To Deal With Knotty Wood.
how Knotty Wood affects the strength and stiffness of structural timber
allow you to get the best from your boat building lumber.
Softwoods such as those used for the framing of interior fitting are
But how the knots will affect the stiffness
and elastic strength of the
will depend on their size location and the type of knot.
Knots are the remains of
branches which died when the tree was younger and have since been
buried within the new growth rings.
The trunk’s new growth rings
simply surround the
dead branch stub often including its bark.
Softwoods such as Conifers are
especially prone to
knotting all the way up the trunk.
Some such as Scots Pine grow
their branches in random
While in others, such as
Spruce the branches and
therefore the knots, grow randomly but more evenly spaced.
How loose the knot is, will
depend on how long the branch
has been dead while the trunk continued to grow.
What is know as a ‘live knot’
will still be
attached to the tree whereas a ‘dead knot’ will no longer be
Dead knots are the ones which
weaken the timber most and
can easily drop out, however both types will affect strength of the
Back to Top
effects of Knotty Wood
In most cases where knotty wood is used
as framing for cabin furniture
strength is not really an issue.
However, if the timber has to
bear any weight, the
position of any knots needs to be considered with care.
A settee support of knotty
wood might be adequate under
normal loads but should a wave rock the boat and someone sit heavily on
beam could break suddenly.
How knots will affect the
strength will depend on their
position, size, number, and condition.
A knot that runs across the
top edge of that settee beam
will be compressed under load and though it will be a weak spot it will
have a serious effect on strength.
The same knot running along
the underside of the beam
which, is subjected to tension will in effect be a hole, greatly
load bearing capacity.
The nearer the
knot is to the
center of the beam the more it will weaken it.
In some instances small knots
along the neutral plane of
a beam can be an asset as they will help prevent longitudinal
Knots in boards and planking
which go through it at right
angles to the broadest surface are unlikely to affect the strength
The other problem caused by
knotty wood becomes apparent
when it comes to finishing.
While that darker patch in the
wood and the way the grain
of the rest of the wood flows around it may look attractive it will
The knotty wood is
usually denser than that
surrounding it making it harder and more brittle.
Back to Top
Softwoods are graded for evenness of
grain and the number of defects
such as knots and Ingrown
Bark which will
mar the look as well
as weaken the board.
are splits that occur in the
wood due to growth defects or shrinkage stress.
Other defects to watch out for
when buying lumber are
caused by the drying process.
Wood which is not dried
carefully can be stressed in
occur inside the
boards when the outside dries and stabilizes before the interior wood,
inside shrinks more than the outside leading to internal checks.
along the rays caused
by too quick drying of the surface.
are caused by too quick
drying of ends, which could have been prevented by sealing the ends
Bowing or warping
is usually due to
stacking boards badly.
restorers the dreaded Wood Rot.
Back to Top
Knotty pine may be
fashionable but it will cause problems when it comes to finishing.
That hard brittle lump and the
changes or direction of
the surrounding grain will make planning smooth a frustrating
their blades set at an angle of 45 degrees.
For dealing with
grain it is possible to buy
planes with a frog which pitches the blade at an angle of 50 degrees or
and some plane makers can supply a high angle frog to convert existing
The other problem with knotty
wood is that the resin from
the knot will seep through paint leaving a circular brown stain.
To prevent this the knot needs
to be painted with a
Knotting Solution before priming.
‘Knotting’ is a shellac based
Another option for small face
knots is to drill out the
knot and replace it with a glued in plug made from the same material as
sawn wood can be made fairly stable, just by ripping it
down the middle.
The outer edges will be vertical grain, progressively
transitioning to flat in the middle, then back to vertical on the other
If you run this through a table saw, ripping it right down the middle,
have two boards, both with a high percentage of quarter sawn material
Now, if you make another pass through the table saw, ripping an inch or
the heart wood edge, you now have a much more stable board.
Back to Top
To comment as a Guest just enter your name (nickname) and then add your Comment.