Clapboard, also known as bevel siding or lap siding or weatherboard (with regional variants as to the exact definitions of these terms), is the cladding or ‘siding’ of a house by installing long thin wooden boards that overlap one another horizontally on the outside of the wall. They were originally riven in triangular or "feather-edged" section, attached thin side up and overlapped thick over thin.
Clapboard was originally split by hand from logs in a radial manner. Later, the boards were radially sawn in a mill. As this technique was common in most parts of the British Isles, it was carried by immigrants to their colonies in the Americas and in Australia and New Zealand.
Clapboards can be cut from trees two different ways: flat-grain boards or vertical-grain boards. Flat-grain boards are to be cut tangent to the annual growth rings of the tree, and vertical-grain boards are to be quartersawn or cut at right angles of the annual growth rings of the lumber. The more commonly used boards in New England are vertical-grain boards. Depending on the diameter of the log, cuts are made from 4 1/2" to 6 1/2" deep the full length of the log. Each time the log turns for the next cut, it is rotated 5/8" until it is rotated a full 360 degrees. This gives the clapboard its taper and true vertical grain.
In some areas, clapboards were traditionally left as raw wood, relying upon good air circulation and the use of 'semi-hardwoods' to keep the boards from rotting. These boards eventually go grey as the tannins are washed out from the wood. More recently clapboard has been tarred or painted; traditionally black or white due to locally occurring minerals or pigments. In modern clapboard these colours remain popular, but with a hugely wider variety due to chemical pigments and stains.
It is good practice to leave the lower part of a wall free of cladding to avoid dampness caused by air not circulating, which could subsequently rot the clapboard. Watermills were traditionally made of brick up to the first floor, and in windmills upper storeys were often timber-framed and only the caps were clapboarded.
Clapboard houses may be found in most parts of the British Isles, and the style may be part of all types of traditional building, from cottages to windmills, shops to workshops, as well as many others.
In New Zealand, clapboard housing dominates buildings before 1960. Clapboard, with a corrugated iron roof, was found to be a cost-effective building style. After the big earthquakes of 1855 and 1931, wooden buildings were perceived as being less vulnerable to damage. Clapboard is always referred to as 'weatherboard' in New Zealand.
- http://www.popularwoodworking.com/article/Finish_Both_Sides_Not_Necessary/ Heart side of boards