Concrete
Building expansion and movement joint systems field guide
What a building movement joint is, why a building that cannot move cracks itself apart, the joint family, how to size it, and how to run it continuously through every layer and keep it dry.
Direct answer
A building movement joint is a deliberate gap that lets two parts of a structure move independently, so the building relieves thermal, moisture, seismic, settlement, and creep movement instead of cracking. The joint must run continuously through every layer it crosses, be sized to the real movement, fire-rated where it breaks a barrier, and kept watertight.
Key takeaways
- A building movement joint is a deliberate gap that lets parts move independently, so the building relieves thermal, moisture, seismic, settlement, and creep movement instead of cracking.
- A movement joint must run continuously through every layer it crosses; a break in any one layer becomes a rigid bridge that cracks, leaks, or tears when the building moves.
- Concrete moves about 5.5 millionths of its length per degree Fahrenheit; thermal joint movement equals temperature swing times segment length times expansion coefficient.
- Standard expansion-joint covers carry about plus or minus 25 percent of nominal width; seismic-rated covers carry roughly plus or minus 50 percent.
- Where a joint crosses a fire-rated floor or wall, use a system tested to UL 2079, installed as listed on every rated layer.
What a building movement joint is
A building movement joint is a deliberate gap that splits a structure into parts that can move independently of each other. The building expands, contracts, settles, and sways whether you plan for it or not. The joint gives that movement somewhere to go. Without it, the movement still happens, but it comes out as cracked walls, spalled concrete, sheared connections, popped tile, and torn finishes.
Think of it as the opposite of a connection. Everywhere else on the building you are tying members together to make them act as one. At a movement joint you are doing the reverse on purpose, letting one part of the building shrug while the part next to it holds still. The gap is the feature, not a defect.
The work is not cutting the gap. The work is carrying that gap continuously through every layer it crosses, the structure, the slab, the wall, the ceiling, the roof, the facade, and the fire rating, then covering it with a system that bridges the gap while still allowing the movement, sizing it to the real movement the engineer calculates, and keeping water out of it for the life of the building. Get the gap and miss any one of those, and the joint either leaks, cracks, or fails to move. The slab and structure that this joint passes through are their own design problems, covered in the slab on grade and the formwork guides; this guide is about the joint that crosses them.
Why do buildings move?
Buildings move because the materials in them respond to heat, moisture, ground, time, and ground motion, and the movement is real and large enough to break things. A restrained building cannot avoid the movement. It can only convert it into stress, and concrete and masonry have very little tension capacity to absorb that stress before they crack.
Thermal movement is the one that drives most expansion joints. Materials grow when they heat and shrink when they cool, every day and every season. Concrete moves roughly 5.5 millionths of its length per degree Fahrenheit, and that small number turns into real inches once you multiply it across a long building and a wide temperature swing. A dark roof or a south wall in full sun runs far hotter than the design air temperature, which widens the swing further.
Moisture is the second driver. Concrete and masonry shrink as they cure and dry, then swell and shrink again as they take on and lose water over the years. Clay brick does the opposite of concrete block over the long term, which is why masonry has its own movement joints. Soils heave when water in them freezes or when expansive clay takes on water, and that pushes the foundation.
Then there is the ground and time. Settlement is the building moving down into the soil, rarely evenly, so one part drops relative to another. Creep is concrete slowly deforming under sustained load for years after it is placed. Seismic motion is the violent case, where the ground throws the building sideways and different parts sway out of step. Each of these is a separate movement with a separate magnitude, and the structural engineer is the one who quantifies them for a given building.
The joint family and what each one does
Four joints get called by each other's names on jobsites, and they do four different jobs. Confusing them is the root of most movement-joint problems, so it is worth fixing the vocabulary before anything else.
An expansion joint is a full gap through the structure that lets parts move apart and back, mainly for thermal and moisture movement. A control or contraction joint is a planned weak line that makes concrete crack where you want it instead of randomly, and it stays within the one element. A construction joint is just a stop in the pour, a boundary where one placement meets the next, and after it bonds the two pours act as one element. A seismic or isolation joint is a wide separation between building sections so they can sway independently in an earthquake.
