dap

A dap is a rectangular notch or recess cut into the face or edge of one timber so that another timber can sit flush against it or slot into it. The cut timber (the one receiving the notch) is said to be "dapped"; the timber that sits in the dap is sometimes called the "dapped piece" or simply "the piece in the dap". The joint creates a larger gluing surface and prevents the pieces from sliding sideways, making it stronger than a simple butt joint.

Daps are most commonly cut in roof carpentry, particularly in the construction of roof trusses and structural frames. A typical application is where a rafter meets a wall plate or beam: the carpenter cuts a dap about 1.5 to 2 inches deep into the rafter, and the rafter then sits down onto the plate with part of its depth occupied by the dap while the remaining timber rests on the plate itself. This bearing arrangement distributes load more evenly and prevents the rafter from sliding down the plate under load or weather stress.

A dap is cut with a handsaw or power saw, with the depth and width determined by the timber sizes and the engineer's calculations. The notch must be square and true; a sloppy dap leaves gaps that weaken the joint and allow water ingress. The timber being dapped must still have sufficient depth remaining below the notch to carry the structural load, which is why daps are limited in depth and are never cut more than one-quarter the full depth of the timber on load-bearing members.

Daps appear in various configurations: full-depth daps that cut completely through the width of a timber, or partial daps that cut only partway across. They are also called "bird's mouths" in certain regional uses, though that term is sometimes reserved for angled notches. The confusion arises because both achieve the same goal of seating one piece on another without slipping.

Poorly cut daps cause timber to split during seasoning or under load, and create stress concentrations that lead to failure in heavy-load situations. Modern engineered timber frames sometimes replace daps with metal plates and bolts for clarity and consistency, but hand-cut daps remain standard in traditional carpentry and in situations where appearance matters.