat-joint failure, the leaks rank by where the detail was hardest. The three-way wall intersection, where a hand-built corner could not survive the movement. The bellows stretched or pinched on installation, so it tore early. The cover flange flashed to an incompatible membrane, so the bond never welded. And the joint left in a low spot or a flat dead zone, so it ponded and the flashing was tested every rain. Each of those is a known failure point, and each shows up on a roof where someone treated the joint as ordinary flashing instead of a moving detail.

The fix is almost never more sealant. Caulk over a moving joint is consumed by the next thermal cycle, and a bead of lap sealant on a torn bellows buys a season at most. When an expansion joint leaks, the question is whether the raised curbs and the bellows cover are actually there and intact, and if they are not, the repair is to build them, not to seal the flat joint one more time. The raised curb is the fix, the same way it was the detail that should have been built originally.

Inspecting the joint: cover, bellows, flashing, and movement

The expansion joint inspection is a hands-on pass over the cover, the bellows, the flashing on both curbs, and the way the joint is actually moving, done before closeout and again on the maintenance cadence. It is not a look from the hatch. The inspector walks the full length of the joint, checks the cover flanges are welded or bonded and locked down on each side, probes the field welds the same way the companion flashing guide probes any seam, and reads whether the bellows is relaxed and intact or stretched, pinched, cracked, or torn.

Read the movement, because the joint is supposed to move and the evidence shows. A bellows that is pulled tight at installation temperature has no room to take contraction in the cold. Cracking, splitting, or a flange working loose along one side says the cover is fighting movement it was not sized for, or was set stretched. At the curbs, the base flashing gets the standard check: top edge protected by the cover flange, termination tight, no exposed membrane edge wicking water behind it. The three-way wall intersection gets the hardest look of all.

Find the wet joint with instruments, not eyes. An expansion joint that has been leaking soaks the insulation along its length, and an infrared scan after sundown or a moisture survey reads it where the probe missed, the same survey work the companion guides cover. The leak history of a roof points straight at the joint when the joint was built flat, so on a problem roof the expansion joint is the first line to re-walk.

The re-roof: matching or upgrading the joint

On a re-roof you inherit whatever expansion joint the last crew left, and the most common find is exactly the flat or low joint that has been leaking for years. The decision at every joint is the same as at every penetration in the companion flashing guide: reflash it new and right, or trust what is there. Trust the old flat joint and the new field of membrane ties into a detail that was already failing, so the roof is new and the same line leaks.

The re-roof is the chance to upgrade the joint, and usually the reason to. If the existing joint was flat membrane or a tired field-built cap, build it up on proper curbs and set a prefab bellows cover as part of the re-roof scope. The added tapered insulation changes the curb height math, so the curbs that suited the old buildup are the wrong height for the new one, and they get rebuilt to land the flashing height and the cover correctly on the new surface. Match the cover style to the geometry, curb-to-curb in the field and curb-to-wall at the parapets.

Coordinate the joint with the structural reality on a re-roof, because the structural expansion joint underneath is still there and still moving. The roof detail has to land on the actual structural gap, anchored to each side, even if the old roofer had stitched it shut. If the previous joint was bridged or buried, the re-roof is when it gets reopened and rebuilt so the building can move the way it was designed to. The point of the re-roof is the details, and the expansion joint is the detail most likely to have been wrong the first time.

Large roofs, data centers, and high-value buildings

On a large or high-value roof, the expansion joints and area dividers carry more weight than they do on a small building, because there is more roof to move and far more cost behind a leak. A data center, a large distribution warehouse, or a manufacturing roof can run hundreds of feet in every direction, so the thermal movement across a single field is large, the structural expansion joints are real and have to be honored, and the area dividers between them are not optional refinements.

Critical-facility roofs raise the stakes under the joint. A leak over a data hall or a clean process space is not a ceiling-tile callback, it is downtime and damaged equipment, so the joints over those spaces get the conservative detail: prefab covers with margin on the movement rating, factory corners at every transition, crickets that keep water well off the joint, and a documented closeout the owner's facilities team can audit. The redundancy that runs the building belongs in the roof detail too.

Plan the joint layout with the structure and the drainage together on these roofs, not as separate exercises. The structural expansion joints land where the engineer put them, the area dividers fill the runs between, and the drains and crickets have to keep all of those raised lines as high points water sheds away from. On a roof this size the tapered layout, the joint layout, and the drain layout are one coordinated plan, and the companion cricket guide covers the tapered side of that coordination.

The maintenance the owner takes on

An expansion joint is a moving part, and moving parts wear, so the joint goes on the owner's maintenance schedule by name the day the roof is finished. The owner inherits a detail that flexes thousands of times a year with the weather, and the cover, the bellows, and the sealant at the flanges all have a service life. A joint that nobody ever looks at is a joint that fails quietly until water shows up inside.

Walk the joint on the maintenance cadence and check the few things that age. The bellows for cracking, splitting, or a spot that has been pinched or pulled. The cover flanges for fasteners backing out or a lap working loose. The lap sealant at the terminations, which is maintenance, not a permanent seal, and gets renewed before it fails. And the drainage around the joint, because debris dams up against any raised line over time and turns the high point the curbs created into a pond if nobody clears it.

