Plate V · Joinery engineering
Mortise-and-tenon joinery specs
Drawbore offsets, riven oak trunnels, housing depths and the shrinkage allowances we cut into every frame.
What a drawbore does
A drawbored mortise-and-tenon joint pulls itself tight. We bore the peg hole through the mortise cheeks first, slide the tenon home, and mark where that hole lands on the tenon. Then we pull the tenon out and bore its hole a little closer to the shoulder. When the tapered oak peg is driven, it has to bend slightly to find the offset hole, and that bend draws the shoulder hard against the face of the mortised timber.
The joint stays closed when the wood shrinks, it needs no glue and no steel, and it can be taken apart a century later by driving the peg back out. That is the whole reason we still cut frames this way.
Shop tolerances
These are the numbers our layout crew works to on 8 in. and 10 in. timbers. Your engineer may tighten them for a particular joint, and we will follow the stamped drawing over this table every time.
| Joint | Tenon thickness | Tenon length | Mortise depth | Pegs | Drawbore offset |
|---|---|---|---|---|---|
| Post to tie beam | 2 in. (+0 / −1/32) | 5 in. | 5-1/4 in. | 2–3 × 1 in. | 1/8 in. |
| Post to plate | 2 in. | 4 in. | 4-1/4 in. | 2 × 1 in. | 1/8 in. |
| Rafter to king post | 2 in. | 4 in. | 4-1/4 in. | 2 × 1 in. | 3/32 in. |
| King post to tie | 2 in. | 4 in. | 4-1/4 in. | 2 × 1 in. | none, hangs |
| Knee brace (each end) | 1-1/2 in. | 3-1/2 in. | 3-3/4 in. | 1 × 1 in. | 3/32 in. |
| Purlin housing | 1-1/2 in. | 3 in. | 3-1/4 in. | 1 × 1 in. | 1/16 in. |
A few rules sit behind the table. Mortises are cut a quarter inch deeper than the tenon is long, so the tenon end never bottoms out before the shoulder closes. Tenons are fitted to the mortise with no more than a thirty-second of slop, checked with a gauge block rather than by eye. The peg hole sits 1-1/2 in. to 2 in. back from the shoulder, and we leave at least 3 in. of wood between the peg and the end of the tenon. That end wood, the relish, is what actually resists the pull. Cut it short and the peg will split its way out under load.
Drawbore offset goes down as the timber gets harder and as the joint gets smaller. In white oak we rarely go past 1/8 in. In softer Douglas fir and hemlock a 1/8 in. offset is still fine for main joints, but on braces we drop to 3/32 in. so the peg does not crush the tenon instead of drawing it.
Riven white-oak trunnels
A trunnel (tree-nail) is the oak peg that locks the joint. Ours are split from straight-grained white oak billets with a froe, never sawn. A split follows the grain from end to end, so the peg has no fibers running out of its side where it could snap. A sawn dowel almost always does.
After splitting we shave the blanks roughly octagonal with a drawknife and let them air-dry under cover until they are well below the moisture content of the frame. Dry pegs go into wetter timber, take up a little moisture, and swell tight in the hole. Before driving, each one gets a long taper on the leading end so it can find the offset hole.
| Use | Diameter | Length | Taper | Notes |
|---|---|---|---|---|
| Main joints (posts, ties, plates) | 1 in. | 10–12 in. | 2-1/2 in. lead | Driven proud and trimmed after the frame is raised |
| Braces and struts | 1 in. | 8–10 in. | 2 in. lead | One peg per end unless drawings call for two |
| Purlins and collars | 3/4–1 in. | 8 in. | 1-1/2 in. lead | Lighter peg where the housing does most of the work |
| Scarf joints | 1 in. | 12 in. | 2-1/2 in. lead | Usually four pegs, plus folding wedges in the key |
We drive pegs with a heavy commander, not a framing hammer. A peg that stops moving before its taper clears the far cheek usually means the offset was too big or the hole was not straight. We back it out, check the hole and, if needed, bore a fresh peg rather than force it.
Housings
A housing is a shallow pocket cut into the face of a timber so the whole end of the joining member sits in it, not just the tenon. It hides gaps as the wood moves, and it gives the joint extra bearing. We house most main joints 1/2 in. deep. Floor joists and purlins that carry real load get a 3/4 in. to 1 in. housing so their full width bears on the timber below.
The rule we teach every apprentice: a housing should be deep enough to hide shrinkage and no deeper. Every extra quarter inch you cut out of a tie beam or a post is wood that is no longer carrying the frame.
Shrinkage and movement allowances
Wood shrinks as it dries below roughly 28 to 30 percent moisture, the point where the cell walls start to give up water. It shrinks most around the growth rings (tangential), about half as much across them (radial), and barely at all along the length. A boxed-heart timber, one with the pith somewhere near the middle, moves differently on every face, and that is where checks come from.
| Species | Tangential | Radial | An 8 in. face, green to about 12% MC |
|---|---|---|---|
| White oak | 10.5% | 5.6% | about 3/8 in. |
| Douglas fir (coast) | 7.6% | 4.8% | about 1/4 in. |
| Eastern hemlock | 6.8% | 3.0% | about 3/16 in. |
Figures are typical values for the species. The right-hand column averages tangential and radial movement and assumes drying takes away a bit over half of the total shrinkage. Real timbers vary.
How we plan for it:
- Kiln-dried stock where it matters. Our white oak and Douglas fir are dried through the section, not just on the surface, so most of the movement has happened before we cut a joint.
- Undercut shoulders. We relieve the inside of each shoulder slightly so the outer edge closes first and stays tight as the face shrinks.
- Housings hide the gap. If a joining timber shrinks 3/16 in. in width, a 1/2 in. housing keeps the gap out of sight.
- Pegs nearer the shoulder. The farther a peg sits from the shoulder, the more the tenon can shrink between them and open the joint.
- Mind the height. Timbers shrink across their depth, so a floor framed into a deep tie can drop a little against one framed into a shallower girt. We keep bearing surfaces at the same depth wherever we can.
When to call the engineer
These joints carry real loads, and the way they fail (relish splitting, tenon shear, peg bending) is well studied. For anything that needs a permit we send our layout to a licensed engineer who designs timber connections. The tables on this page are how we cut joints in the shop. They are not a substitute for a stamped connection design.