Timber basics

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Planed wood 383-4.jpg

This article is a quick guide to the key concepts relating to timber. For more depth and further topics, see Timber.

While basic woodwork isn't too hard to do, woodwork is quite a complex subject, and the information below is more basic guidance than the last word on the subject. Experienced woodworkers will note there are various exceptions to be found to the information presented here.


Timber Sizes

The timber industry has long operated in imperial sizes, and imperial terms are still in widespread use today. However legislation has outlawed the sale of goods in inches and feet, so timber is now sold in the nearest metric sizes to the old standard imperial sizes.

Timber is still widely bought in imperial sizes, while being sold in metric. For example 2x4 means 2"x4". Sellers will supply 50mmx100m, which is near enough identical. 2x4 and 4x2 are the same thing, with both terms in widespread use. Metric sizes are also sold that don't correspond to the popular imperial sizes.

For rough sawn wood the nominal size is normally very close to the real size supplied. However this isn't so in all cases, especially when dealing with used wood. Its not unusual to find historic timber that's nearer 1.5" x 3.5", but still tends to be described as 2x4.

Planed wood is a little smaller than rough, usually by about ¼" or 5-7mm each way. 2x4 PSE is simply 2x4 rough sawn that has been planed to make it smooth.

When timber is sold in metric, the stated size is the size you get.

Length

Timber is sold in various lengths that are multiples of 30cm (apx a foot). Most common are 1.8m, 2.4m & 3m (apx 6', 8' & 10'). Several longer sizes are also sold. (Note that the metric equivalent lengths are very slightly shorter than the imperial.)

Price

Most timber is sold by price per length, and some by price per volume. Some example price lists:

These are not company recommendations


Terms

Rough sawn
Usually a splintery finish, but smoother sawn surfaces are seen on some goods.
Dimension sawn
Sawn to exact size. Same rough finish as rough sawn.
PAR
Planed All Round. Nice smooth finish. There's no guarantee of accurate squareness with PAR, a lot of PAR is true, some not. PAR has mostly been supplanted by PSE today. Hand held planes produce PAR.
PSE
Planed Square Edge. This is planed all round with sides accurately at 90 degrees.
PFS
CLS
Canadian Lumber Standard. CLS is planed smooth, has radiused (rounded) corners, and is free of large knots at the edge of the wood. These features reduce the spread of fire in timber frame wall cavities and make it safer to handle.
ALS
American Lumber Standard, like CLS
Regularised
Radiused corners, similar to CLS but the planed surface is not consistently smooth as its planed to size spec rather than planed smooth. It may thus be rough sawn in some areas. Its also graded, eg to C16 etc.
Kiln dried, KD
timber dried to a specified moisture content. However poor storage by the merchant (after drying) may result in higher moisture content.

Qualities

Species

Most wood used for DIY is spruce, fir or pine. Other species are also used, but command a higher price, limiting their use.

Whitewood

Spruce

Whitewood is spruce, douglas fir, hemlock, pine and several other species of timber intended for first fix use, ie domestic woodwork that will not be seen when the project is complete. It may have some splits & stains and some warp. Its also used for floorboards, being more stable than redwood and cheaper. Spruce knots however are unstable.

DIY sheds sell a lot of whitewood.

The quality of whitewood on sale has improved over the years, and a percentage is good enough for second fix.

Whitewood doesn't take dyes evenly, and preservatives have limited penetration.

Redwood

Redwood is Scots Pine intended for second fix use, ie domestic woodwork that will be seen when the project is complete. Its mostly free from splits & stains, and generally has much less warp than whitewood, though warp is still an issue.

Pine darkens over the years, eventually ending up quite a dark wood.

Joinery

Generally joinery timber is clear, with a knot-free surface. Its used for furniture.

Pine

Clear Pine

Pine is a genus of conifers covering many different varieties of tree. Timber described as pine isn't always a pine species at all, but it will have much the same appearance and properties, making the end result the same. Redwood is Scot's pine.