Hand the owner the record and the schedule, not just the roof. The closeout package names the joint, its cover, and the sealants used, and the maintenance schedule says to walk it, clear the debris, and renew the sealant on a cadence. The owner who knows the joint is a moving detail keeps it; the owner who was never told treats it as flat roof and finds out the hard way.

What to document

The expansion joint record is what defends the roof when the joint leaks and the question is whose fault it is. A warranty claim and a callback both turn on whether the joint was built to the manufacturer's detail, on proper curbs, with the right cover for the movement, and the only proof is the log made while the work happened. The cover and the flashing are set the day the crew leaves, so the record made on the roof is the evidence.

Capture each joint by location and type: structural expansion joint or area divider, the curb height on each side, the cover and bellows used with its movement rating, the membrane and how the flange tied in, the cricket and drainage that keep water off it, and the inspection result. Note the seismic joints separately with the engineer's movement numbers, and tie the package to the manufacturer's detail and the structural drawings, because that is the file the rep and the engineer read when a claim is judged years out.

Common mistakes

• Running a flat, membrane-only joint across the deck, so the moving seam sits in standing water and tears.
• Building no raised curbs, so the cover and the flexing membrane have no height above the water.
• Leaving the joint in a low spot or flat dead zone, so water dams against it instead of draining away.
• Skipping area dividers on a large adhered roof, so a single field gets long enough to over-stress its seams.
• Stretching, pinching, or pulling the bellows tight on installation, so it tears on the first cold contraction.
• Field-building the three-way wall intersection by hand instead of using the factory curb-to-wall corner.
• Anchoring a curb across the gap or bridging the two sides, so the structural joint can no longer move.
• Flashing the cover flange to an incompatible membrane, so the bond never welds and the flange leaks.
• Dropping a standard thermal cover on a seismic joint that needs the rated lateral and larger movement.
• Caulking a torn or flat joint instead of rebuilding it on curbs, and calling the repair done.

Field checklist

Standards and references

The membrane manufacturer's published detail and warranty govern the expansion joint, the same as every other flashing on the roof. They specify the cover and bellows, the flange and how it ties to the membrane, the curb height, the area-divider spacing for their system, and the corner and transition pieces. The closeout inspection is run against those details, so confirm every figure in this guide against the actual product literature and the warranted accessory list before you build the joint.

The NRCA Roofing Manual is the practical trade reference for expansion joints, area dividers, and curbs. It is where the raised double-curb detail, the area-divider concept for relieving membrane stress on large roofs, and the cricket definition that puts the expansion joint on the divert-water list are laid out for the trade. Treat its figures, including the commonly cited 8 in base-flashing height and the 150 to 200 ft area-divider spacing, as recommended industry practice, confirm them against the current edition because the manual is revised across cycles, and never cite a detail number you have not verified.

The structural side belongs to the engineer and the structural code. The structural expansion joint and the seismic joint are located, sized, and rated for movement by the structural engineer under the building code and ASCE 7 for seismic design, and the roof has to accommodate the movement they designed, not the other way around. SMACNA architectural sheet metal guidance covers the metal cover and flange work, ASTM material specifications define the membrane and cover materials, and FM Global data sheets add requirements on insured roofs. The code is adopted and amended by jurisdiction, so confirm the requirement against the adopted edition and any local amendments, and let the manufacturer's detail and the structural drawings override any rule of thumb.

Units, terms, and conversions

Expansion joints use a specific vocabulary, and the same part reads differently across a detail drawing, a product sheet, a structural drawing, and a spec, so the terms are worth pinning down.

Curb height is given in inches above the finished roof, commonly at least 8 in, which is about 200 mm on a metric drawing. The cover's movement is rated as how far the joint can open and close, and on a seismic joint how far it can shear sideways, in inches or millimeters. The bellows is the flexible loop, EPDM commonly at 60 mil, on a closed-cell foam core. A curb-to-curb cover spans a joint in the field; a curb-to-wall cover carries it up a parapet. An area divider is the raised member that breaks a large roof field without spanning a structural gap, and a cricket or saddle is the diverter that sheds water around the joint.

Roof expansion joint

The watertight roof detail over a structural expansion joint, built on two raised curbs with a flexible cover spanning the gap to take the building's movement

Area divider

A raised double member on a continuous deck that relieves thermal stress by breaking a large membrane field into sections; it does not allow movement

Bellows

The flexible membrane loop over a closed-cell foam core that compresses and stretches to absorb the joint's movement, commonly EPDM at 60 mil

Expansion joint cover

The factory or field assembly of a bellows and two metal flanges that spans the curbs and flashes to the membrane on each side

Curb-to-curb / curb-to-wall

Cover styles for a joint running through the field versus a joint carried up a parapet or a higher wall

Seismic joint

An expansion joint sized for the large, multidirectional movement of a seismic event, rated to the structural engineer's design

Movement rating

How far a cover can let the joint open, close, and on a seismic joint shear, before the bellows is overworked

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