The distinction that matters most: a control joint keeps a single member acting as one element while steering its cracks, while an expansion or seismic joint splits the building into separate elements that move on their own. One manages cracking within a piece. The other separates pieces.
| Joint | What it does | Through the structure? |
|---|---|---|
| Expansion joint | Lets parts move apart and back for thermal and moisture movement | Yes, full gap through every layer |
| Control / contraction joint | Steers shrinkage cracks to a planned line within one element | No, partial, within the member |
| Construction joint | A planned stop between pours; bonds back to one element | No, it transmits load across |
| Seismic / isolation joint | Separates building sections so they sway independently | Yes, wide gap through every layer |
The expansion joint
An expansion joint cuts the building into segments that can grow and shrink without pushing on each other. It is a real gap, open through the whole assembly, with no reinforcement crossing it and nothing rigid spanning it. The two sides are independent structures from the joint's point of view.
Because the joint is open, both sides usually need their own support at the gap. That is often a double column line or a double bearing wall, one column or wall for each side, rather than one member trying to carry both. The structural engineer sets that arrangement, because how the building is framed at the joint decides whether each side is actually free to move or is quietly still tied together.
The width of the gap is sized to the movement, which on a thermal expansion joint comes from the temperature swing times the length of the segment times the material's expansion coefficient. A long segment in a climate with big seasonal swings needs a wider gap than a short segment in a mild one. Under-cut the width and the two sides close the gap and start bearing on each other on the hottest day, which is exactly the load the joint was supposed to prevent.
What is the difference between a control joint and an expansion joint?
A control joint makes concrete crack on a planned line; an expansion joint lets parts of the building move apart. They sound similar and they are opposites in what they do.
A control joint, also called a contraction joint, is a tooled or sawn groove that thins the concrete at a chosen line so the shrinkage crack forms there instead of wandering across the slab or wall. The two sides do not separate. They stay one element, and aggregate interlock and any continuous reinforcement still transfer load across the crack. Control joints handle the small movement of curing shrinkage and minor thermal and moisture cycling. They live within the slab or the wall and do not pass through the rest of the building. The placement and spacing of slab control joints is its own subject, covered in the slab on grade guide.
An expansion joint is a full separation. The two sides are meant to move apart and back, the gap is open, and it carries through the structure, not just the concrete. You fill and cover an expansion joint with something that can stretch and compress. You do not fill a control joint to make it move, because it is not supposed to move apart in the first place. Detail an expansion joint as if it were a control joint and you have built a crack control feature where the building needed a separation, and the building will find its own separation by cracking somewhere you did not choose.
What is a seismic joint?
A seismic joint is a wide separation between two building sections, or between two adjacent buildings, sized so they can sway out of phase in an earthquake without slamming into each other. It is the same idea as an expansion joint, taken to a much larger and more violent movement.
The driver is story drift. Codes let a structure deflect sideways by a fraction of its story height, commonly on the order of one to two percent, so a tall section can move several inches at the top relative to a section next to it. Two sections that are not tied together will move differently, and if the gap between them is too narrow they pound, which is its own failure mode that wrecks corners, stairs, and cladding. The seismic gap has to be wide enough to keep them apart through the design earthquake.
Two things make seismic joints harder than thermal ones. The movement is multi-directional, so the joint and its cover have to take movement in and out, side to side, and up and down at the same time. And the spacing is set by the seismic demand and the building's shape, not by how long the building is, so a seismic joint can appear in a building that would never need a thermal expansion joint. The structural engineer calculates the required separation under the adopted code, and that width is not a number to estimate in the field.
The joint has to run continuously through every layer
A movement joint is only as good as its weakest layer, and it has to be continuous through all of them. The gap that exists in the structural slab has to carry up through the topping, the floor finish, the partitions, the corridor walls, the ceiling, the roof deck, the membrane, the parapet, and the exterior facade, in line, the whole way. A break in any one layer is a rigid bridge across a moving joint, and that layer cracks, leaks, or tears the first time the building moves.
This is the part that gets lost between trades. The structural drawings show the joint in the frame, but the joint also lives on the architectural drawings, the roofing details, the firestop submittals, and the finish schedule. If the drywall contractor runs a continuous wall across the structural joint because nobody told him it was there, that wall is the new failure point. If the roofer laps the membrane straight over the gap with no expansion detail, the roof is the leak.
