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Wood buildings: log or frame
WOO11 RllILBlINGS - LOG OR PIIAblE*
by
N . R . llutcheon a n d S . G . Flattar
Log buildings have l o n g been a symbol o f pioneer l i f e and s e t t l e m e n s
in North America. There is now an i n c r e a s i n g i n t e r e s t i n t h e i r p r e s e r v a - t i o n and reconstruction a s c u l t u r a l a r t i f a c t s . They a r e a l s o b e i n g
favored i n some q u a r t e r s a s a means o f r e v e r t i n g to a simpler way of l i f e a n d even as s u i t a b l e housing f o r c u r r e n t life styles. T h i s renewed
i n t e r e s t i n l o g housing justifies c a r e f u l s t u d y o f the r e a s o n s -
technical, economic and s o c i a l - f o r t h e o r i g i n a l d e c l i n e in t h e i r u s e
s i n c e some o r all o f t h e s e reasons may still be v a l i d t o d a y . Since frame c o n s t r u c t i o n was t h e p r i n c i p a l replacement f o r logs and is s t i l l t h e most c o m o n form o f c o n s t r u c t i o n for s i n g l e - f a m i l y d w e l l i n g s , a comparison o f l o g and frame will not be o u t of p l a c e .
The e a r l y s e t t l e r i n N o r t h America was faced everywhere, except o n the P r a i r i e s , w i t h d e n s e f o r e s t s . Wood was at o n c e h i s g r e a t e s t r e s o u r c c , f o r f u e l and shelter, a s \ c e l l a s h i s greatest problem s i n c e a l l land had t o b e cleared of t r e e s b e f o r e it cotild be u s e d . There was reason t o be
extravagant and not e c o n o m i c a l i n t h e use o f wood f o r housing as well as f u e l . I n i t i a l l y t h e r e were no b e t t e r a l t e r n a t i v e s t o wood i n l o g form. Sawn lumber, n a i l s , lime, c u t s t o n e , b r i c k s and g l a s s o n l y became
a v a i l a b l e as local and regional economies developed.
The sequence
QE
development i n s e t t l e r s ' housing from t h e f i r s t c r u d e s h a n t y w i t h a l a r g e h o l e in the roof for t h e escape o f smoke, to l a r g e rlag c a b i n s with chimneys, t o more e l a b o r a t e and more permanent h o u s e s of
frame, s t o n e and b r i c k is r e f l e c t e d in t h e w r i t i n g s of pioneers such as S t r i c k l a n d (1) a n d Dunlop [ 2 ) i n O n t a r i o in t h e 1 8 3 0 f s , t h e n know as Canada West. Dunlop, an o f f i c e s of t h e Canada Company, favored the
European t r a d i t i o n of d u r a b l e masonry c o n s t r u c t i o n . In doing s o , he underestimated t h e economics to b e achieved w i t 1 1 wood c o n s t r u c t i o n in
h e a t i n g and i n first c o s t u n d e r Canadian c o n d i t i a n s . T h e appearance of b r i c k masonry i s s t i l l h i g h l y valued today f o r Canadian houses b u t i s p r o v i d e d most commonly as b r i c k v e n e e r over i n s u l a t e d frame construction.
* P r e s e n t e d a t the C o n f e r e n c e on Log S t r u c t u r e s i n Canada, h e l d i n Banff, A l b e r t a , October 1977 and i s s u e d i n thjs form w i t h permission of t h e authors a n d The Resource C c n t r c , Faculty of linvironmcntal Design, The
T h e r e i s l i t t l e doubt t h a t t h e l o g hause l a c k e d p r e s t i g e and was
o f t e n replaced b y timber frame a n d masonry as soon as t h e economic p o s i t i o n o f t h e owner p e r m i t t e d . Vermin were a problem ( 3 , 43, some types b e i n g c a r r i e d b y t h e visltors and travellers who were always welcomed w i t h o u t r e g a r d f o r p e r s o n a l hygiene. Smooth f l o o r s and crack-
f r e e , plastered walls, a t t a i n a b l e only w i t h sawn lumber and plaster, were undoubtedly held i n h i g h r e g a r d by t h e housewife f o r p u r e l y
p r a c t i c a l s e a s o n s a s well a s f o r p r e s t t g e . Many o f t h e log houses which have s u r v i v e d f o r 100 y e a r s or more have been discovered b e h i n d clapboard s h e a t h i n g , and l a t h and p l a s t e r o r p a n e l i n g .
