Timber Joints at a Glance: Mortise & Tenon, Halving, Step Joint & Joist Hanger
Whether it is a mortise and tenon, a step joint or a joist hanger, the joint decides how load-bearing and durable your timber structure will be. This overview shows you the most important traditional and modern timber joints, with guide values for dimensions and costs, and answers the question of when to use which.
What matters most in a timber joint
Every joint in timber construction has to transfer forces: compression, tension and shear, often several at once. A rafter presses diagonally onto the purlin, a brace channels compression into the post, a floor joist hangs with its full load on the beam. Before you decide between a mortise and tenon and a joist hanger, you therefore need to know which force is acting at that point. Traditional carpentry joints transfer forces through precisely worked timber faces, while modern connectors use steel plates and screws with tested load ratings. Both work, but not both work equally well in every location.
Traditional carpentry joints
Traditional joints use little or no metal at all. They demand more skill and time, but they look clean and, on exposed structures such as roof frames, timber framing or carports, they are the best-looking solution. All the dimensions in this section are rule-of-thumb values from carpentry practice.
Mortise and tenon: the standard for posts and rails
With a mortise-and-tenon joint, a rectangular tenon is left standing on the end of one timber and fits into the mortise of the other, typically where a post meets a sill or where rails sit in a timber frame. As a rule of thumb the tenon is about one third as thick as the timber, so roughly 4 cm on a 12 x 12 cm post, and 4 to 6 cm long. The tenon locates the timber and transfers shear; it does little to resist pull-out, which is the job of a wooden peg or an extra screw.
Halving joint: a flat overlap
With a halving joint both timbers are notched out by half their thickness and overlap flat against each other, for example at a lengthwise joint between purlins or where sill plates cross. The straight halving joint is the easiest to cut; the splayed halving and above all the dovetail halving also take up light tension forces. As a rule of thumb the timbers overlap by two to three times the timber depth, so 40 to 60 cm on a 10 x 20 cm purlin. Bear in mind: a halving joint halves the cross-section, so at heavily loaded points this should be calculated beforehand.
Step joint (Versatz): the compression joint for braces
The step joint, known in German carpentry as the Versatz, is the classic way to connect compression braces, for example a knee brace at 45 degrees between a post and a purlin. The brace is let in end-on into a notch in the main timber and transfers the force directly through the pressed end grain. Guide values: the depth of the notch is one sixth to at most one quarter of the timber depth, so 2 to 3 cm on a 12 cm timber, and ahead of the notch at least eight times the notch depth of timber (the so-called relish) must remain, or the end of the timber will shear off. Cut cleanly, a front step joint carries enormous loads, entirely without steel.
Birdsmouth: the rafter's seat
The birdsmouth is the triangular notch on the underside of the rafter that lets it sit snugly and slip-free on the purlin. As a rule of thumb it may be at most one quarter to one third of the rafter depth deep, so about 4 to 5 cm on an 8 x 16 cm rafter. Cut any deeper and you weaken the rafter right at the bearing. To resist uplift under wind suction, the birdsmouth is today almost always combined with a long screw or a rafter-to-purlin anchor.
Modern connectors: steel instead of the chisel
Joist hangers, angle brackets and fully threaded screws have made timber construction much faster: no chopping out mortises, load ratings approved by the building authority, and fitting in minutes. The price for this is visible metal, and the duty to follow the manufacturers' fixing specifications to the letter.
Joist hangers
Joist hangers made of galvanised steel (usually 2 mm thick) carry a beam end-on against a supporting beam, ideal for floor joists, trimmers or a decking substructure. They come for beam widths of roughly 40 to 200 mm; depending on size they cost around 1.50 to 8 EUR each (guide price). The key point: fill every hole with 4.0 x 50 mm ring-shank nails or approved connector nails. A joist hanger that is only half nailed will not reach its catalogue values.