Walk the joint from the foundation to the roof on the drawings and confirm it is shown, in line, on every layer it crosses, including the fire-rated assemblies. A joint that jogs, stops, or disappears in one trade's scope is not a continuous joint. It is a hidden restraint waiting to crack the building at the spot where the line was broken.
How do you size a movement joint?
You size the joint to the movement it has to absorb, then to the movement capacity of the cover that bridges it. The structural engineer owns the movement number; the manufacturer owns the cover's capacity. Your job in the field is to build the width that was designed, not to round it off.
For a thermal expansion joint, the movement is the temperature swing times the segment length times the material's coefficient of expansion. Concrete runs about 5.5 millionths per degree Fahrenheit, varying with the aggregate, and steel framing is in the same range. Multiply across a long segment and a real temperature range, including the surface temperature in sun, not just air temperature, and the result is the thermal movement the joint must take. Seismic joints add the calculated story drift on top, and below-grade joints add structural movement and construction tolerance.
Then the cover has to keep up. A standard expansion joint cover commonly carries movement of about plus or minus 25 percent of its nominal width, while a system rated to seismic standards can carry roughly plus or minus 50 percent. So a 4 inch nominal joint cover that moves 25 percent handles about an inch of total movement; if the design movement is larger, the joint and its cover both grow. The numbers above are typical orders of magnitude, not design values. The required width and the cover selection are the engineer's and the manufacturer's calls under the project specification and the adopted code. Do not under-size a joint to make a cover fit. Size the cover to the joint.
The expansion-joint cover system
An expansion-joint cover is the manufactured assembly that bridges the open gap, carries whatever crosses it, and still lets the two sides move. Below the cover the gap stays open. The cover is what makes the gap usable, by giving a floor something to walk on, a wall a finished face, a roof a watertight bridge, and a fire barrier a path that stays rated.
These are engineered systems, not site-fabricated details, and they come from a handful of specialty manufacturers. Sika Emseal, Construction Specialties, Nystrom, Balco, and others publish covers for floors, walls, ceilings, roofs, and below-grade conditions, each rated for a movement class, a traffic or load condition, and where required a fire rating and a watertight rating. The specification under section 07 95 00, expansion control, is where these get called out.
The mistake is treating the cover as a finish accessory picked late. The cover has to match the actual movement, the actual joint width, the traffic or load, the finish on both sides, and the fire and water ratings, and those decisions interact. Pick the cover with the engineer's movement number and the manufacturer's tables in hand, early enough that the framing and the substrate are built to receive it. A cover ordered to fit a gap that was poured wrong is a compromise on day one.
Floor joint covers
A floor expansion joint cover has to take foot traffic, rolling loads, and sometimes forklifts and trucks, stay close to flush so it is not a trip hazard, and still move. Those goals pull against each other, which is why floor covers are the most varied part of the catalog.
The common types are a metal cover plate that slides over the gap, a flush gland or seal set into the floor, and for heavy traffic a load-rated assembly with a center bar that shares the wheel load across the gap. The right one depends on the load and the finish. A lobby with stone or tile wants a flush, narrow seal that disappears into the floor pattern. A loading dock or a parking deck wants a load-rated cover that will not get torn out by a pallet jack or a tire.
The detail that gets botched is the transition to the finish floor. The cover has to land flush with the tile or the slab on both sides, not proud of it and not sunk below it, or it becomes the thing everyone catches a cart on and eventually a wheel rips it loose. Set the cover height to the finished floor, on both sides, and confirm it before the floor finish locks the elevation in.
Wall and ceiling covers
Wall and ceiling covers bridge the joint on vertical and overhead surfaces, where the demand is a clean finished look that still lets the two sides move, rather than a load. They are usually a metal cover plate or an extrusion over a flexible center, sized to the movement and finished to match the wall or ceiling.
The movement still has to be respected even though nobody walks on these. On a seismic joint the wall cover has to take the same multi-directional drift as the floor below it, so the cover slides and flexes rather than bolting tight on both sides. Screw a wall cover solidly to both sides of a moving joint and you have stitched the joint shut at that line, and the cover buckles or shears the first time the building moves.
