Where the extra timber goes
The splice count is the part everybody gets right and the material figure is the part everybody gets wrong. Suppose a sixty foot plate out of twenty-four foot stock. Sixty over twenty-four is two and a half, so three sticks, two splices. That much is obvious. What is less obvious is that three twenty-four foot sticks give seventy-two feet of timber and only sixty feet of plate, because at each splice two pieces are lying on top of each other for the whole length of the scarf. With the four foot two inch scarf this page defaults to, two splices eat 8.33 feet, and the sums close: sixty plus 8.33 is 68.33 feet, which is what you have to buy, and the remaining 3.67 feet of the seventy-two comes off the ends.
That is why this page reports the material length separately from the finished length. On a shallow scarf — and shallow is the usual case in timber framing, because a shallow taper puts more long grain in contact — the overlap can be several feet each. Four splices at five feet is twenty feet of timber that exists in the frame and adds nothing to its length.
Slope, and what it costs
The scarf length is the timber depth multiplied by the slope. An eight inch deep plate at six to one gives a forty-eight inch taper; at three to one it gives twenty-four. The shallow one uses twice the timber and takes far longer to cut. Both of those are real costs, and neither of them is the reason to choose one over the other. The slope is a structural choice about how the joint transfers load along the grain, and it belongs to whoever engineered the frame. This page takes the figure and does arithmetic with it.
| Depth | 4:1 | 6:1 | 8:1 |
|---|---|---|---|
| 6 in | 24 in | 36 in | 48 in |
| 8 in | 32 in | 48 in | 64 in |
| 10 in | 40 in | 60 in | 80 in |
| 12 in | 48 in | 72 in | 96 in |
Add the square abutments to those. A scarf that runs out to a knife edge is fragile at the tip, hard to cut cleanly and prone to breaking off during handling, so the tapers are normally stopped square a little short of nothing — an inch at each end is a working figure, and it adds two inches to the joint length.
How the piece count is derived
With every piece cut to the full stock length, the run covered by n pieces is n times the stock, minus the (n-1) overlaps. Setting that equal to the run and solving gives n equal to the run minus one scarf, divided by the stock minus one scarf, rounded up. The subtraction in the denominator is the important part: what each additional stick buys you is not its full length but its length less a scarf. With a twenty-four foot stock and a four foot two inch scarf each stick past the first advances the run by 19.83 feet, not twenty-four, and if the scarf ever gets as long as the stock the arithmetic stops working entirely because a stick can no longer reach past its own joints.
Splice positions
The positions this page reports follow directly from that: the first joint centre sits one advance plus half a scarf from the start, and each one after that is one advance further along. It compares them to a bay grid only so you can see the relationship, because whether a splice may land in a bay, over a post, or somewhere specific relative to a bent is decided by the structure and not by the packing. If the drawing wants the joints somewhere else, cut the pieces to suit and use the manual splice count instead.
Questions people ask
Why does a spliced run need more timber than its finished length?
Because at every splice two pieces of timber occupy the same stretch of the run. The overlap is the scarf length, it is present twice and counts once toward the finished dimension, so every splice adds one scarf length to the material. On a sixty foot run with two four foot scarfs, that is eight feet of timber that does not lengthen the plate.
What slope should the scarf be?
This page does not supply one, and there is no single right answer. The slope governs how load passes along the grain across the joint, and the choice is made by the frame designer or engineer for the member, the loading and the species. Shallow tapers use more timber and take longer to cut; that is a cost consideration and not the deciding one.
Can a splice sit anywhere along the run?
That is a structural question, not a layout one. Where a joint may fall relative to the supports is part of the frame design, and this page reports the distance to the nearest bent purely so you can hold the layout against your drawing. It does not move joints, and it has no view on where they should be.
What is the square abutment for?
It stops the taper running out to a feather edge. A knife-edged tip is weak, splits off while the timber is being handled, and gives you nothing to work to when cutting. Stopping the taper square an inch or so short leaves a small blunt end at each tip. It adds twice that to the joint length, which is why it has its own field.
My run fits in one stick. Do I still need any of this?
No, and the calculator says so. The scarf figures still get reported in case you are splicing for another reason — a timber that will not come down the access road in one piece is a common one — but there is nothing forcing a joint into a run that fits.