As pioneer commntties developed, logs were l e s s readily available,
w h i l e mills f o r t h e p s o d u c t i o n of lumber became nore common. This gave frame c o n s t r u c t i o n a n advantage since t h e volume o f wood r e q u i r e d was o n l y a f r a c t i o n o f that i n l o g c o n s t r u c t i o n . Sawn lumber a l s o o f f e r e d t h e c l a d d i n g and f i n i s h e s which were d e s i r e d a n d was a l s o adaptah l e to a w i d e r sange o f forms and c o n s t r u c t i o n s . It could a l s o b e arranged to m i n i m i z e the more s e r i o u s e f f e c t s o f t h e s u b s t a n t i a l shrinkage and s w e l l i n g o f wood across t h e g r a i n w i t h changes in m o i s t u r e c o n t e n t . These and o t h e r t e c h n i c a l c o n s i d e r a t i o n s o f i n s u l a t i o n and h e a t i n g
economy a r e t h e b a s i s f o r much o f the d i s c u s s i o n to f o l l o w .
INSULATION AND IJEATING
One a f t h e advantages claimed f o r l o g h o u s e s i s t h a t t h e y a r e warm i n w i n t e r . T h i s c l a i m is commonly based on consideration of t h e
i n s u l a t i n g v a l u e , o r t h e R v a l u e , p r o v i d e d b y t h e s u b s t a t r t i a l t h i c k n e s s e s of wood making up t h e e x t e r i o r w a l l s . Table T provides a basis f o r
comparing various w a l l c o n s t r u c t i o n s .
I t can b e c o n c l u d e d that a s o l i d wood w a l l 8 i n c h e s t h i c k w i l l be s u p e r i o r in i n s u l a t i n g v a l u e t o m o s t o t h e r uninsulated w a l l s , brat
i n f e r i o r t o f u l l y i n s u l a t e d wood frame ~ v n l l s . An u n i n s u l a t e d wood w a l l 1 3 inches thick, involving f r o m 5 to 10 times a s much wood as i s
r e q u i r e d to frame and s h e a t h a wood framc wall w i l l barely meet the
requirements of t h e Residential Standards 1977 ( 5 3 . It h a s been proposed
t h a t i n a n t i c i p a t i o n o f f u t u r e e n e r g y c o s t i n c r e a s e s , i n s u l a t i o n up to R 20 can b e j u s t i f i e d (63
.
This requires framc w a l l s t h i c k enough to accommodate 5 i n c h e s o r more of i n s u l a t i o n .Walls of s o l i d wood, a s l o g s , o r in some o t h e r form, w i t h m t i n s u l a t i o n , a r e n o t capable of meeting present-day i n s u l a t i o n r e q u i r e - ments w i t h reasonable w a l l t h i c k n e s s e s . The a d d i t i o n of i n s u l a t i o n t o
them means a d d e d c o s t and l o s s o f the advantage of simp1 i c i t y o f c o n s t r u c -
tion, w h i l e t h e s h r i n k a g e c l l a r a c t e r i s t i c s of wood may i n t r o d u c e cornplica- t i o n s when additional l a y e r s must be added t o a simple, load-bearing wall of horizontal elements. O n l y when wood o r some o t h e r f u e l is readily
a v a i l a b l e , and has little value for use elsei~ihere, i s it p o s s i b l e t o i g n o r e t h e i n s u l a t i o n s t a n d a r d s t h a t a r e made necessary b y the need t o
FUEL ECONOMY
F u e l economy is dependent on much more t h a n t h c IE v a l u e of w , ? I l 5 . Ileat can a l s o b e lost by t~ansmission t h r o ~ r g h a l l o t h e r e n c l o s i n g
s u r f a c e s , including windows and d o o r s ; c e i l i n g s and f l o o r s , Walls, when