On a ceiling, the cover also keeps the joint from being a continuous open slot that dumps light, sound, and air between spaces, while still moving. Keep the fastening to one side floating against the other, follow the manufacturer's anchorage, and do not let a finisher caulk the cover rigidly to both sides to make it look tidy.
The roof expansion joint
A roof expansion joint carries the building's movement joint up through the roof and keeps water out while it moves. The standard detail is a pair of raised curbs, one on each side of the gap, with a flexible cover or bellows spanning between them, so the membrane never has to stretch across an open moving gap at deck level.
Raising the joint on curbs is the point. Water runs to the low field of the roof, so lifting the joint above the drainage plane keeps standing water and flow away from the one line in the roof that is designed to move. The membrane turns up each curb and ties into the cover or the bellows, and the cover takes the movement above the water line.
This roof condition is its own detail with its own failure modes, and it deserves the same attention as the rest of the roof. The first thing a roof inspection looks at is terminations and flashings, because that is where most leaks start, and a roof expansion joint is all termination. Do not let the roofer flatten the joint into the field or lap straight over the gap to save a curb. Keep it raised, keep the cover free to move, and tie the membrane to the curbs the way the roofing manufacturer details it.
Do movement joints need a fire rating?
Where a movement joint crosses a fire-rated floor or wall, the joint needs a tested fire-rated joint system, because the open gap is a hole straight through the fire barrier. An ordinary firestop will not do, because a normal firestop is meant to seal a static opening, and this opening moves. You need a firestop that moves with the joint and still holds the rating.
The test standard is UL 2079, tests for fire resistance of building joint systems, which evaluates floor-to-floor, floor-to-wall, wall-to-wall, and head-of-wall joints. The part that matters: the system is cycled through its rated movement, commonly several hundred cycles, before the fire endurance test, so it has to keep the rating after it has moved, not just when new. A fire-rated expansion joint system pairs the movement capacity and the fire rating in one tested assembly. Manufacturers list these as fire-rated, often watertight and traffic-bearing as well, in single-install products.
Two things make this go wrong on the job. The fire-rated joint is specified for the structural and architectural joint but not carried through every rated assembly the joint crosses, so there is a rated wall somewhere with an unrated moving gap in it. Or a tested system is installed outside its listing, wrong depth, wrong backing, wrong width, which voids the rating that the test established. The required rating and the listed system are set by the code, the architect, and the manufacturer's listing. Install the tested assembly as listed, and confirm it is shown on every rated layer the joint crosses.
Why do expansion joints leak?
Expansion joints leak because they are an open path straight through the building envelope that also has to move, and a seal that has to stretch and compress for decades is the hardest seal to keep watertight. Waterproofing the joint is the single most common place these systems fail, and the leak usually shows up far from the joint, after water has traveled inside the assembly.
On any exterior, plaza, or below-grade joint, the waterproofing has to be continuous and tied into the building's waterproofing on both sides, so there is no break in the water plane at the gap. The usual elements are a water bar or a flexible gland in the joint and a membrane on each side lapped into it, or a manufactured watertight expansion joint that builds the seal and the movement into one product. The leak path is always the lap, the transition, and the change of plane, not the middle of a run.
The field reality is that a joint sealed with a bead of caulk across a moving gap fails fast, because no sealant joint that wide survives that much cyclic movement for long. Use a tested watertight joint system rated for the movement, lap it into the waterproofing on both sides, and treat every inside and outside corner and every transition as the place it will leak if you rush it. Skip the redundancy and the detailing at the transitions and you own the leak, and on a buried joint the repair means excavating to reach it.
Below-grade and plaza joints
Below-grade and plaza expansion joints carry the joint through a deck that has water sitting on it or soil and groundwater pushing against it, so the waterproofing is under constant pressure and the joint is buried where you cannot easily reach it to fix. This is the worst place to get a joint wrong, and it is where the cost of a failure is highest.
Hydrostatic pressure changes the problem. Below grade, water is not just draining past the joint, it is pressing on it, so the seal has to hold against head, not just shed runoff. Manufacturers make dedicated below-grade joint systems for exactly this, designed to be tied into the below-grade waterproofing membrane as a continuous water plane. The detailing of the surrounding waterproofing is its own subject, but the joint has to integrate with it, not sit beside it.
