Timber & Roundwood Performance
Unique Performance Characteristics of Roundwood & Timber Products
Timber is a natural product, this is a part of its appeal. As a consequence ) it exhibits some unique characteristics when compared to manufactured alternatives such as steel and concrete. Customers should be aware of these and plan for them.
Checking in Posts & Poles
Post or pole checking refers to vertical cracks which open up lengthways in dry products.
Why does wood, in particular, round posts and poles check
As wood dries it shrinks with the rate of shrinkage different in each direction (see below).
Rates of wood shrinkage during drying:
D: is pole diameter (radial shrinkage)
C: is pole Circumference (tangential shrinkage)
As treated product dries following wood treatment.
(from treatment to 12% Moisture)
Scion (Forest Research) verify that checking in posts, strainers and poles has practically no effect on strength since checks follow the grain along the wood fibres and this has no effect on bending or compression strength of posts or poles.
Checking has practically no effect on product durability since checking is into sapwood which is required to be fully treated with preservative. Checking will increase as the product dries out as atmospheric moisture changes checks will expand and contract. quarter round and half round posts and rails check only a little as timber shrinkage during drying is concentrated on the cut face.
Twisting in Timber Products
Twisting in sawn timber and round post/pole products is a consequence of the grain in trees not always being straight up and down the tree. It is a genetic trait and is more prevalent in the early years of tree growth. This is referred to as spiral grain. As timber dries it has a tendency to twist (some pieces more than others) which will always be in a clockwise direction. Sawn timber products produced from the centre of logs referred to as "core wood", twist significantly more than sapwood.
Poles will also twist as they dry following treatment. PermaPine recommends seasoning poles (through air drying) prior to installation and using heavy fixings/fasteners, which will restrain timber products as they dry.
Timber rails are especially susceptible to twisting. To reduce the incidence of twisting there are a number of things that can be done:
Air season sawn timber by filleting it, ensure it is strapped or weighted during drying. Dry in a location with good air flow. Cover timber to reduce exposure to direct sunlight while drying.
When a packet is opened and partially used, re-strap or weight the remaining timber down.
Try and keep packets of timber out of the sun especially when the packet is open.
For roundwood and pole products, we recommend air seasoning the product prior to installation. This will minimise the potential for product twisting after installation. When air seasoning product, checking will be concentrated on the product faces exposed to direct sunlight.
Resin on Surface of Timber
Resin is removed from timber during the steam drying process. Resin on the surface of the timber product is an indication the product has been well steamed. The presence of this resin is unique to steaming, it is not present with the two other methods of drying wood; air drying and kiln drying.
For timber products containing corewood where it is necessary to achieve higher chemical retentions such as H4, H5 and H6, PermaPine choose to steam wood rather than air dry or kiln dry. This ensures effective treatment.