w e l l i n s u l a t e d , often a c c o u n t f o r less than 30 per cent of the annual heating requirement in single-family dwellings. The heat l o a d i n v o l v e d in warming she c o l d outdoor air, which l e a k s i n t o a house as infiltra-
tion, can account for a s much as 4 0 p e r c e n t of the a n n u a l h e a t i n g r e q u i r e m e n t . Infiltration i s dependent a n t h e l e a k a g e c h a r a c t e r i s t i c s
of t h e b u i l d i n g , and on t h e wind and s t a c k e f f e c t r e s u l t i n g when the i n d o o r air is warmer t h a n t h a t o u t d o o r s . It i s current practice i n mast small buildings t o rely on t h i s n a t u r a l u n c o n t r o l l e d air leakage t o provide t h e a i r needed f o r v e n t i l a t i o n a n d for f u e l - b u r n i n g appliances. The d i f f i c u l t y i s t h a t t h e infiltration rate is not always matched t o t h e v e n t i l a t i o n r e q u i r e m e n t and a l s o t h a t the leakage characteristics of
b u i l d i n g s may vary markedly from one to a n o t h e r , depending on workmanship
a n d c o n s t r u c t i o n . Good practice i n s i n g l e - f a m i l y frame d w e l l i n g s h a s reduced the w i n t e r a i r leakage t o about 150 c u b i c feet per minute, or
a b o u t 0 . 3 a i r changes p e r hour in an average-size house, i n c l u d i n g b a s e - ment, u n d e r w i n t e r c o n d i t i o n s ( 7 ) . A f u r t h e r reduction c o u l d l e a d at
times to inadequate n a t u r a l ventilation while a n i,ncreasc would r e s u l t
in an i n c r e a s e d f u e l r e q u i r e m e n t . I t will b e e v i d e n t t h a t p r o p e r control of a i r leakage is a s i m p o r t a n t a s t h e R v a l u e o f w a l l s and c e i l i n g s i n d e t e r m i n i n g f u e l economy, and l o g d w e l l i n g s m s t h e c a r e f u l l y c o n s i d e r e d from t h i s p o i n t of view. FIREPLACES F i r e p l a c e s seem t o go w i t h l o g b u i l d i n g s . T h i s a s s o c i a t i o n is a n a t u r a l cne since b o t h f i r e p l a c e s and l o g c o n s t r u c t i o n w e r e promoted b y t h c same factor, a p l e n t i f u l s u p p l y o f cheap timber. I t i s u n f o r t u n a t e t h a t f i r e p l a c e s a r e s t i l l s o p o p u l a r and so ~ i d e l y regarded as a pleasant
amenity s i n c e they are i n f a c t t e c h n i c a l l y c r u d e h e a t i n g devices and can even b e a hazard t o che s a f e and efficient o p e r a t i o n of a d w e l l i n g
n o r m a l l y h e a t e d by o t h e r t y p e s o f systems.
The energy from a f i r e p l a c e a v a i l a b l e for h e a t i n g c o n s i s t s m a i n l y
of t h a t which emanates d i r e c t l y by radiation f r o m t h e glowing f i r e t o w a r d s t h e room. The major p a r t i o n o f t h e h e a t i n the f u e l is c a r r i e d
off
by t h e gases and t h c a i r f l o w i n g t h r o u g h t h e f i r c ~ l l a c c and up thechimney. Fleasurcments made many y e a r s ago on c o a l f i r e s showed t h a t t h e p o r t i o n o f energ). r e a c h i n g t h e room by radiation from thc f r o n t
O F
t h e f i r e p l a c e v a r i e d f r o m 1 2 to 2 2 p e r cent of t h e h e a t i n t h e f u e l (8).