Because the repair means digging up the plaza or the structure to reach a buried joint, redundancy is worth its cost here. The structural engineer and the waterproofing designer set the system, and the field discipline is to protect the installed joint and its membrane laps from damage during backfill and topping, since a punctured membrane at a buried joint is invisible until the leak appears inside months later.
Where the joints go
Movement joints go where the building wants to move differently from one part to the next: at length intervals on a long building, at re-entrant corners, where wings meet the main mass, and where the structure or its height changes. The engineer sets the actual locations, but the patterns are predictable.
Length is the classic trigger for a thermal expansion joint. The long-standing reference is the National Academy of Sciences Federal Construction Council Technical Report No. 65, which relates the maximum building length without a joint to the design temperature change. As rules of thumb that flow from it, concrete buildings are commonly jointed around 200 ft and steel-framed buildings around 300 ft, with the real spacing depending on temperature swing, framing, and restraint. Treat those as starting points, not as the answer.
Shape matters as much as length. A re-entrant corner, an L, T, or U plan where a wing sticks out past a notch, concentrates stress and makes the wings move differently, so joints are often placed to separate the wings at the notch. The code flags re-entrant corners as a plan irregularity when the projection past the corner is large, and for seismic design that drives a separation. Building separations and the seismic gap width are calculated under the adopted building code; confirm both the locations and the widths with the structural engineer and the code, not from a rule of thumb.
Coordinating the joint across trades
The joint is a single line that lives in every trade's scope at once, and coordinating it is most of the work. The structural engineer sets the gap and the framing at it. The architect carries it through the walls, ceilings, floors, roof, and facade and picks the covers. The fire-rating designer makes it keep its rating. And every trade whose work crosses that line has to detail their work to move with it.
The failure is always a handoff. Each trade builds its own scope correctly in isolation, and the joint dies at the seam between two of them, where a wall, a topping, a membrane, or a conduit was run straight across without anyone realizing a movement joint was there. The joint does not show up as a single coordinated line on anyone's drawing unless someone makes it.
Make the joint a coordination item in its own right. Mark it on a composite drawing, walk it through every assembly it crosses, and confirm that every trade whose work touches the line knows it is there and has a moving detail for it. Flexible connections at the joint, in the structure, the finishes, and the MEP, are what let the building move as designed. The list of trades and the moving details belong on one page, owned by one person, not scattered across a dozen submittals.
MEP crossing the joint
Any pipe, duct, or conduit that crosses a movement joint has to cross it with a flexible loop or connector, because a rigid line run straight across a moving joint breaks at the joint when the two sides move. The MEP that crosses the joint is one of the most overlooked details, and a sheared sprinkler main or gas line at a seismic joint is a life-safety problem, not just a leak.
The fix is a flexible element sized to the same movement the joint takes. Piping uses flexible expansion loops or seismic V-loops, ductwork uses flexible connectors, and conduit uses expansion-deflection fittings that allow both length change and angular movement. The connector has to handle the full multi-directional movement at a seismic joint, not just in-and-out, and it has to be installed near the joint, with enough slack to absorb the drift. For sprinkler piping, for instance, the seismic separation assembly is installed close to the joint so it actually takes the movement.
The detail to enforce is simple to state and easy to skip: no rigid line crosses the joint. Inspect for it, because a conduit strapped tight on both sides of a seismic joint looks finished and is a failure waiting for the first real movement. Provide the flexible crossing, give it slack, and support it so the slack is not strapped out of it by someone who thought it looked loose.
Maintaining the joint over its life
Joint covers, seals, and water bars are wearing parts, and they need inspection and maintenance like any other moving component on the building. They move thousands of cycles a year, take traffic and weather, and they fail gradually, so the leak or the trip hazard arrives quietly unless someone is looking.
Floor covers loosen and their fasteners back out under rolling loads. Glands and sealants harden, shrink, and pull away from the substrate after years of cycling and sun. Water bars and membrane laps are the parts you cannot see, and they are the ones that matter most, because their failure is the leak that shows up inside. A joint that was watertight at handover is not watertight forever.
