In building construction, an expansion joint is a mid-structure separation designed to relieve stress on building materials caused by building movement induced by:

- thermal expansion and contraction caused by temperature changes,
- sway caused by wind,
- seismic events, etc.

Because the joint bisects the entire structure, it marks a gap through all building assemblies--walls, floors, roofs, decks, planters, plazas, etc.  This gap must be filled to restore the waterproofing, fire proofing, sound proofing, air barrier, roof membrane, trafficable surface and other functions of the building elements.