Some a d d i t i o n a l heat may b e recavered by t h e f i s c p l n c c and chimney znd d e l i v e r e d to the room. The t o t a l heat r e a c h i n g t h e room from t h e traditional open wood-burning f i r e p l a c e used i n Canada w i l l seldomexceed 20 p e r c e n t o f t h e heat i n t h e fuel and may o f t e n b e much l e s s . Much h i g h e r r e c o v e r i e s , up to 50 p e r cent w i t h stoves and up t o 7 0 per
c l o s e l y controlled so t h a t t h e h o t gases are n o t immediately diluted a n d cooled by an excess b f a i r as I n an open Eire. S t o v e s o p e r a t e d w i t h a n
open front so t h a t tlie f i r e can b e seen, w i l l always b c l e s s e f f i c i e n t
than when o p e r a t e d c l o s e d : ~ n d w i t h t h e air supply p r o p e r l y c o n t r o l l e d .
Another difficult f e a c u r c of t h e f i r e p l a c e i s t h e large s u p p l y of
air that must be provided t o e n s u r e t h a t smoke w i l l not e n t e r the room.
It is recommended that a fireplace chimney should be capable of main- t a i n i n g a v e l o c i t y of a i r flow t h r o u g h t h e fireplace opening o f 0.8 f e e t per second (93. T o achicve t h i s with a f i r e p l a c e opening of 3 ft by
24
ft
c a l l s f o r a t o t a l f l o w o f 360 c u b i c f e e t per minute. T h i s may b e compared w i t h the a i r lcakage of 150 c u b i c feet per minute f o r an average h o u s e . T h i s e x t r a air must come from outside and r e p r e s e n t s anincreased heat 10s s
.
When f i r e p l a c e s are operated in a house t h a t h a s o t h e r fuel-hurning
d e v i c e s w i t h separate chimneys, t h e r e will h e competition for t h e a i r
available from i n f i l t r a t i o n . T h i s may resula: in d r a f t failure f o r one
or t h e o t h e r o f t h e chimneys in a t i g h t l y c o n s t r u c t e d house. If t h e f i r e p l a c e f i r e d i e s down w h i l e the o t h e r system continues to o p e r a t e , t h e d r a f t i n t h e fireplace chimney will decrease and, under c e r t a i n c o n d i t i o n s , t h e r e may b e a b a c k flaw d a m t h e chimney and o v e r the
glowing embers i n t h e grate, delivering s u b s t a n t i a l quantities o f carbon monoxide t o t h e room. There must a l w a y s be adequate o p e n i n g s to ensure a generous a t r supply t o t h e room in which a fireplace i s being
o p e r a t e d (10)
.
Fireplaces a r e best used to provide increased radiation l e v e l s f o r improved comfort conditions at l o c a t i o n s in front of the f i r e p l a c e , in compensation f o r low general room a i r temperatures under mild w i n t e r weather condi t i e n s [ I I ) . Stoves a r e more s u i t a b l e from the p o i n t of v i e w
of e f f i c i e n c y , s a f e t y and comfort.
SHRINKAGE AND SWELLING OF 1V00D
By d e f i n i t i o n , moisture c o n t e n t is the ratis o f the weight of m o i s t u r c i n t h e wood to t h e rVeight of d r y wood erpresscd as percentage.
Tn g r e e n t i m b e r , the moisture c o n t e n t may bc 40 to 200 p e r c e n t . IVItcn
~ ~ o o d d r i c s o u t , water i n the c e l l c a v i t i e s is l o s t first, while t h e c e l l walls s t a y s a t u r a t e d down to the fibre saturation point. ?his point is commonly reached at a b o u t 25 t o 30 p e r c e n t f o r a l l woods ( 1 2 3 .
