Expansion Joint Glossary (A-Z)

Expansion Joint Glossary (A–Z)

Plain-language definitions of the terms used in building expansion joint systems, EPDM seals and waterproofing membranes.

Anchorage: The fixings and embedment that hold a joint system to the structure and transfer load into it.
Backer rod: A compressible foam rod placed in a joint to control sealant depth and provide a bond-breaker.
Bellows: A flexible convoluted seal (rubber, fabric or metal) used mainly in pipe and duct expansion joints.
Blockout: A recess formed in the slab edge to receive and anchor a floor joint frame flush with the finished surface.
Centre plate (cover plate): The plate that spans the joint gap in a metal cover system, sliding or self-centring as the structure moves.
Coefficient of thermal expansion (α): How much a material expands per degree of temperature change — e.g. ~11×10⁻⁶/°C for concrete. Drives thermal movement.
Compression seal: A pre-compressed elastomeric profile wedged into a joint; it seals by remaining in compression.
Control (contraction) joint: A planned line of weakness that forces concrete to crack in a chosen location as it shrinks. Not a moving joint.
Deflection: Vertical movement of a floor, deck or beam under load — one of the movements a joint may need to accommodate.
Elastomer: A rubber-like polymer (EPDM, neoprene, silicone, TPV) that flexes and returns to shape; the sealing element of most joints.
EPDM: Ethylene propylene diene monomer — the workhorse building-joint elastomer, with excellent weather, UV and ozone resistance.
Fire barrier: A joint system component that maintains the fire-resistance rating of a wall or floor across the joint (e.g. 2–4 hr).
Finger joint: A bridge deck joint with interlocking steel fingers for very large movement. A structural, not architectural, type.
Gland: The elastomeric sealing element (often EPDM) retained by the frame in a watertight or deck joint.
Intumescent: A material that expands when heated to seal a gap against fire and smoke; used in fire-rated joints.
Movement class: A system’s rated movement capability, classified by ASTM E1399 as Category I–IV (IV is highest).
Neoprene (CR): Chloroprene rubber — a general-purpose sealing elastomer with moderate oil and weather resistance.
Nosing: The edge material (often elastomeric concrete) protecting the joint arris against traffic and impact.
Plaza deck / split slab: A landscaped or trafficked deck over occupied space, requiring a watertight, often insulated, joint.
Seismic joint: A high-movement, multi-directional joint that separates building sections to absorb earthquake movement.
Self-centring: A mechanism (spring or bar) that returns a cover plate to its central position after movement.
Shore hardness: A measure of elastomer hardness (e.g. Shore A 65 for a joint gasket).
Strip seal: A bridge deck joint with a single elastomeric strip in steel edge rails, for moderate movement.
Thermal movement: Expansion and contraction of a structure with temperature; the most common driver of joint design.
TPV / TPE: Thermoplastic vulcanisate / elastomer — recyclable, weldable seal materials used in floor and roof joints.
UL 2079: The standard fire test for building joint systems, giving hourly ratings (1–4 hr).
Vapour barrier: A membrane under a floor joint cover that resists moisture and vapour migration.
Watertight: A joint that resists water passage, verified by test (fire-hose or pressure) — essential for decks and roofs.
Wind sway: Lateral, cyclic movement of a tall building under wind load, felt at facade and seismic joints.