Like other joints of its kind, a half-lap scarf joint is used to join two timbers end-to-end to span a length greater than the lumber at hand. The half-lap is probably the simplest of all the scarf joints to cut, as it is merely two timbers reduced to half of their thickness where they lap over each other. This provides a face-grain-to-face-grain joint with plenty of connecting surface. The joint is usually secured with screws or pegs.
This is my kind of joint: dumb simple.
Are there any limitations for its use, in terms of timber size or span? For instance, I’m wondering if it can be used for a 24′ long tie beam in a 24’x36′ garage/workshop.
For a tie-in spanner beam – if that is all the beam is used for – yes. The problem is when you want to attach anything to it ( like a chain hoist)- then -no. Since the best scarf joint only gives the attached pieces 60% of the load-bearing capacity of a regular piece of lumber- ( if a piece of 2x10x10 has a load bearing of 200 lbs- at its centre- to extend the piece to 18′ – the l/b capacity is reduced by almost half. Therefore to make a beam 24′ long – you would need to overlap the scarf joint by approx 2′ ; and to get the strength for any kind of attachment you would have to laminate 3 more pieces of wood (of the same size) together to attain the original strenth (l/b capacity) of the beam.