Scarf joint
Also called: scarf, splayed scarf, tabled scarf, hooked scarf, bevelled scarf
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A scarf joint, or splice, is a kind of end joint: a joint that connects two timbers end to end along their length so that together they act as one continuous piece longer than either could be cut alone.
How a splice is formed
A splice is a form of lap joint, an overlapping joint, and ties several shorter lengths together into one long timber. Wood is joined end to end in one of two main ways: with a finger-shaped interlocking cut, called a finger joint, or with a sloped splice, the scarf proper, in which the two ends are tapered or bevelled to matching sloping faces, usually down to a feather edge, a taper so thin it comes almost to nothing. The English word traces back to an Old Norse term for an obliquely cut beam end. However well it is made, a timber built up with a splice ends up weaker than one single, unbroken length would be, though a well-manufactured scarf, finger joint or end-grain lap can reach up to 90 percent of clear, defect-free wood's tensile strength. Compared with a plain butt joint, where two square-cut ends simply meet, a splice's sloped faces offer far more long-grain glue area, meaning surface glued along the direction of the wood fibres rather than across them, giving extra strength and a cleaner look. The shallower the angle, the broader those long-grain faces become, so a shallow-angled glued splice makes for a particularly strong glued bond.
Choosing the type
Splices are grouped by the kind of load they must resist. A bending-compression scarf, the plainest form, resists bending and compression; a tension-and-bending scarf cuts one member's lap end square and mitres the other, much like the bending version. For loads combining tension and cross strain, a sideways bending load, one traditional form is the tabled and splayed scarf, combining a stepped table, a flat notched shoulder, with a sloped face; another, used in beams and posts, is the vertical scarf, simply a joint halved through both members. Carpenters have cut scarf joints by several methods. A squint-butted scarf is formed by trimming the ends at seventy degrees on a mitre gauge and cutting each piece to its own centre line; today a circular saw or a chop saw does the job just as often. In Japanese joinery the splice joint is called sogi tsugi; where a pin needs fitting, a rabbeted half-splice known as miyajima tsugi is used instead, and a lap joint, better suited to two heavy timbers, is generally preferred over a simple scarf for anything too heavy to hold and cut by hand. One documented example, a stop-splayed, undersquinted, cogged scarf, meaning a splayed splice with unmatched squared shoulders and a cog, a projecting block locked into a matching notch, held up the posts of an 1835 Shaker Village building at Hancock, Massachusetts. Fitting a cog into such a joint improves its resistance to sideways bending, and combining a table with wedges can produce a splice of great strength.
Proportion and strength
How long a splice needs to be depends on its proportions, and choosing them takes sound principle, an eye for looks, and the demands of the job at hand. The engineer Tredgold (1871) published influential rules for proportioning a splice's parts to a timber's tensile, compressive and shear strength. By his figures, fir left unbolted needs an overall splice length of roughly twelve times the beam's thickness; oak, ash or elm, also left unbolted, need only around six times the beam's depth, being stronger woods for the purpose. Where straps or bolts reinforce a splice, they should be sized to match the strength of the timber parts they reinforce. Where a spliced beam also carries a lengthwise pull, a wrought-iron plate fixed beneath the joint supplies the extra tensile resistance needed. Writing later, Hodgson (1902) judged the ordinary splice the strongest and most economical choice overall for lengthening a tie, a timber under tension.
Fit, placement and use
On a well-made splice every face sits level with its neighbour, none standing proud, raised above the surrounding surface. In a long timber-framed plate, the horizontal timber running along the top of a wall frame, the position of a needed splice is chosen near posts and braces so the frame's overall strength holds up. Where a splice will be seen, such as along a long run of moulding or trim, a seamless finish matters most.