Timber - Structural Timber - Types, Sizes, Cuts, & Uses
In building and construction timber can be split into two areas of use, structural (carcassing) and joinery timber.
Structural softwood timber has a lower visual quality than timber used in joinery products; it may contain more knots and have a less attractive grain pattern.
There are several species of structural softwood such as Douglas Fir, Spruce, and Pine.These are intended for first fix use i.e. domestic woodwork that will not be visible once the project is complete.
Structural timber can be either sawn treated or untreated, in a regularised or eased-edge finish. If the timber is to be used outside, treated timber should be used to avoid rot. Most sawn timber can be bought as tanalised. This means that it has been treated with Tanalith E wood preservative to give long term protection against both fungal decay and insect attack.
Regularisation, and eased-edges finishes. Regularisation refers to the trimming of all sides of the sawn timber so that every length is of a consistent size. This will generally take 5mm or so off the nominal or pre-machined size of the timber. Eased edges are rounded edges to help with manual handling of the timber.
Structural carcassing timber can be used for internal building timbers, general carpentry, cladding, fencing, gates and garden furniture.
Another type of structural timber is CLS (Canadian Lumber Standard). This is used primarily in the construction of timber frame houses and also in internal and partition walls. Although it is of Canadian origin CLS is now also sourced from Europe. It is planed to more finished dimensions and precise tolerances than carcassing timber and it is planed smooth with rounded corners at the edge of the wood.
These features reduce the spread of fire in timber frame wall cavities and make it safer to handle.
Carcassing and CLS timbers are produced from kiln dried whitewood in accordance with British standards to either C16 or C24 grade.
Common uses for softwoods are as follows :-
-
Pine – joinery, flooring, furniture;
-
Spruce – construction, panelling, cladding;
-
Larch – cladding, boats;
-
Cedar – joinery, garden products;
-
Douglas Fir – doors, joinery, construction;
-
Hemlock – doors, joinery, construction;
-
Western Red Cedar – cladding, shingles.
Timber Sizes - Nominal and Finished Sizes, Customary Sizes and Cuts
Although timber structures and non-structural applications can be designed using many sizes, the sawmills producing timber generally do not know the intended end-use for that material at the time it is cut.
They therefore produce a range of ‘customary’ sizes. Specifying timber in these is the most economic method both in terms of material usage and cost.
Timber is now sold in the nearest metric sizes to the old standard imperial sizes, and lengths are sold in sizes of 300mm increments that approximate to an imperial foot measure. Hardwoods are available in lengths and increments from 1.20m – 4.80m, and softwoods are available in lengths and increments from 1.20m – 7.30m. Hardwoods range from approximately 12mm to 100mm in thickness and from 38mm to 275mm in width, and softwoods range from approximately 16mm to 300mm in thickness and from 75mm to 300mm in width.
Nominal size is the actual size of timber that the board is machined from, and , as such, the Finished size of the product will have a smaller size than the nominal size.
This can be demonstrated by using a dimension of 50mm x 100mm as a ‘standard’ rough sawn piece of timber, it is ‘fully dimensioned’ as it has not been machined in any way, it is the nominal size. If that piece of timber was regularised i.e. planed on all sides with the corners rounded the finished size would be 45mm x 90mm.
If it was CLS grade, which is very similar to regularised in appearance, the finished size would be approximately 38mm x 87mm, and if it was PAR/PSE, planed all round or planed square edge, the finished size would approximate to 45mm x 90mm.
The finished size will usually only be a few millimetres different to the nominal size of the wood, however, if a project requires precise measurements then this could be important.
When specifying timber it is important to note that the dimensions of a piece of wood can vary depending upon its moisture content and how it has been treated.
The moisture content in European standards relating to sizes is 20%. The change in size with change in moisture content is calculated as follows :-
-
there shall be a 0.25% increase in thickness and width for every 1% increase in moisture content over 20% up to 30% (above 30% no further increase in size occurs);
-
and, conversely, there shall be a 0.25% decrease in thickness and width for every 1% decrease in moisture content below 20% e.g. a 200mm joist could shrink by 4mm if it dries from 20% to 12% in use.
The Range Of Customary Sizes Of UK Timber
Examples Of Customary Timber Cuts For Cladding, Flooring, Decking, & Mouldings
For further detailed information the TRADA Wood Species Database contains the physical properties of more that 150 species of timber, and this site's Guides page details the workability, durability, and price indicators of many timber species.
