[Strawbale] oak and wood drying

[Derek Roff]

--On Monday, May 7, 2007 9:51 AM -0400 Corwyn <corwyn at midcoast.com> 
wrote:

> Are solar kilns better than fossil fueled kilns (other than the
> obvious reasons)?  It was my understanding that kiln drying large
> timbers was basically impossible.  Something to do with case
> hardening.

Solar kilns cannot dry wood as quickly nor as safely as kilns that 
incorporate modern, fuel-fueled technology.  I know of no way the 
owner-builder can come close to the results of a multi-million dollar 
computer-controlled kiln managed by an experienced staff.  On the other 
hand, with care, the owner builder can be successful using site-built 
timbers.  But it does take care and attention.

Fully drying large timbers takes longer than most of us can wait.  The 
usual rule of thumb for air drying is one inch of thickness per year. 
Kiln drying can accelerate that, but overly fast drying increases 
problems.  Obviously, better timber, a better kiln, and a better the 
operator will give better results.  In any case, partial drying will 
reduce weight, increase strength, and give a hint as to how each timber 
is planning to twist and warp.

Wood will eventually dry, and in large timbers, case hardening is 
likely.  Case hardening refers to a condition where the outside of a 
piece of wood has dried and shrunk, compressing the inside fibers, 
which, for some period of time, retain more moisture.  Differential 
drying is the primary cause of distortion as wood dries.  If case 
hardening exists, the wood will contain internal stresses, even after 
it is fully dry.  Case hardening can cause severe distortion when the 
wood is milled.  Timbers used in construction are unlikely to be 
remilled, but internal stresses should be kept in mind when remodeling 
or reusing old timbers.

I'm always disappointed to see inaccurate, poorly-worded or incomplete 
information posted to these lists.  Especially when the author is me. 
My last message contained several errors, omissions and statements 
likely to mislead.  Let me try again.

Obviously, timber framers have been building sturdy, reliable buildings 
for more than a thousand years.  They succeed through an understanding 
of how wood behaves, and by following procedures developed through the 
centuries.  Few of us will have an opportunity to acquire that 
knowledge and expertise.  When we amateurs attempt to build with green 
lumber/timber, without the foundational knowledge of wood, we risk 
failures.

Timber framers know that you can't do much to restrain wood from moving 
as it wants to move.  Traditionally, joints were designed to work in 
harmony with wood movement, and wood was selected and prepared to 
minimize problems.  Tree selection and proper milling are essential 
parts of the timber framing process.  Nowadays, we seldom have any 
chance to influence tree selection and milling of the wood that we buy. 
Most lumber today comes from trees that are swift-growing and young. 
Both characteristics increase the risk of problems.

I said that cross-grained joints made with green wood will be stressed 
and may crack.  This is true if the joint connections try to restrain 
wood movement.  An example is mortise and tenon joint fixed with glue, 
multiple pins, or metal reinforcement using multiple bolts, screws or 
nails.  A traditional timber-framed mortise and tenon, secured with a 
single, large-diameter dowel, will not have these stresses.

I said that most joints and fasteners will loosen as wood moves from 
green to dry.  I should have said "some."  The traditional dovetail 
joint will loosen if it involves cross-grain construction, but should 
stay tight if the grain is parallel in both members.  A mortise and 
tenon joint will loosen quite a bit in one dimension, slightly in the 
second and scarcely at all in the third dimension.  Properly oriented, 
pinned and integrated into the structure, this differential shrinkage 
is not a problem.  Otherwise, it may cause structural or cosmetic 
issues.

Fasteners such as nails, screws and bolts are held more tightly as wood 
dries.  They may become impossible to remove.  Fasteners such as 
braces, brackets and supports may loosen, depending on how they are 
connected relative to the shrinkage of the wood members.

Sorry for the previous errors, and this overly long reiteration.  I 
hope this message won't require another set of corrections.

Derelict

Derek Roff
Language Learning Center
Ortega Hall 129, MSC03-2100
University of New Mexico
Albuquerque, NM 87131-0001
505/277-7368, fax 505/277-3885
Internet: derek at unm.edu