Butt joints are a nifty trick that lets you plank a row that is longer than the longest board you have available. Siding planks tend to come in twelve foot sections. On regular houses, a single wall could be much longer than twelve feet and butt joints are necessary. On tiny houses, butt joints are used for different reasons.
Due to windows, doors, and other features on the side of a tiny house, there are rarely sections where a twelve foot plank will not reach. Instead, butt joints are utilized to use off cut siding planks. For example:
The windward side is seven feet wide.
Three inches on each side is needed for trim space, so each plank needs to be six feet and six inches long.
If you cut a twelve foot plank at the six foot six inch mark, you now have a perfectly good plank that measures five foot six inches left over as an off cut. Instead of throwing these off cuts out, you can reuse them on the leeward side.
If you use a plank that is five foot six inches long, you need an additional piece that is one foot long. Connecting them with a butt joint will fill in the necessary length and give the tiny house the appearance you are looking for.
Butt joints are merely edge setting two planks next to each other close enough that they appear to blend together. The corners on the lower end of the butt joint are weaker and can be lifted by strong winds (which is why they should only be used on the leeward side and never on the windward side).
To guarantee a tight butt joint, be certain to only use "primed edge" sides for the butt joint. The siding planks come coated in primer, and the edges that have primer on them are perfectly square and flat. Edges that you cut with a hand saw may be off by a few degrees. While it may look straight enough in your hand, they will result in an open looking butt joint that will draw a lot of attention.
By placing two "primed edges" next to each other, you are sure to have a very tight butt joint that will blend into the house and not be noticed.
Butt joints also allow water to penetrate the siding, which is why it is a good idea to place extra tar paper under each butt joint. A tar paper shingle will help shed any water that works its way in between the butt joint.
Carefully staggering butt joints on the leeward side of the tiny house will allow you to reuse off cut sections of siding that would otherwise be lost as waste. Reusing your materials will help reduce the cost of materials by reducing the amount of waste you generate. Reducing your costs and wastes is a big part of tiny house living, so why not start this philosophy during the construction process?