The most common building expansion joint is a simple sealed joint: a gap formed during casting, filled for most of its depth with a compressible joint filler board, with a backer rod and a flexible sealant capping the top. Each component has a job. The filler board (commonly closed-cell or impregnated fibre/foam boards, or bituminous fibreboard) occupies the gap so debris and concrete do not bridge it, and compresses as the joint closes. The backer rod — a closed-cell polyethylene round cord — sits below the sealant to control the sealant depth, give it the correct hourglass profile and prevent three-sided adhesion (which causes premature tearing). The sealant is the weatherproofing, flexible skin. Sealant chemistry is chosen by exposure and orientation: polysulphide sealants are a long-proven workhorse for general building and horizontal joints; polyurethane (PU) sealants give excellent flexibility and abrasion resistance for floors and facades; and silyl-modified polymer (SMP / MS polymer) hybrids cure bubble-free, are paintable, low-odour and tolerant of slightly damp substrates. Use a non-sag (gun grade) sealant for vertical joints so it does not slump, and a self-levelling (pour grade) sealant for floors and pavements. Brand options across the nine manufacturers include Fosroc Thioflex 600 (polysulphide) and Nitoseal MS / PU ranges, Sika Polysulphide GG (gun grade) and PG/Pour Grade plus the Sikaflex PU range, MC-Bauchemie sealants, Master Builders Solutions MasterSeal joint sealants, and PU/polysulphide sealants from STP, UltraTech and Dr. Fixit. Always confirm the exact movement rating and primer requirement from the product TDS.

When joints are wide, must carry pedestrian or vehicle traffic, or must look clean in a finished space, a sealed sealant joint is replaced or supplemented by an engineered expansion joint cover or profile system. Floor and wall cover-plate assemblies typically use anodised aluminium (commonly 6063 alloy) bases with aluminium or stainless steel cover plates and a centre gasket or insert that flexes with movement; they bridge the gap, sit flush with the finish and survive foot and trolley traffic. Profile systems with a replaceable elastomeric insert suit tiled floors, malls and hospitals where hygiene and a wipeable surface matter. For roads, parking decks and bridge approaches, heavier modular or strip-seal joints (a steel-edged channel with a locked-in elastomeric seal) handle large movements and traffic loads. The trade-off versus a simple sealed joint is cost and complexity: cover systems are more expensive and need accurate setting-out and embedment during casting, but they are durable, serviceable (the insert/gasket can be replaced) and far better where the joint is exposed and walked on. Many of these mechanical systems are proprietary hardware specified alongside the chemical waterproofing; Space Arc can advise on pairing a cover or profile system with compatible sealants and waterproofing from the construction-chemical brands we distribute, so the hardware and the membrane work as one detail.

Even the right joint system fails if installed badly, and most joint failures are workmanship, not product. Follow a disciplined sequence. Surface preparation: the joint arrises must be sound, clean, dry and free of laitance, oil and old sealant — grind or wire-brush, then remove dust; weak or spalled edges should be repaired with a polymer-modified mortar before sealing. Backer rod: insert a closed-cell backer rod of the correct diameter (slightly larger than the gap) to the right depth so the sealant achieves the recommended width-to-depth ratio — typically a controlled, shallow profile rather than a deep slug of sealant. Use a bond-breaker tape instead of backer rod where the joint is too shallow, to still prevent three-sided adhesion. Priming: prime the joint faces if the TDS calls for it — skipping primer is a leading cause of adhesion loss. Sealant application: gun the sealant in one continuous pass, tool it firmly to wet both faces and form a slightly concave surface, and respect the joint width limits and minimum cure time before exposing to water or traffic. Common mistakes to avoid: bridging the filler with mortar or tile adhesive (which locks the joint and forces cracking elsewhere), using interior-grade plus-minus 25 percent sealant on an exposed plus-minus 50 percent facade joint, omitting the backer rod, sealing over a wet or dusty joint, and running tiles or screed straight across a structural movement joint. When to call a professional applicator: large or seismic joints, submerged and below-grade joints, traffic-bearing joints, and any joint feeding into a waterproofing system — these reward correct detailing. Space Arc Engineering provides both the matched materials and trained applicator support for exactly these situations.