A way to create a longer board out of two shorter ones.
A scarf joint is an inelegant but convenient means of joining two boards, i.e. two pieces of framing lumber, end to end, resulting in a single longer piece. In short, a portion of each board is thinned to half its thickness, then the two are overlapped and joined. The following instructions (no guarantees, of course) should make it possible for the do-it-yourselfer to accomplish a scarf joint. Practice on scraps before applying this trick to important work.
A typical scenario where this joint comes in handy: you're building something that requires a thirteen foot long two by four, but your local lumberyard is out of anything longer than twelve-foot lengths. Say you have some two or three foot scraps around. By scarfing together a twelve-footer plus one of your scraps, you can make your own longer board, thereby avoiding a costly and time-consuming trip to a faraway supplier.
You will need:
Assemble these necessities in a well lit place with good means of support. (Adequate physical support for wood that is being worked is essential to your safety.) Choose a length for the joint: this should be two to three multiples of the width of the lumber you're using, and four to eight times the thickness. For example, a scarf joint in two by four lumber is often eight inches long or more. A longer joint wastes more wood and takes more work to create, but will be stronger due to more contact area between the boards. Consider that the length of the joint will represent an overall loss of length since the scarf joint is essentially an overlap; don't miss this fact and run out of wood. If you are scarfing together a twelve-foot piece and a two-foot piece and you use a 12-inch joint, the result will be thirteen feet long; one foot of length is taken up by the joint itself.
Now complete the following steps, in order.
- Using tape measure, pencil, and framing square, mark off a section, equal to the length of joint desired, at one end of each piece to be joined.
- Hand saw option: if you have a good saw or two (ideally you'd have both a ripsaw and a crosscut saw and good skill with them, you can produce this joint easily and cleanly thereby; most people, however, would find it took them longer that way and turned out with a noticeable lack of precision. The following method is for the growing majority who don't keep well-sharpened hand saws around.
- With tape measure, great care, and a few practice cuts on scraps (measured afterwards to determine actual depth), set your circular saw's depth of cut to exactly half the thickness of the lumber you're working with. On "two by" lumber* this will be about three quarters of an inch.
* See two by four for explanation of nominal vs. actual lumber dimensions in the U.S.
- With each piece of lumber well supported, make a series of half-thickness crosscuts across each piece, each crosscut being parallel to the previous one and about a quarter inch distant. The first of the series of crosscuts should be right next to the pencil line that marks the end of the joint area, and the last should be within a quarter inch of the end of the piece of wood.
- Using the hammer, strike the remaining tongues of wood between the cuts you just made, breaking them away from each board. This is best done with the striking face of the hammer, but after the majority of the wood has been broken away, you may find it useful to use the claw of the hammer (if yours is not overly curved) to remove more of the smaller remaining bits, as a sort of a crude chisel.
- With hammer (or mallet) and chisel, knock out most of the remaining bits of wood in the joint area. Use good chisel technique, trying not to cut too deeply. When finished with this stage, the bottoms of all the crosscuts will still be visible.
- Using the block plane, clean and trim the joint area further until it is somewhat smooth. For simple framing it will not need to be terribly smooth, but you still want good contact between the two halves of the joint, so going for some degree of smoothness is worth a minute's work.
- Test the fit of the two pieces to be joined, inverting one and overlapping its cutaway area with that of the other piece. If needed, repeat some of steps 4 through 6 to improve the fit.
- Once you are satisfied with the joint, apply wood glue to both halves, spread evenly, wait a few minutes for tackiness to occur, and clamp the joint together. You may also wish to fasten the joint with nails, screws, bolts, or other fasteners. Use at least two strong clamps distributed evenly within the joint area, and leave the clamps in place for the length of time recommended by the glue manufacturer.
- Eventually, remove the clamps and admire your newly created longer piece of lumber. Sand the edges around the joint if necessary for appearance or fit. Fill any gaps if necessary for appearance. Cut your new piece to final length and continue with the rest of your project.
- Don't forget to celebrate with beer (optional).
A rough illustration:
piece 1 | | piece 2
(before) | | (before)
piece 1 (modified) | |
__________| __________| piece 2
| | (modified)
__________| (both pieces now joined)
Further information for those seeking details...
Some of the nodes hardlinked below may yet need to be created.
Joint location: there can be reasons for choosing to make a scarf joint more in the middle of the resulting board, or more toward one end or another. A full treatment of this could involve aesthetics (in lighter boards used only for appearance or trim) or structural engineering (in cases where load-bearing structures are at stake) and it's somewhat beyond the scope of this writeup. Suffice it to say that when considering the scarf joint as a way to make a longer board out of multiple shorter scraps, some thought should be given to whether it will be stronger or weaker, or look better or worse, to place the joint(s) in one location or another.
Choice of glue: a good quality wood glue can be recommended by a trusted lumber yard or hardware store. For outdoor or wet-area applications you should use a waterproof wood glue.
Variations on the scarf joint: when the joint needs to be as invisible as possible, the 90 degree angles in the joint can be changed to 45 degrees; this helps the joint be less obvious. This can be useful when the resulting board is going to be stained, or primed and painted. Another variation is found in traditional timber framing: here the interface between the boards is more complicated, with a shape that locks the two pieces together mechanically even in the absence of glue. Graphics and photos of this can be found with a web search.
Strength of the joint: the greater the surface area in contact between the two original scraps, the greater the strength of the finished joint. Boatbuilders typically choose the length of the joint to be 8 times the thickness of the pieces of wood. They need the extra length because their boards are commonly required to maintain strength while bent into a curved shape. Other factors contributing to the strength of the joint are the quality of glue and the type of fasteners. For basic household/DIY applications of the scarf joint, the strongest result you can easily produce is with a good wood glue generously applied, plus fastening in several locations across the joint with wood screws. This "screw and glue" method is important because modern wood glues can be stronger than the materials that they bond together, thus you can create a perfectly good joint that still permits breakage of the wood a short distance away. The mechanical fasteners spread the stresses of the joint so that more of the available wood can dissipate those stresses.
Pro and Con: don't use the scarf joint for anything too serious if you don't have a full grasp of the structural implications. For example, if you're building a load-bearing wall and are short of materials, don't scarf two shorter boards together to make a stud or a header. Scarf joints are more typically used in applications where appearance, and not strength, is at a premium. For example in various types of fascia around the outside of a house, or in soffits, siding, barge boards, and so forth.
Production methods: This writeup was created with the underequipped home-improvement hobbyist or starving carpenter in mind, who often carries little more than measuring tools, a saw, a hammer and a chisel. It has been asked whether a simple set of jigsaw (reciprocating saw) cuts, if that tool were available, would not produce a scarf joint more quickly and cleanly. In response: consider that while making the longer of the necessary cuts, the "foot" of a hand-held jigsaw would be seating itself against the narrowest dimension of the lumber at hand, a somewhat dangerous situation that also lends to inaccuracy. Actually, the ideal machine to produce the long "face cut" of the scarf joint is a stationary band saw, again outside the current scope of this writeup.