In the production of timber, there is a requirement to ensure that timber has the dimensions and shape that will enable it to do its job properly. If it won’t fit in, then it can’t be relied upon to look good or carry loads or both. This is the subject of utility of product.
Timber dimensions must be within tolerances. The tolerances are defined in the appropriate product standards.
Highest tolerances apply to fitted appearance products such as floorboards or lining boards. Tongues and grooves must be of correct dimension and shape to fit together and the boards must be the right thickness and width to form a plana surface when all joined together.
High tolerances also apply to mouldings (machined timber products such as dowels and joinery timbers). This is necessary to ensure that there is a good fit in a completed assembly.
Seasoned structural timber products have a tolerance of –0 on all cross sectional dimensions. There is a small allowance for some over-size, but none for undersized members. This means that nail plates will make a full penetration in seasoned timber in trusses, and the face of stud walls will also be planar.
Unseasoned timber has some negative tolerance – generally –3mm. This allows for some variation in the manufacture, which is required as some unexpected shrinkage of the timber during production can happen on occasions.
Structural designers should always assume that the dimensions are the minimum dimensions within tolerance.
As well as limits on the size of timber elements, there are also limits on the shape of the timber. Squareness of the cross section must be very good for joinery timbers, quite good for seasoned structural timber, and there is less expectation of unseasoned timber.
Because of the “prestressing” that occurs during the growth of the tree, timber contains residual stresses which affect its behaviour when it is cut into structural elements. Hardwood trees have compression on the inside of the trunk and tension in the newer wood on the outside of the trunk. In old trees, the compression in the juvenile wood can cause compression failure of that wood. This is one cause of “brittle heart”. These wood fibres may have already ruptured during growth of the tree and have low strength and brittle behaviour.
Softwood trees have tension on the inside and compression in the outer layers. If the tree is moved out of alignment for any reason it will lay down extra compression wood on the low side to restore the tree to alignment.
The stresses in the wood are released when the trunk is cut into lengths of sawn timber. If there is a residual stress differential across a piece of timber, this may lead to twist, cup, bow or spring in the piece. The movement takes place to minimise the residual stresses in the wood. Most sawmill operations use cutting patterns that minimise the residual stress differential in individual lengths. As the movement of the timber is accentuated by moisture movement, the deformation becomes more apparent when the wood dries.
Cup - Deformation of the cross-section - edges move upwards.
Bow - Deformation over the length of a board in the minor axis direction.
Spring - Deformation over the length of a board in the major axis direction.
Twist or warp - Twisting of the board so that successive cross-sections are not parallel.
All product standards have limits on these utility issues. The stringency of the limit tends to follow the same pattern as the stringency of the dimensional tolerances. Many appearance products have higher expectations of straightness compared with structural timber. This reflects the difficulty in using pieces with curvature or twist in high appearance applications.
• An upwards cup can trap water in exposed timber. In thin decking, the effect of cupping can be minimised by using nails or screws near each edge of the piece.
• With thin pieces, a bow can be removed by fastening to closely spaced framing. In beams, blocking pieces between adjacent pieces can minimise the effect of a bow and give straight lines of fasteners in the flooring or lining. In studs, the blocking pieces are known as noggings and are illustrated in Figure
• A spring in timber can only be removed by deforming the timber by bending about its major axis. This is very difficult, so tolerances for spring are very small. Where there is a little spring in timber, carpenters generally place the timber so that the curvature is upwards, and weight of the structure will reduce the spring.