What is ‘quarter sawn’ planking? and why is it best used for decking?
To explain quarter sawn timber we need to go back to the log from which it is cut . If you think of the log as a circular section then quarter sawn planks are cut on the radius, that is the plank width is from the centre to the edge. This is important because the grain direction effects the characteristic of the plank in its use in boat construction.
All timbers will expand and contract as the ambient humidity causes the cellular structure to swell with moisture absorption. Different species of timber react in different degrees to moisture absorption but all follow a general rule. For each one percent that timber expands along the grain, that is the direction of growth, it will expand three percent across the grain and ten percent around the grain. This is simply imagined by thinking of the log section as being roughly circular. As the diameter grows, the circumference will grow by 3.142 times, i.e. the value of π.
That differential expansion means that when the timber is sliced into narrow planks it will warp and distort dependent on the run of the grain. This can be harnessed for good or bad and planks need to be selected carefully for particular applications. There are three ways that planks can be classified.
Flat sawn planks are cut with the grain running across the widest dimension. The grain must run within 0° and 30° of the longest side of the plank and it will expand more across the width than the thickness. This is best used for hull planking as the expansion will take up most to tighten the seams. Also the plank will be the strongest of all the cuts and least prone to splits or cracks.
Planks cut between 30° and 60° are called rift sawn, or sometimes bastard cut. The differential expansion due to moisture will cause the plank to warp or curl. These planks have no best use as structural boat building timbers and are often sliced for veneers.
Quarter sawn planks are cut with the grain running across the shortest dimension. For them to be classed as quarter sawn the grain must be within 60° to 90° of the longest side of the plank. These planks will expand more across the thickness than across the width and so are best used for deck planks. There will be minimum stress across the bond between the polysuphide caulking seams. The planks will also exhibit the best abrasion resistance as the grain presents the majority of the hard summerwood to the surface. Good quarter sawn decking planks will appear to have close parallel grain running along the length and no signs of figuring or shaping.