Below t h e f i b r e saturation p o i n t , t h e moisxurc c o n t e n t h a s a n
equilibrium value which depends on t h e a m b i e n t conditions: mainly on t h e relative h u m i d i t y and t o a l e s s e r e x t e n t on t h e temperature. When t h e ambient c o n d i t i o n s change, time i s needed f o r m o i s t u r e t o move i n and o u t
of t h e wood to bring t h e moisture content t o the new equilibrium value. The time required f o r t h i s depends on t h e s i z e o f t h e timber, w h e t h e r t h e
In d r y i n g down t o TFrc f j h r r l w t r t r a t i o n p o i n t , t h e r e is nu c l l n n ~ c ir? t h e p r o p e r t i e s of t h e wood, a p a r t froin clccrcnsc i n brrlk density. l<cIe~rr t h e f i b r e s a t u r a t i o n point, s t r e n g t l l s a n d c l a s t i c m o d u l i i n c r e a s e a s
moisture content d e c r e a s e s ; vulnerability to r o t and t o i n s e c t attack a l s o decreases. A t t h e same tjrne, wood s l ~ r i n k s progressively as tlre m o i s t u r e content f a l l s below t h e f i b r e s a t u r a t i o n p o i n t and swells lwhen
t h e moisture c o n t e n t increases. The movement i s r e v e r s i b l e a n d c o n t i n u e s t h r o u g h o u t t h e life o f the wood. When wood changes in
d i m e n s i o n with v a r i a t i o n s i n moisture c o n t e n t , there is l i t t l e change i n t h e longitudinal d i r e c t i o n . Average v a l u e s f o r shrinkage from t h e f i b r e
saturation p o i n t t o t h e o v e n - d r y c o n d i t i o n a r e h e t n e e n 0 . 1 a n d 0 . 3 per c e n t along the g r a l n . Tllood, holrevcr, displays a n i s o t r o p i c properties
w i t h respect t o rnoisturc movement. bh.1~11 l a r g e r changes take p l a c e in the
t a n g e n t i a l d i r e c t i o n (bctwccn 3 . 7 t o 10 p e r c e n t ) while the r a d i a l
s h r i n k a g e t e n d s t o h e two-thirds as much (hetween 1 . 7 t o 6 . 7 per c e n t )
.
These i n h e r e n t c h a r a c t e r i s t i c s form t h e b a s i s o f i m p o r t a n t c o n s i d e r a t i o n si n t h e d e s i g n and use of wood ( 2 3 ) .
C o n s i d e r a g r e e n l o g w i t h a moisture c o n t e n t above t h e f i b r e
s a t u r a t i o n p o i n t which is :low a l l o l v e d to d r y . Circumferential shrinkage stresses w i l l develop in the log because t h e o u t e r layers will d r y a n d s h r i n k more r a p i d l y t h a n t h c corc, a n d a l s o because t h ~ tangential s h r i n k a g e i s g r e a t e r than t h e r a d i a l s h r i n k a g e . As d r y i n g o f t h e l o g progresses, t h e s e c i r c u m f e r e n t i a l s h r i n k a g e s t r e s s e s i n c r e a s e to t h e c r i t i c a l p o i n t where t h e y exceed t h e capabiP i t y of t h e wood t o
accommodate t h e m , apLd t h e i n e v i t n l ~ l c r a d i a l s p l i t r e s u l t s . A c l o s e i f i s p e c t i a n of l o g s t h a t have d r i e d i n v a r i a b l y r e v e a l s r a d i a l c r n c L 5
( F i g u r c 1 ) . I n d i v i d u a l p i e c e s formed by s p l i t t i n g o r sawing t h e l o g
lonqitudinally b e f o r e it is d r i e d can a d j u s t more r e a d i l y to the d r y i n g
shrinkages, thus a v o i d i n g some, if n o t a l l , of the c r a c k i n g that t a k e s p l a c c i n whole l o g s .
A preliminary c o n c l u s i o n , w h i c h can b e d x w n from t h e d i s c u s s i o n on rnoisturc movement t h u s far, is t h a t l o g s uscd i n c o n s t r u c t i o n may them- s e l v e s n o t h e a i r t i g h t . 'This i s especially t h e c a s e i n stovewood
c o n s t r u c t i o n ( 1 4 ) where l o g pieces, i n s t e a d of s t o n e s , are eml~edded in m o r t a r . The r a d i a l cracks i n t h e l o g pieces that a r e p l a c e d across the p l a n e o f this wall p r o v i d e a direct: p a t h f o r a i r l e a k a g e . Differences
i n t h c dimcnsionnl c h n n ~ c s o f t h c z\;ooil anil ~l!o~t;lr w i tll m ~ i s t u r c 2nd t c n q ~ c r n t u r c v n r i ; ~ t i o ~ ~ r ; i 11 sc 1.r; i c.c a n y tlccrc:~sc thc n i r t i !:Iitncss o f t l ~ c L-on5 t r ~ ~ r t i o i i s t i 1 I I-izrtlwr.
