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Although this varies through different species, the deformation is always greater in the direction tangential to the rings than in the radial direction.
Grain orientation has a massive impact of the stability and performance of wood in exterior applications.
Heartwood performs differently during the drying process, and also has an impact of performance of exterior timber.
Certain timber cuts can be targeted to improve the drying and the overall exterior performance of exterior timbers. Some of the common sawmilling cuts can be found below:
A vertical grain orientation has three main benefits:
As wood tends to shrink and expand more tangentally compared to radially, the shrinkage and expansion is halved with vertical grain. This means that coatings will last longer on timbers with vertical grain orientation.
In exterior applications, wood typically cracks perpendicular to the grain. This means that flat sawn timbers will have more visible cracking than quarter sawn, vertical grain boards. Surface cracks can harbour moisture, leading to premature decay - this means that vertical grain products last longer, and hold coatings better than those that are flat sawn.
Grain orientation has a significant effect on long term weathering. Flat grained timbers are more likely to show more surface cracking over time, while vertical grain pieces are less likely to crack. Coatings will last longer on timbers with vertical grain orientation.
The image below shows heavily weathered treated radiata pine decking.
The top piece is flat sawn, and exhibits severe surface checking. The bottom piece was cut with a vertical grain.