Hardwood

Hardwood closeup 0394-2.jpg

Hardwoods are generally denser, harder and slower growing than softwoods, but there are exceptions. Thus they have close growth ring spacing and fine grain. Hardwoods are mostly used for appearance, rot resistance (some species) or toughness, and cost significantly more than pine & spruce.

Mouldings

Timber is available pre-planed to many shapes. These are called mouldings, and all have their own uses.

Many are shown here. There are also some less popular shapes here.


Grading

Grading is an assessment of the structural strength of the timber. Key features assessed in grading are splits and knots, especially large knots at the edge of the wood.

Small timber bought for DIY use is mostly ungraded. Graded wood is stamped with the grading details.

For new floor joists and roofing BR requires use of graded timber.

C16 is the most common grade, but C24 and some less common timber grades are also available. The higher spec grades may be used where dimensions need to be minimised.

Buying Wood

Ordering wood to be delivered means you don't pick the timber. This is ok for 1st fix, but with work where the wood needs to be straight it can be a problem. Timber yards are known for sometimes using new customers to clear junk.

Problem Wood

Bent stock is a regular problem.

Warp

Docking (selective cutting) deals with a lot of warp. Wood often warps at points (knots) rather than all along, so cutting it at those points gives shorter pieces of straight wood.

Warped wood is good for framing, where considerable warping is tolerable. When the warp is too bad, the wood can be cut short and used for noggings.

Twist

Twisted wood can sometimes be made good enough by using in very short lengths, as the amount of twist on each piece is then much smaller. This often works for first fix, but there's always more junk wood than uses for it.

Other ways to deal with twisted wood are:

  • Using it fixed firmly to something much stronger, thus forcing it to untwist
  • Use it for framing, which is somewhat tolerant of twist
  • Plane it to give smaller straight timber.
  • Don't buy it in the first place!

Cupping

Planks are prone to cupping, whereby one side becomes convex and the other concave. If its desired to fix it, wetting the dished side will expand it a little, and it can then be dried while weighted flat. The thinner the plank, the more chance of success.

Stability

Timber defects can often be worked around, but warped or twisted timber has bent since cutting, and is thus unstable. Changes in moisture content are prone to producing movement again. This further restricts the uses for such wood.

Uses for junk wood

Timber framing is the main use. All sorts of defects can be hidden behind plasterboard once finished.

  • Wood with paint, nails or damage will all be hidden
  • Bent wood can be fitted bending sideways
  • Even wood bent both ways will only cause gentle undulation on the plasterboard if not too bad, and this usually isn't noticeable.
  • Split wood can be used too, adding a few screws to fix it together.
  • Almost anything can be used as noggings: undersize, odd shaped, badly bent, even glued or screwed together offcuts.

Mildly bent wood can also be used in timber framed shed construction. Its hidden by the cladding.


Water Content

All timber contains some water. Where stabilty matters, which is most applications, timber should ideally be either purchased with water content similar to final use, or else acclimatised before use. If ignored, warp and twist are more likely after fitting.

Timber used in new build for structural elements is required to have a maximum of 18% water content (though many houses have been built with green timber).

Green & Seasoned

Most wood for DIY work is seasoned. Green wood (meaning unseasoned rather than green in colour) has high water content, and is liable to move during drying, making it of limited use for DIY. The main exception is green oak used for oak frame. Timber frame sheds can also be built with green, as some movement on drying is usually acceptable.


Durability

Most DIY timber is not durable, meaning it soon rots if used outdoors without protection from water. The options for outdoor timber are:

Well known durable species include

  • oak (very durable)
  • red cedar

Note that the sapwood of all species is non-durable, its heartwood that's durable. The majority of timber is heartwood, with only the outer layer of the tree (under the bark) being sapwood.

Treatment

Timber is available ready treated against rot. There are a number of different preservatives used, some applied by dipping and some by vacuum impregnation. Cutting it exposes unpreserved ends, which should be treated for best life expectancy.

When applying preservative, the cut ends need the most attention, as they soak up water like a sponge. Cut ends will usually sponge up several coats of preservative, which helps it last longer.


Board

Timber also comes in board form. The most common types are hardboard, chipboard, MDF, plywood and timberboard. These are described in more detail in Sheet Materials.

