We have trialled and tested a number of jointing techniques over the last fifteen years for our external wood products. One of the biggest issues for timber exposed to the weather is the natural expansion and contraction that occurs from season too season.
Timber responds to climatic conditions and as it absorbs and gives up moisture, it naturally changes size in all directions. Different woods have individual characteristics, but all have this trait. Too add to the complexity, timber expands more across it's grain and less along the board length.
Our boards come from a wide range of sources, timber types vary and even the original way a board is sawn affect this movement. Often timber finishes are not recoated when required, affecting the boards ability to react to moisture uptake more.
Most of our stock are reclaimed floor boards, normally machined with a tongue on one board edge and a groove on the mating edge of the next board. This is a perfectly acceptable technique for new timbers installed inside buildings and finished appropriately. But by the time we receive these boards (often a 100 years later!) the tongue and grooves have been damaged, and often split and full of accumulated dirt and grime.
Certainly no candidate for satisfactory and long lasting joints. On our first bird houses and bird feeders, we would remachine the board edges by ripping off the damaged tongue and grooves on a table saw, and rejointing by cutting new pockets along the edge and using wood biscuits with a waterproof glue. This proved entirely satisfactory for these products, the biscuit joints were secure, even if the board edges opened up slightly. Some of our bird houses are 15 years old, and still in sound condition.
About four years ago we revisited our jointing technique, and now use a new style that even if the board edges separate slightly, no water ingress occurs to the inside.
We still rip the old tongue and groove edges off, but now we machine a matching slot into each edge of the boards, install a timber tongue and glue the tongue and boards together with a waterproof polyurethane glue.
This gives us a mechanically secure joint, and even if the boards absorb and lose moisture during the seasons which can open up the joints, the tongue inside the slots provides a barrier to water getting into the interior. We started using this technique on our rustic timber letterboxes, and because of its simplicity and reliability, our bird houses and feeders as well.
A particular advantage to this technique for us, is that we can use boards of differing thicknesses to each other. Once our boards are selected from our stock, we cut to length first, then match the best faces too one side. As long as we reference this face side when we cut the slots, our boards join together presenting a uniform appearance on the chosen side. The inside, which is seen less or not at all, is not so critical in its appearance if the edges are not level.