7 Ways to join wood to obtain long and strong pieces

There are works where long wooden elements are needed that have very good strength. It is difficult to have such pieces in one piece so it is necessary to have several pieces of wood joined together so that the final product is very strong. For the joint to be strong, the contact surface between the two pieces that are joined together must be as large as possible. An extension by butting the pieces together and gluing only on the perfectly straight surface at a 90° angle from the end will not have the necessary strength. Therefore more elaborate butt joints must be found in which the profiles provide a much larger contact surface.

Below are some of these joints, the most commonly used and among the easiest to make for items used to make furniture pieces or to extend pieces of wood to be used for cladding. There are other, more complicated butt joints for extending beams in construction. In this case the forces acting on the joint are very strong. More about them in another article. Now let's see how we can extend wooden elements used for simpler work.

Joining in teeth

It is the most commonly used joint in the industry for extending wooden elements. It is used for both solid wood panels, when the joined elements are shorter and smaller, but also when obtaining lamellar beams (glulam), where the elements to be joined are much larger.

The joint is much stronger than a butt joint. With the advent of splicing teeth, the way of splicing changes completely compared to butt splicing, the contact area becomes much larger and the splice is made parallel to the fibre, not at the end of the fibre. It is known that gluing along the grain, when the fibres of the two pieces of wood are parallel, is very strong.

To make the joint, however, it is necessary to make the joint profile, a difficult operation, especially in factories, where the facilities make this operation possible. However, in workshops or for DIY projects, other types of joints are chosen that are strong enough to allow the project to be carried out. There is a article entirely dedicated to the joint in teeth and those interested can find out more by reading it.

Joining with dowels (billets)

It is the joint used to extend the elements and in the case of wood-derived boards. It is basically a way to make their alignment perfect and is doubled by the use of screws when more strength is needed.

Joining can be done quickly and easily with a drill, a suitable drill bit, dowels and wood glue and is therefore recommended for beginners, for extending short elements that are not subject to high compressive, stretching or twisting forces. However, specialists in the field consider it weak and suitable mainly for household work.

A major disadvantage of the joint is that moisture variation can lead to dimensional shrinkage of the dowels. Combined with ageing of the adhesive for various reasons (temperature, unsuitable conditions, unsuitable adhesive for the existing conditions), this leads to play and ultimately to joint failure.

And for dowel jointing has been dedicated an article where you can learn more about the advantages and disadvantages of the method.

Oblique joint

Another very simple joint is the one in which the elements are cut obliquely. This increases the contact area between the two pieces and the joint will have greater strength. However, this is not a spectacular increase, so it should be used for elements that are not subject to strong forces.

Cutting can be done using a circular whose blade can be tilted. The greater the angle, the greater the bonding surface and the greater the strength of the joint. For example, when cutting at an angle of 45° the bonding surface increases by 40% compared to the straight surface.

For the joint to be perfect, one of the pieces should be cut face up and the other, at the same blade angle, face down. This way the fit of the edges will be very good.

The two cut surfaces are glued together, the edges are matched and then clamped with cleme until the adhesive hardens. In order to make the joint as firm as possible, wooden strips can be placed on either side to stiffen it and the clamps can be used to hold it in place.

Slanting lengthwise joint

In order to increase the joint area even more, the elements can be cut obliquely, but not perpendicular to the piece, as before, but along the element. The cut has to be made very accurately so that the elements fit together perfectly. To do this, use an ordinary echelon, like the one used in technical drawing, or a template in the shape of an echelon.

As with the other oblique cut, the cutting line is drawn on one piece on the front and the other on the back. The width of each piece must be at a perfect 90° angle to the length. To draw the cutting line the echelon fits at right angles to the angle made by the length and width of the piece. The cut must be made very cleanly, exactly on the drawn line.

After cutting, dust is removed and adhesive is applied to the entire cut surface. The pieces are placed one after the other on a perfectly straight surface. They are fitted together and clamped until the adhesive is fully cured. It is mandatory to put clamps on the end areas of the glue (the tips of the elements) so that the elements are correctly glued.

Joining with log and notch

The mortise and tenon joint is one of the most commonly used in furniture making. It is mainly used for right-angled joints, but also for extending elements when they are thick enough to make the mortise and tenon.

For the joint to be perfect, the notch must be slightly deeper than the stock. The gap left when the elements are put together is also needed for the adhesive with which this butt joint is stiffened.

The length of the stump depends on the subsequent strength of the joint. The longer it is, the more pressure can be applied to the joint without it coming loose or breaking.

The stump and the notch can also be handmade by those who are passionate about woodworking and joints made this way. But it is easier to use router with milling tools that can do both the notch and the stump.

Wedge joint

A similar variant to the stock and socket joint is the wedge joint. Instead of a log, a wooden wedge (flange) is used which fits perfectly into the grooves made in the two elements to be joined. The wedge is much easier to make and so the joint becomes simpler to make.

The joint can be classic or pierced. In the latter case, a groove is made at the ends of the two elements to be joined in the groove. A wedge of width equal to the depth of the two grooves is used to bring the two elements together. The wedge must be made in such a way that it fits firmly into the grooves. To make the joint secure the wedge is glued. The joined elements are placed between two planks and clamped until the adhesive hardens.

It is a very suitable butt joint for items that need high bending strength.

Half-thickness jointing in wood

It's an aesthetic, strong and easy to make joint using a circular with table or a router. It's a great and durable way to make butt joints. Overlapping surfaces allow for parallel fibre splicing. The larger the overlapping surface, the stronger the splice.

To make the perfect extension the area to be overlapped must be measured and marked. The cut should be made up to half the thickness of the elements. The tools must be sharp so that the cut is precise and smooth. Once the cutting has been done, check that the elements fit perfectly. Then glue is applied and the elements are clamped in the clamps until the glue is fully cured.

Variants of the same joint are those in which the cut in the wood is oblique or has a profile that further increases the contact surface.

The method can also be applied for the extension of elements in Chipboard or MDF.

Other types of butt joints

They are not the only joints that allow the extension of elements used in the production of furniture, flooring, guardrails or wall cladding, but they are among the easiest to make. Joints such as butt to butt with bevelled butt, butt to butt with false butt, butt to butt with square butt and the like are much more elaborate and difficult to do. But the more sophisticated they are, the larger the contact surface and the stronger the joints.

It is very important that the choice of the joining method is based on how the extended element will be used. Compressive strength, tensile strength, twisting strength, bending strength are all important when making a butt joint. When it is well made, the contact surface is large and defects that can be weak points are removed. (knots, cracks, resin bags), the element resulting from the joining of two pieces of wood can be stronger than a single piece of the same length and thickness.