Hardboard: thin non-rigid brown board, typically 3mm thick. Most used as low cost drawer bottoms.

Chipboard: wood chippings glued together, and sometimes coated with white melamine, brown imitation wood veneer etc. Most common furniture board in Britain. Usually fairly weak and vulnerable to water.

MDF: a uniform brown material, can be machined and worked without grain being an issue. Vulnerable to water and not very strong.

Plywood: Available in many grades for different purposes. Dimensionally stable, strong in both directions, and one of the stronger wooden board types. Lower cost plywoods are vulnerable to water and delaminate fairly easily.

Timberboard: Strips of wood glued side by side to create flat board. Gives a real wood finish. Strong along the grain, less so across. Cups badly if exposed to water on one side for a day, but normal cup spills don't do this.


Drilling holes

Most holes drilled in wood are either pilot holes, clearance holes or countersink holes.

Clearance holes allow the screw to slide through freely. An ideal size for this is the full width of the screw shank plus half a millimetre. Hole size isn't critical, but if too large the head may sink into the hole when tightened, enlarging the hole in the process.

Pilot holes are holes that enable screws to be driven in without difficulty or risk of splitting the wood. A good size for pilot holes is half a milimetre larger than the narrowest width seen on the screw spiral.

Countersink holes are very shallow tapering holes for the screw head to sit in. These allow a counterunk head to sit flush with the surface. They are generally drilled using a countersink, but can also be made with a large drill bit. In most cases the size and shape of the countersinking hole need not match the screw head well, as the head will distort the wood under it to some extent.


                                 /| 
            ____    /\/\/\/\/\/\/ |
pilot hole  ____   <              |
                    \/\/\/\/\/\/\ |
                                 \|

               ____                 /| 
                       /\/\/\/\/\/\/ |
clearance hole        <              |
               ____    \/\/\/\/\/\/\ |
                                    \|

                    ____
               /| 
  /\/\/\/\/\/\/ |
 <              |        countersink
  \/\/\/\/\/\/\ |
               \|   ____


        Approximate Hole Sizes

Pilot Holes

Experience will soon tell you when to use a pilot hole and when not. Generally speaking, small screws in medium timber or bigger don't need pilot holes, but with medium to big screws or small timber the chances are a pilot hole would be wise. Lack of pilot hole can cause splitting in small wood, or jamming with medium to large screws. Thinner screws go in a good deal easier.

Countersink holes

Again sometimes they're needed, sometimes not. Plasterboard screws have heads that penetrate less than traditional countersunk heads, and can usually be sunk fine without drilling first.

Knots

Knots are made of much tougher material than the surrounding wood. Screwing into knots with standard size pilot holes causes the wood to split. Its generally best to avoid knots when fixing, but sometimes a screw is needed there. A simple solution is to use a slightly larger pilot hole, then they behave fine, good grip and no splits. Never try to screw a knot with no pilot hole.

Nails may be driven through knots if a clearance hole is drilled first - though this is rarely necessary. Don't attempt to nail a knot without pre-drilling.

Difficult screws

Going in

Awkward screws that are proving difficult to get in mean you need a pilot hole, or a bigger pilot hole. If the right drill bit isn't to hand, dipping the screw in oil makes some difference and is often enough. Dipping in oil before driving reduces friction, requiring less energy to get the job done. Various substitutes can be used, such as margerine, chocolate, etc (anything high in fat).

Don't labour over a tough screw, if it won't go in just take it out and fix the problem. If you keep at it you'll only end up with a well jammed screw that requires repeated curses to get out.

Coming out

There are numerous ways to get stuck screws out. See Removing a Damaged Screw


Stiffness

The stiffness of wood is proportional to width x depth cubed. Joists etc are normally rated for stiffness rather than strength.


Used wood

There is a gotcha with used wood: most power saws and embedded nails really don't mix well. If you regularly use old timber, nail-safe circular saw blades are available. For occasional work one can use a hand saw or jigsaw. Nails can damage the blades on these, but not the operator.

Planes are also vulnerable, so used wood is best not planed.

See Also