Manual of Purpose-Made Woodworking Joinery

Timber grading 7

shakes), or towards the pith from the circumference
(star shakes). They are caused by shrinkage in an over-
mature tree.

Live knots and dead knots

Knots in converted timber are the remaining roots
of the tree’s branches. Small, so- called live knots are
acceptable in commercial grading of joinery timber –
but large live knots are unacceptable in good- class
joinery. Knots that are loose, broken or decayed are
known as dead knots and are unacceptable in any class
of joinery. An ominous black ring around a knot is
usually a sign that it is likely to fall out when the
moisture content of the timber changes.

DEFECTS DURING SEASONING

Checks or splits

So- called end- checks or end- splits – usually of a very
short length – in the ends of seasoned timbers are
caused by unequal drying. When ordering timber for
a particular job, it is wise to add at least 100mm for
end- check wastage. Internal- checks or splits – usually
short in length – are caused by errors in kilning tem-
peratures. Such errors are also responsible for occa-
sionally found surface- checks or splits on the mid- area
face or edge of kiln- seasoned timbers.

DEFECTS AFTER SEASONING

Wane

This is caused by the log being too economically
converted, leaving part of the log’s round shape and
inner bark (bast) on the edges of boards; producing a
so- called waney edge.

Bowing

Figure 1.4(a): Bow, bowed or bowing in timber ter-
minology refers to a segmental- shaped bend or warp
in the length of a board or section of timber, which
springs from the wide face of the material.

Cupping

Figure 1.4(b): Cup, cupped or cupping – covered previ-
ously, but shown here in 3D form – refers to a concave

or convex distortion across the face of a board, usually

caused by the board’s face being tangential to the

annual growth rings.

Springing

Figure 1.4(c): Spring, sprung or springing refers to a

segmental- shaped bend or warp in the length of a

board or section of timber, which springs from the

narrow edge of the material.

Twisting

Figure 1.4(d): Twisting, twist or twisted can refer

either to a distortion in the length of a piece of timber,

whereby it has developed a spiral- like propeller- shape;

or, in joinery terms, it refers to distortion of a framed-

up unit such as a door, caused by one or more of the

framed stiles or rails being twisted – or by ill- made

corner joints.

Warping

Warp, warped or warping refers to any of the above

four timber defects.

TIMBER GRADING

Although the grading of timber (according to defects,

strength and appearance) varies to some extent

between the different countries supplying it to bulk

purchasers, it is mainly sold in six grades – although

very often the first three or four grades are not

Figure 1.4 (a) Exaggerated example of a bowed board;

and (b) Exaggerated example of cupping.

Figure 1.4 (c) Exaggerated example of sprung edges;

and (d) Exaggerated example of a twisted board.

(a) (b)

(c) (d)