Angle brackets and nail plates
Angle brackets, for example 90 x 90 x 65 mm with a stiffening rib (swage), join timbers around a corner, for instance a purlin onto the top of a post. Depending on size they cost 0.50 to 3 EUR each and are fixed in seconds, but they stay visible. Nail plates and flat connectors suit joints loaded in tension. Outdoors, always choose hot-dip galvanised or stainless-steel versions.
Fully threaded screws
Modern fully threaded screws (6 to 12 mm diameter, lengths of over 600 mm) are almost invisible high-performance connectors: driven in at an angle of about 45 degrees, they transfer tension and shear forces directly from timber to timber. An 8 x 300 mm screw costs around 1 to 3 EUR (guide price). Edge and spacing distances to the manufacturer's specification are important; pre-drill for hardwood and large diameters.
Traditional or modern? A direct comparison
| Joint | Typical use | Mainly transfers | Effort | Cost per joint (guide) |
|---|---|---|---|---|
| Mortise and tenon | Post on sill, rails | Compression, shear | high | 0-2 EUR |
| Halving joint | Purlin joint, sill crossing | Compression, limited tension | medium to high | 0-3 EUR |
| Step joint (Versatz) | Braces, knee braces | high compression forces | medium | 0-2 EUR |
| Birdsmouth | Rafter on purlin | Bearing compression | low to medium | 1-3 EUR (with screw) |
| Joist hanger | Floor joists, trimmers, decking | Shear | low | 3-10 EUR (with nails) |
| Angle bracket | Corner joints, post head | Shear and tension forces | very low | 1-4 EUR |
| Fully threaded screw | almost universal | Tension and shear | low | 1-3 EUR |
The cost figures are material prices as rough guide values; the real cost factor with traditional joints is the labour time. A skilled carpenter needs 15 to 30 minutes for a clean step joint and about 5 for a joist hanger.
Which joint when? A decision guide
- Exposed roof frame, timber framing or a carport done properly: traditional joints (mortise and tenon, step joint, birdsmouth) keep any metal from spoiling the look, and compression forces run straight through the timber.
- Floor joist or trimmer onto a beam: a joist hanger, because end-on shear transfer is exactly what it is made for.
- Compression brace or knee brace: a front step joint, secured with a screw against slipping.
- Rafter on purlin: a birdsmouth plus a fully threaded screw or a rafter-to-purlin anchor against wind uplift.
- Quick project with few tools: angle brackets and screws, which are solid and cheap but visible.
- Joint loaded in tension (e.g. collar ties, extensions): modern connectors or a dovetail halving; a plain halving joint alone is not enough here.
In practice almost everything is combined: a modern roof frame uses birdsmouths and step joints where compression acts, and screws or anchors where tension and wind forces have to be secured.
Typical mistakes, and how to avoid them
- Birdsmouth cut too deep: more than one third of the rafter depth weakens the bearing massively. Better to notch shallower and, if need be, choose a wider purlin.
- Relish too short on the step joint: below eight times the notch depth the end of the timber is liable to shear off, so bear it in mind when you cut to length.
- Joist hanger only partly nailed: the approved load capacity only applies when every hole is fully nailed.
- Wrong screws: ordinary chipboard screws are not approved structural timber screws, so in load-bearing structures always use screws with an approval (ETA).
- Steel without corrosion protection outdoors: outside, hot-dip galvanised or stainless-steel connectors belong on the timber, otherwise nails and plates rust within a few years.
Structural design, standards and clean planning
All the dimensions and prices given are guide values. How deep a birdsmouth may really be, and which joist hanger carries which load, are governed by Eurocode 5 (EN 1995) and the manufacturers' approvals, depending on span, snow load and wind zone. For load-bearing components, larger spans or projects that need approval, there is no way around a structural engineer and your local building authority. For your own preliminary planning it pays to draw the whole structure through beforehand: once you have seen every joint in three dimensions, you cut fewer of them wrong. Tip: in a 3D planner such as HolzBau 3D you place posts, purlins and rafters true to scale and see immediately where timbers meet, and you then set the right joint type deliberately.