Put the joints on the building's inspection list. Check the covers for loose fasteners and damage, check the seals for hardening and separation, and on any exterior or buried joint watch for the staining and interior leaks that say the waterproofing is going. Replace the seal or the cover with the manufacturer's matching part before the leak does interior damage, not after.
How these joints fail in the field
The failures cluster, and they trace back to the same handful of root causes every time. Name them and you can catch most of them before they are built.
A long building with no expansion joint cracks itself, usually at the weakest plan feature, because the thermal movement had nowhere to go. A control joint gets detailed where the building needed an expansion joint, so the planned crack line becomes a planned restraint and the building separates somewhere else. The joint is continuous in the structure but a wall, topping, ceiling, or membrane was run rigidly across it in one trade's scope, and that layer is the crack or the leak. The gap was under-sized for the real movement, so the two sides close and bear on each other on the hottest day or in the design earthquake. The joint was never properly waterproofed or the waterproofing was not lapped at the transitions, and it leaks. And a rigid pipe, duct, or conduit was run straight across the joint, so it shears when the building moves. Each one is preventable, and each one is expensive to fix after the building is closed in.
What to document
Write down what each joint is and what went into it, because on a buried or rated joint the record is the only way to know the assembly without tearing it open, and the crew that maintains or replaces the cover years out works straight off your notes. Capture what the joint is, what it does, and the system that was installed, for each joint on the building.
Record the joint type and the movement it was designed for, the locations, the engineer's required width and the actual built width, the cover system and its manufacturer and model with the movement and fire and water ratings, the fire-rated assemblies it crosses and the listed system used in each, the waterproofing tie-in detail, and the flexible MEP crossings provided. Tie each to the drawing and the submittal so the next person can find the part to replace it.
| Item to record | Why it matters |
|---|---|
| Joint type and design movement | Says what the joint is for and how much it must take |
| Required vs built width | Confirms the gap matches the engineer's movement number |
| Cover system, make, and model | The part to match when it wears out |
| Movement, fire, and water ratings | Ties the cover to what the joint had to do |
| Fire-rated assemblies crossed and listed system | Proves the rating carries through every barrier |
| Waterproofing tie-in detail | Shows how the water plane stayed continuous |
| Flexible MEP crossings provided | Confirms no rigid line crosses the joint |
Common mistakes
- Building a long structure with no expansion joint, so the thermal movement cracks it at the weakest point.
- Detailing a control joint where the building needed an expansion joint, confusing crack control with separation.
- Running a joint that is continuous in the structure but broken by a wall, topping, ceiling, or membrane in one trade's scope.
- Under-sizing the gap for the real thermal or seismic movement, so the sides close and bear on each other.
- Failing to waterproof the joint, or not lapping the waterproofing into it at corners and transitions, so it leaks.
- Running rigid pipe, duct, or conduit straight across the joint with no flexible loop or connector.
- Carrying a fire-rated joint system on the main joint but not through every rated assembly the joint crosses, or installing it outside its listing.
Field checklist
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Standards and references
The structural engineer owns the movement design. The engineer calculates the thermal, moisture, settlement, creep, and seismic movement for the specific building, sets where the joints go, and sizes the gap and the seismic separation. Length-based spacing traces to the National Academy of Sciences Federal Construction Council Technical Report No. 65, which relates maximum building length without a joint to design temperature change, and to the spacing practices in the concrete and steel literature. Those are starting points; the engineer's calculation and the project specification control the locations and widths.
The expansion-joint-cover manufacturer owns the cover. Specialty makers such as Sika Emseal, Construction Specialties, Nystrom, and Balco publish floor, wall, ceiling, roof, and below-grade systems rated for movement class, load, fire, and water, specified under section 07 95 00, expansion control. Select and install to the manufacturer's listing and movement tables.
Fire-rated joints are tested to UL 2079, tests for fire resistance of building joint systems, which cycles the system through its rated movement before the fire endurance test and covers floor-to-floor, floor-to-wall, wall-to-wall, and head-of-wall joints. The International Building Code governs the required fire rating, the seismic separation, and where joints are required, with provisions that reach down to items like a seismic separation in large continuous ceilings. Three rules carry the whole subject: the joint must run continuously through every layer it crosses; it has to be sized to the real movement and fire-rated where it breaks a barrier; and it has to be waterproofed with the MEP crossings detailed to move. The required spacing, sizing, and fire rating are calls for the structural engineer, the manufacturer, and the adopted code with local amendments. Confirm them against the current edition before you build.
