Scarf joint

What is a scarf joint?

A scarf joint is a timber splice that lengthens a beam, plate or post by joining two pieces end to end. Each end is cut to a long, shallow slope, the sloped faces overlap and bear on each other, and the pair is fixed with bolts, screws, nails, steel plates or structural adhesive. Slovak carpenters call the technique preplátovanie (lap splicing) or šikmý plát (sloped splice). The purpose is to make one long member from shorter lengths when no single piece of timber is long enough.

The joint is not limited to beams. It appears in wall plates, ridge members and tie beams.

How does a scarf joint carry load?

A scarf joint transfers force through its sloped contact faces and through its fasteners. Under compression, the sloped faces press against each other and much of the force passes by direct bearing. Under tension, the faces cannot hold the two pieces together on their own, so the fasteners carry the pull across the overlap. A long slope spreads the transfer over a large area, which is why a long, shallow scarf is preferred to a steep one.

Because the slope runs across the grain, the joint needs careful proportioning. The thin feathered tip at the end of the slope has short grain and splits easily, so good practice avoids very sharp tips and keeps the cut flat so the faces bed fully. The overlap length, the fastener layout and the position of the joint along the member are design decisions. The designer checks them against the loads, and the carpenter should not change them on site without asking.

Where do scarf joints appear in roofs and log houses?

In roofs, a scarf is most often found where a long plate or ridge member has to be made from shorter lengths. A wall plate running along the top of a masonry wall, a ridge beam over a long roof and a tie beam at the bottom of a truss can all be spliced this way instead of waiting for a single timber of full length. In Slovak krov (traditional roof frames) the splice sits alongside the pegged connections described in the mortise-and-tenon joint article, and the two are often used in the same frame.

In log houses the picture is different. Walls are built from stacked logs that are joined at the corners by notching, not by splicing along their length. A scarf is used there only in special cases, for example to extend a wall plate, a sill or a ridge log that must be longer than the available timber. Builders generally avoid it, because every joint is a line where water and settlement can get in. Where a splice cannot be avoided, it belongs in a low-stress zone and must be sealed carefully.

How is a scarf joint used to repair rotten beam ends?

In old roofs and floors, the most common use of a scarf is repair. Water from a failed gutter, a damp wall head or a blocked ventilation path can rot the last part of a beam where it bears on masonry. The usual approach is to cut back the decayed end to sound timber, shape a new piece with a matching scarf, and fix it to the sound part with bolts, screws or structural adhesive. The new piece takes over the bearing, and the sound remainder of the original beam keeps its role.

The sequence matters. Decay is often deeper than it looks from outside, so the carpenter probes the timber and removes samples before deciding how far to cut back. The water source is fixed first, because a new splice in a wet wall will rot just as the old one did. The scarf is marked and cut, the new piece is treated with preservative, and fasteners are placed symmetrically so the joint is not loaded eccentrically. Where the original joint could not carry the new load, a steel bracket or threaded rods are often added.

How is a scarf joint different from a mortise-and-tenon joint?

Both joints connect timber without metal plates in their traditional form, but they do different jobs. A scarf joins two pieces in line to make one longer member. A mortise-and-tenon joint connects members at an angle, for example a post into a sill or a brace into a post, and it locks through a peg into a pocket. The scarf relies on the sloped overlap and its fasteners, while the mortise relies on a cut pocket and interlocking shoulders.

Feature	Scarf joint	Mortise-and-tenon joint
Main purpose	Extends a member in line	Connects members at an angle
Load path	Sloped faces and fasteners carry compression and tension	Shoulders and tenon bear, the peg resists separation
Typical location	Plates, ridge and tie members, beam ends	Roof frames, posts, braces and sills
Weak point	Thin feathered tip and badly placed fasteners	Pocket removes timber and can split the member
Typical repair role	Replaces a decayed beam end	Reassembled or re-pegged after movement

When must a structural engineer decide the repair?

A carpenter can handle a simple repair on a lightly loaded secondary member once the water source is fixed. The structural engineer (statik) must decide when the element carries real load, when the splice sits in a main member, or when the bearing or load path changes. The engineer checks the new and old timber, the fasteners and the connection to the wall, and the decision usually turns on the situations below.

Situation	Simple repair may proceed	Engineer should decide
Decay in a secondary plate or lightly loaded member	Yes, with the water source fixed and a clear scarf detail	Only if the load path is unclear
Decay at a main beam bearing on masonry	No	Yes, including the bearing on the wall
Splice that must carry a new or larger load	No	Yes, because the design changes
Rot that extends into the wall or whose extent is unknown	No	Yes, after opening up and probing
Change to the roof structure or to spans	No	Yes, and the works may need approval

Alterations to structures are governed by the building act 25/2025 Z. z., in force since 1 April 2025. Many older records still use the terms of the 1976 act, so ask the local building authority which procedure applies before work starts. This article describes the joint in general and is not a design basis for a specific building.