grafting

Grafting in carpentry is the process of joining the end of one timber to the end of another timber to extend length, create a continuous piece, or repair damage. Unlike scarfing, which cuts diagonal faces on both pieces to overlap them, a graft typically involves end-grain contact, sometimes with an intervening block or tenon. The joint must transfer load along the grain direction, which makes the glue bond and mechanical fastening critical to strength.

The simplest grafting method uses a square butt joint with glue and dowels or wooden pegs driven through both pieces perpendicular to the grain. A stronger variant, the end-lap graft, overlaps the timber ends partially and glues the side surfaces in contact. For structural timber in beams or posts, a mortise and tenon graft cuts a tenon on one piece and a mortise in the other, creating mechanical interlock before gluing. Some practitioners use a T-graft or cross-graft where a tenon on one piece slides into a mortise cut into the side or end of another.

The wood species, grain direction, and moisture content all affect graft longevity. End grain absorbs glue unevenly, so grain-raising after wetting is common; sanding between coats and clamping under consistent pressure helps close gaps. The joint is weakest immediately after gluing, before the adhesive cures fully, which typically takes 24 hours for hide glue and longer for epoxy. Movement between pieces due to seasonal wood shrinkage can split a weak graft.

The term graft likely derives from horticultural grafting, where plant tissue is joined to stock. In timber work, it has been used at least since medieval carpentry, when extending beam lengths or joining pieces for ship hulls was essential. Modern industrial joinery uses biscuits, dowels, or pocket-hole fasteners rather than traditional mortise-and-tenon grafts, but the principle remains: end-grain joining is always considered a joint of last resort due to poor load transfer compared to long-grain connections.