Units and terms
Movement joints carry several names that get used interchangeably across structural, architectural, and waterproofing drawings, so the same line can read differently from one sheet to the next.
Movement is given in inches in the US tables and millimeters in metric ones, and a cover's capacity is stated as a movement class or a plus-or-minus percentage of its nominal width. The coefficient of thermal expansion is in length-per-length per degree, about 5.5 millionths per degree Fahrenheit for typical concrete. Seismic separation is given as a calculated drift in inches. Match the unit to the source before you trust the number.
- Expansion joint
- A full gap through the structure that lets parts move apart and back, mainly for thermal and moisture movement
- Control / contraction joint
- A planned weak line that makes concrete crack where you want it, within a single element, without separating it
- Construction joint
- A planned stop between two pours that bonds back to act as one element and transmits load across
- Seismic / isolation joint
- A wide separation between building sections so they can sway independently in an earthquake without pounding
- Expansion-joint cover
- The manufactured assembly that bridges the open gap, carries traffic or finish, and still allows the movement
- Fire-rated joint
- A tested joint system, commonly to UL 2079, that keeps a fire barrier rated while the joint moves
- Movement range
- The total movement the joint must absorb, from thermal swing, moisture, settlement, creep, and seismic drift combined
FAQ
What is a building expansion joint?
A building expansion joint is a full gap through the structure that splits a building into segments so they can grow and shrink with temperature and moisture without pushing on each other. It runs continuously through every layer, carries no reinforcement across, and is covered by a system that bridges the gap while still moving.
What is the difference between a control joint and an expansion joint?
A control joint steers a shrinkage crack to a planned line within one element, which stays a single piece. An expansion joint is a full separation that lets two parts of the building move apart and back, carried through the whole assembly. One manages cracking in a member; the other separates members.
What is a seismic joint?
A seismic joint is a wide separation between building sections, or adjacent buildings, sized so they can sway out of phase in an earthquake without pounding into each other. It takes multi-directional movement, and its width is the structural engineer's calculation of story drift under the adopted code, not a field estimate.
Why do expansion joints leak?
Expansion joints leak because they are an open path through the envelope that also has to move, and the leak almost always starts at a lap, corner, or transition, not mid-run. A bead of caulk across a moving gap fails fast. Use a tested watertight joint system lapped into the waterproofing on both sides.
How do you size a building movement joint?
Size it to the movement, which for a thermal joint is the temperature swing times the segment length times the expansion coefficient, plus seismic drift where it applies. Then match a cover rated for that movement. The structural engineer sets the width and the manufacturer sets the cover capacity; do not under-size the gap to fit a cover.
Does a movement joint need a fire rating?
Where a movement joint crosses a fire-rated floor or wall, yes. The open gap is a hole through the fire barrier, and an ordinary firestop will not move with it. Use a system tested to UL 2079 that keeps its rating after being cycled through its movement, installed as listed on every rated layer the joint crosses.
How far apart do expansion joints go in a building?
It depends on the building, but as rules of thumb concrete buildings are often jointed near 200 ft and steel near 300 ft, tracing to Technical Report No. 65, which ties spacing to the design temperature change. Shape matters too, since re-entrant corners and wings often need joints. The structural engineer and the code set the actual locations.
What happens when pipe or conduit crosses a movement joint?
A rigid pipe, duct, or conduit run straight across a movement joint shears when the two sides move, which at a seismic joint can be a life-safety failure. Cross it with a flexible loop or connector sized to the full movement, installed near the joint with slack, so the line moves with the building instead of breaking.
Why does a long building crack if it has no expansion joint?
A long building still expands and contracts with temperature, and if it is restrained the movement turns into stress instead of motion. Concrete and masonry have little tension capacity, so the building relieves that stress by cracking, usually at the weakest plan feature. An expansion joint gives the movement a place to go instead.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.