A c t il;t l ~ n n v r ~ i ~ ~ c ~ t ~ t _.; o tm wnotl i 11 scr-v i c c w i l I t l r [ w ~ l i l (111 t l ~ r r:lngp of-
111oi sttirc c o ~ i t c ~ ~ t w l ~ i ~ - l i i 11 t ~ 1 1 - 1 1 ~ l ~ l i ~ ! i ~ l s oil t- i ~ ~ i l s ~ - : I L . ~ ( > I - S i 11f1 L I C I I C i I I ~
w c t t i r ~ ~ r:nr~rI clryiiig, incltfitliiig rl~c* r c l n t i v c h ~ r ~ ~ i ~ l i t y , ;in11 ; I W h:lril t n p r ~ c I i ~ t . Rakcr (15) , IIOIVCVLII., 11si 112 1 ; 1 1 > 0 ~ - i t t i > r ~ :111d f i r l d ~ l i l t a
representative of C a n a d i a n c o n c l i t i o n s , Elas Fwcn d? l e t o r l r ; ~ w some S c n c r ; ] 1
guidelines a b o u t t h e ~ n o i s t u r c c o n t e n t to b c cxpcctcd i n p a r t i c u l a r environments. F i g u r e 3 g i v e s moisture c o n t e n t v a r i a t i o n s f o r wood
i n d o o r s and o u t d o o r s f o r b o t h w i n t e r and summer c o n d i t i o n s . The f i g u r e does n o t r e p r e s e n t any p a r t i c u l a r t e s t o r s i t u a t i o n and i s n o t s t r i c t l y a p p l i c a b l e t o l o g w a l l s s i n c e t h e effect-5 o f r c d i s t r i b u r i o n from o n e
s i d e o f a l o g to the o t h e r b y means o f c a p i l l a r i t y a r e n o t known. It
does, however, p r o v i d e a Eair assumption on which to base some approximate calculations concerning dimensional change i n l o g
constructions.
The moisture c o n t e n t o f woad i n d o o r s is shown t o change from 5 per
cent i n w i n t e r to 15 p e r ccnt in summer f o r a scasonal change of 1 0 per c e n t . During thc same p e r i o d , t h e moisture content o f wood o u t d o o r s changes from 1 8 p e r c e n t t o 1 0 p e r c e n t f o r a seasonal change o f 8 p e r c e n t . T h e s e values do n o t take into account wetting by c o n d e n s a t i o n or by r a i n f a l l i n g directly on a w a l l . Thc cross-grain shrinkage f o r woods
in c o n o n u s e is on t h e average about 5 per c e n t f o r a m o i s t u r e c o n t e n t change of a b o u t 30 p e r cent from the f i b r e s a t u r a t i o n p o i n t t o oven d r y ,
or a b o u t 0.17 per c e n t dimensional change for each percentage change i n moisture c o n t e n t , These seasonal m i s t u r e changes o f 8 to 10 p e r c e n t
can produce dimensional changes a c r o s s t h e g r a i n of about 1 . 5 p e r c c n t , c o r r e s p o n d i n g to 0 . 1 2 i n c h e s i n an 8 - i n c h l o g .
