sister

To sister a beam means to bolt, lag-screw, or bolt a second timber or steel member directly against an existing beam, creating a composite section that shares the load. The new member, called the sister beam, runs parallel to the original for part or all of its length. This is structural repair and reinforcement in one operation, commonly used when a primary beam has developed damage, sagging, or insufficient capacity for changed loads.

The sister beam must be sized and positioned to address the specific failure mode. If the original beam is cracked or split along the grain, the sister typically runs its full length. If it sags under dead load, a shorter sister placed under the worst deflection may suffice. Fasteners matter greatly: lag bolts (0.5 to 1 inch diameter) or through-bolts spaced 12 to 24 inches apart along the beam are standard; spacing depends on load transfer demands and the rigidity of the connection.

Sistering works best on timber joists, rim boards, and floor beams in occupied buildings where removing the original beam is impractical or impossible. It is less common on steel beams, where welding a plate or bolting a channel is often preferred. The technique also applies to joist repairs in older homes where framing has rotted or twisted; a new joist sistered to the damaged one lets you avoid cutting out the failure entirely.

The weakness of sistering is that the original and sister beams must remain in intimate contact under all load conditions. If the original beam has already sagged significantly, installing a sister without shoring and jacking will not erase the deflection; the sister will only prevent further movement. Poor fastener placement or spacing allows the two members to slip relative to each other, defeating the composite action and concentrating stress at the fasteners.

The name likely arises from the two beams functioning as parallel members, side by side, like siblings. It is distinct from splicing (joining end to end) or bracing (adding lateral support). In timber framing, sistering is quick and reversible compared to structural surgery; in modern building codes, it is recognized as a valid repair method when properly engineered and inspected.