A Structural Engineers Look at Timber
Richard Harris in
his excellent book 'Discovering Timber Framed Buildings' suggests that
"it is generally misleading to analyze traditional carpentry using
modern structural analysis", James Bentwod in his SPAB pamphlet
'Repairing Timber Framed Buildings' only mentions engineering
calculations in cases of eroded timbers and suggests a repairer with
experience can "rely on an intuitive feeling for the structural
stability of the frame he is working on".
Do Structural Engineers have a contribution to make to preserving and
even illuminating the function of timber framed buildings or should
everybody (as one Conservation Officer told me he did), try to keep
Engineers well away from old buildings?
I am probably too closely involved to give an unbiased answer to these
questions but I can say a few things about timber framed buildings that
have struck me when working with them.
It is said that the essential character of nations remains the same and
Tacitus (the Roman historian writing of the German tribes who later as
the Angles and Saxons came to England) said of their houses 'They do
not even make use of stones or wall-tiles; for all purposes they employ
rough hewn timbers, ugly and unattractive looking'!
The English liking for timber frame has a long pedigree!
When a Structural Engineer who has worked on modern designs first
encounters traditional timber framing the first reaction can easily be
one of bafflement.
Why are the floor joists and rafters wider than they are deep rather
than the other way round as in modern construction? (see Figures 1
& 2) - the traditional way would seem to be just be a real waste of
Modern style timber
frame construction (Early 20th Century Felixstowe).
sawn regular sized softwood timbers (generally depth much greater than
width of joists and studs), and simple nailed joints. In this case
studs are tenoned into plates, but later butt jointing with skew nails
is generally used.
at Kelsale) joists of width greater than depth housed into main beam
with mortice & tenon joints.
Why are the wall
plates and the rest of the wall framing timber so large?
Why are all the roof timbers so small - and usually - so distorted?
A little more investigation will reveal the intricacy of many of the
joints - some have a complexity more reminiscent of those puzzles where
you are given half a dozen elaborately shaped pieces of wood and are
asked to make a cube, than the product of a workaday carpenter.
(see Fig 3 & Fig 4)
Timber Frame Construction Main Beam /Post joint, formerly jettied (post
now removed) House at Kelsale
traditional wall plate scarf joint.
As well as the
complexity of the joints, they appear far stiffer than needed to resist
the likely loads on them. Richard Harris said that a giant could lift
up a timber framed building and turn it upside down without breaking it
up. The obvious question is why on earth would they be constructed like
this? Even in the remoter parts of Suffolk giants have rarely been
known to invert buildings!
The English, and dare I say especially their businessmen and farmers,
are not known for spending more money than is strictly necessary on
their buildings, except when using the building as a symbol of status,
or in present day parlance giving it 'pavement appeal'. It should be
remembered that when built, most buildings will be erected to fulfill a
function at a price, and nowadays a great deal of ingenuity and hard
work is put into finding easier and cheaper ways of making houses. A
good example is trussed rafters - a brilliantly cheap method of putting
pitched roofs on houses, (but a method unlikely to last for a thousand
So were master carpenters simply ignorant in the middle ages
constructing heavy and elaborately jointed timber buildings, or were
they so bound by tradition they could see no other way of constructing
I am sure the answer must be no. At any point in history, the most
economic solution to a problem will depend on the availability of
materials and the technology available to process them. In the time of
the dominance of timber framing up to the 18th Century, oak (which was
almost invariably used for timber framing) was plentiful and other
timbers were unlikely to last as long in the conditions of use, (no
reliable ways of keeping moisture away from the outside of timber
frames were available).
However the means of converting the oak into houses were hand tools,
saws, axes, and adzes; and oak could only be easily worked with these
tools when green (i.e. before drying out from its natural moisture
content to the lower moisture content likely within a house).
The reason for the elaborate jointing I suspect is hinted at in the
Spring 2001 issue of the "Eavesdropper" in an article by Alan Bayford
reporting Andrew Moore's talk on the Management and Conversion of
Timber for Buildings. He describes how timber twists as the material
dries and this locks mortice and tenon joints together. This is correct
as far as it goes, but I believe the real reason for the elaborate
joints is to prevent the timber framing from twisting and turning out
of line while this shrinkage takes place; the locking together of the
joints is a way of achieving this. Some of the timber members then end
up seemingly oversized simply because they are sized to allow the
construction of the complex torsion resisting joints rather than simply
to bear the loads applied to them.
(see Fig 5 showing a traditional torsion resisting rafter / plate joint
and Fig 6 showing the equivalent modern detail).
Traditional rafter/ wall plate joint.
dovetail on the underside of the rafter allows compressive force in
rafter to be transferred into the wall plate and provides torsional
restraint to the rafter.
House at Fressingfield.
Diagram Photo of
typical modern cut roof rafter / plate joint - rafters are notched over
the top of plate with no torsional restraint.
Structural Engineers know that when designing for instance timber roof
trusses, the critical thing is to get the joints right, and often if
there is enough timber to adequately form the joints the remainder of
the structure will be satisfactory.
I was recently asked to look at a failed jettied gable built onto a
house around the 1950s. Here a large oak beam spanned between two
concrete brackets and a blockwork wall of about 5 metres height was
built off it.
The oak beam had twisted by at least 10 degrees over its length and the
twist was sufficient to lift the beam at one end, so it was bearing on
one corner on the supporting brackets rather than bearing on edge, in
spite of the weight of blockwork on top of it.
This gives some idea of the forces within the timber twisting it, which
have to be resisted by the framing joints.
Certainly when you look at the traditional joints - mortice and tenon,
lap dovetail, the splice joints used in wall plates, they are all
torsionally stiff i.e. they are able to resist this twisting effect.
It would be interesting in due course to examine the function of each
joint in detail and hopefully a later article will attempt this.
Of course resisting shrinkage and twist is not the only function of the
joints in traditional timber framing, they also transfer loading
through the frame and help tie it together against loads including wind
loading and outward thrust from roofs.
What are the lessons to be learnt from this? Firstly, as well as the
beauty and historic interest to timber framing there is also a
The elegant and appropriate use of the materials available, and the
craft tradition refined over many hundreds of years gave elegance and
intelligence to the way timber was turned into buildings. What at first
appears a heavy handed way of using oak turns out to be far from this.
Secondly, when repairing timber framing, the repairs (especially if in
green oak) need to respect the way the building works as a structural
frame, and to be jointed in such a way as to lock together with the
remainder of the framing and not allow it to subsequently distort, or
twist off line as the green oak dries out. This is where a Structural
Engineer's knowledge of the way timber behaves when loaded and how
frames work can be put to good use in helping to produce appropriate
repairs to existing timber framed buildings - or, for that matter, help
in building new ones in the traditional manner (fig 7).