A uniform m o i s t u r e change o f 1 0 p e r c e n t t f ~ r o u g l ~ o u t a wall of
h o r i z o n t a l l o g s w i l l causc t h e wall t o change in h e i g h t by as much as 1.5 i n c h e s in 8 f e e t i f t h e r e a r e no restraining ~ Q T C ~ S . Under s e a s o n a l
m o i s t u r e changes w i t h the l o g s expanding on o n e s i d e while shrinking on
the other, t h e wall may n o t change g r e a t l y i n h e i g h t . The j o i n t s between
logs, however, w i l l open on t h e i n s i d e w h i l e c l o s i n g on the o u t s i d e from s u m e r t o w i n t e r , w i t h t h e r e v e r s e a c t i o n t a k i n g p l a c e from w i n t e r to summer. T h i s i n e v f t a b l e working of joints t h r o u g h o u t l o g c o n s t r u c t i o n can have a marked influence on airtightness, d e p e n d i n g on the d e t a i l s o f
construction and in particular o n t h e way i n which t h e j o i n t s between l o g s a r e caulked. Frequent maintenance may b e necessary when a h i g h dcgrec of a i r t i g h t n e s s i s r e q u i r e d . Shcrwood (161, i n h i s account o f e a r l y b u i l d i n g techniques i n t h e Canadian N o r t h w e s t ( a l m o s t a l l b u i l d i n g s were a f l o g s ) , describes t h e "mudding" o f l o g b u i l d i n g s which t o o k place every year. T h i s involved p u t t i n g c l a y s l u r r y a n t h e b u i l d i n g a f t e r
freezing weather had s e t i n , u s u a l l y in l a t e November. In Sherwoodts
words : "The mud was to f i l l a l l c r a c k s and soon f r o ze h a r d , it made t h e b u i l d i n g almost airtight..
. ."
CONSTRUCTION IMPLICATIONS
C l e a r l y , the d i m e n s i o n a l c h a n g e s t h a t take p l a c e r + i tll variations i TI
moisture content can bc considered t h e most s i g n i f i c a n t e f f e c t o f
moisture on wood. Since t h e moisture content o f ~ c o o d t e n d s t o f o l l o w a n y c h a n g e s i n t h e r e l a t i v e humidity to which it is exposed, l o g s taken from outdoor s t o r a g e t o be u s e d i n c o n s t r u c t i o n w i l l u s u a l l y a d j u s t to a lower
moisture c o n t e n t i n s e r v i c e . This w i l l r e s u l t i n an i n i t i a l shrinkage.
In l o g b u i l d i n g s , however, t h e marked d i f f e r e n c e between t h e small
moisture-induced movement a l o n g the g r a i n and t h o s e movements across d ~ e
The walls o f l o g h u i l r l i n j i s can s h r i n k up t o 3 inches p e r s t o r e y a s tlre wood d r i e s . T h i s may Ilc of l i t t l e conscquencc when the waI1 consists
o f l o g s er planks and tllc s t r u c t u r e is a sjmplc o n c . tlowevcr, when othcr members arc a t t a c h e d o r framed into t h e w a l l , many c o m p l i c a t i o n s can
arise. In the framing o f o p e n i n g s , f o r example, vertical p o s t s
( 2 in. x 4
i n . ,
nominal) a r e usually s p e c i f i e d in c o n s t r u c t i o n rnant~als (17)to h o l d t h e l o g cnds, and f o r t h e window o r d o o r jamb to be n a i l e d to it-
Above the e n d of t h e p o s t a "space t o a l l o w f o r s e t t l i n g " i s s p e c i f i e d . A m p l e room fs a l s o s p e c i f i e d above t h e hcad o f windows and d o o r s t o p e r m i t s e t t l e m e n t . Mackie (18) s u g g e s t s a minimum o f f i n c h per log.
I t s h o u l d b e n o t e d t h a t cross-grain movement on a 10-inch l o g c o u l d b e a s
much a s
3
inch, a 5 p e r c e n t change i n dimension c o r r e s p o n d i n g t o achange i n moisture content from the f i b e r s a t u r a t i o n p o i n t to 5 p e r cent,
while the change along t h e g r a i n is o n l y about 0.1 per c e n t . The r a n g e
of moisture c o n d i t i o n s normally e x p e c t e d i n c o n s t r u c t i o n i s from a
relatively h i g h moisture content of approximately 20 per cent to a low o f about 5 p e r c e n t , and t h e corresponding s h r i n k a g e is about: 23 per c e n t o r
2
inch in a 1 0 - i n c h l o g . T h u s t h e c o n s t r u c t i . o n d e t a i l s t h a t '"allow f o r settlement" impliclzly account for t h e dimensional change due t o moisturea n d a v o i d t h e problem o f the p o s t b u c k l i n g o r excessive s t r e s s e s a c t i n g on the windows o r d o o r s . Any o t h e r v e r t i c a l members b u i l t i n t o t h e wall would not change in t h e same way as the l u g s o r p l a n k s and must b e
p r o v i d e d with a d e q u a t e clearances t o a v o i d having t h e h o r i z o n t a l members p r i e d a p a r t when s h r i n k a g e takes p l a c e and t h e logs tend to c l o s e up
u n d e r t h e i r own w e i g h t . It i s significant to n o t e t h a t l o g s have rarely, i f e v e r , been placed vertically, Apart from t h e obvious s t r u c t u r a l and c o n s t r u c t i o n a l problems, t h i s t y p e of c o n s t r u c t i o n can on1 y show c r a c k s between t h e l o g s a s t h e wood s h r i n k s on d r y i n g .
