Sometimes interesting structural load paths present themselves in unexpected places. The photograph below is such an example. It shows a cross section of timber cladding boards at the door threshold on my parent’s shed, which is located at the bottom of their garden behind the garage. You can see the end of the garage through the glass panels in the door.
What is interesting is trying to understand why the timber cladding board in the centre of the picture has started to cup, which has in turn caused the tongue and groove’s cut into the board’s long edges to begin separating from adjacent boards
This requires us to know something about how trees are converted into boards and why this affects the way that they move and warp. It also requires us to understand how this affects the way cladding boards should be detailed.
There are three methods of converting a log into boards. The most common is known as plain / flat-sawn, which essentially involves slicing the log into vertical strips. It is the most common choice because it minimises the amount of timber wastage and maximises the number of boards. It is therefore cheap.
The second most common method is known as quarter-sawn. As the name suggests this involves cutting the log into quarters and then flat-sawing each quarter. This produces a little more waste than conventional, flat-sawn timber and is therefore more expensive.
Rift-sawn boards are reasonably uncommon, because there is high waste and therefore they are expensive. In this case the boards are sawn in a radial pattern.
The reason that various methods of forming boards exist is because they each intersect a tree’s growth rings in different way. Flat-sawn boards are cut tangential to the growth rings; rift-sawn boards are cut radially and quarter-sawn somewhere in between. The benefit this brings will shortly become clear.
When a tree is felled its moisture content could well be 100%. As it starts to dry out free water will evaporate from its cells until it reaches somewhere between 25 and 35%. Beyond this point water is lost from the cell walls of the timber fibres themselves. This causes the timber to shrink. Since shrinkage tangential to the growth rings is roughly twice that in the radial direction the method used to form the boards becomes really important.
Flat-sawn boards will distort out of plane causing the middle of the boards to move towards what would have been the heart of the tree. Quarter-sawn boards will tend to warp in plane. In contrast to both other forms of cut rift-sawn boards tend to be dimensional stable.
Knowing what we now do we can return to the garden shed cladding.
If we look closely at the board edges we can detect from the pattern of growth rings that the boards were flat-sawn. They were always going to vulnerable to distortion. If we look even more carefully we can also see that the growth rings in the middle board are orientated in the opposite direction to the boards either side. This has had the effect of causing the middle board to pull the edges of the two adjacent boards outwards.
This has of course been exacerbated, because the builder did not understand how to fix the boards so as to minimise the effect of shrinkage. The photo above shows that the boards have tongue and groove edges that are meant to interlock while being allowed to slip past each other.
To minimise the effect of shrinkage this arrangement is supposed to be nailed through the upper shoulder of each board with the lower edge being free to move while being restrained by the tongue and groove joint at the bottom of the board.
Of course you can see in the photo below that the clown that built this structure has double nailed the centre portion of each board forcing the shrinkage movements to the outer edges.
That clown was of course a teenager who later became an engineer and decided to write an engineering blog.
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