Tl~c anisotropic property o f wood with r e s p e c t to m o i s t u s c movement can c a u s e o t h e r problems in log c o n s t r u c t i o n s . lt'hen o t h e r members, SUCTI as plywood o r e t h e r v e r t i c a l b o a r d i n g elements, are a t t a c h e d t o t h e wall
e i t h e r i n s i d e or o u t s i d e , thcy a r e unlikely to s h r i n k and expand e x a c t l y i n accordance w i t h t h e basic log structure (191
.
I tis
a l s o h i g h 1 y prsbab l e that interior wall s connected s t r u c t u r a l l y to t h e e x t e r i o r w a l l s may n o t be subject t o similar v a r i a t i o n s i n moisture c o n t e n t (and, hence, d i m e n s i o n a l changes) o r a t t h e same time (see F i g u r e 2 2 . It i s necessary, t h e r e f o r e , t o c o n s t r u c t these elements i n such a way a s to a v o i d t h e conflicts that may a r i s e a s a r e s u l t o f d i f f e r e n t dimensfonal changes.The development of wood-frame c o n s t r u c t i o n has a l s o been i n f l u e n c e d by the large cross-grain dimensional changes in wood. The f i r s t g r e a t
improvenierrt was i n t h e adoption of framing p r a c t i c e s s u c l ~ a s t h e p l a t f o n n f r a m e a n d b a l l o o n frame so t h a t t h e b a s i c framing d i m c n s i o n s were
determined l a r g e l y by wood used i n t h e s t n h l e l o n g - g r a i n direction. C r o s s - g r : ~ i n shrinkage in h o a r d s u s e d f o r s h e a t h i n g w;is the11 accolnmodatccl b y v a r i o u s developments s u c l ~ a s l a p j o i n t s , t o n g u e - n n d - g r o o v e j o i n t s ,
s p l i n e s and b a t t e n s . Airtightness was f u r t h e r improved b y t h e e x t e n s i v e use ef s h e a t h i n g papers and s t u c c o a n d p l a s t e r . Cianufactured products
c x t e n s i v e l y for s h e a t h i n g a r e sufficient 1 y s t a b l e dirnensi wnal ly so t h a t t h e y a r e compatible with wood i n the long-grain direction. T h e g r a d u a l e v o l u t i o n of wood-frame p r a c t i c e s and m a t e r i a l s h a s produced v e r s a t i l e and r e f i n e d systems of c o n s t r u c t i o n f o r small b u i l d i n g s which, i n most cases, u t i l i z e t h e basic material t o t h e b e s t advantage, both
technically and economically.
I N CONC LUS ION
Buildings made of logs wcra most s u i t a b l c in t h c e a r l y days o f settlement in f o r e s t e d a r e a s . Rempel (20) confirms t h a t the l o g house
"...
was t h e quickest, c h e a p e s t and e a s i e s t form of construction.'"lVood-f xame c o n s t r u c t i o n began ta replace log c o n s t r u c t i o n as sawn lumber
and other materials became available at favorable p r i c e s . Naf 1 s, which were e s s e n t i a l i n t h e u s e o f sawn lumber, were so valuable a t times in
the pioneer economy that, it is s a i d , settlers a b o u t to move on to a new f r o n t i e r location recovered them by burning t h c houses t h e y were v a c a t i n g .
Log construction continued t o be used f o r simple s t r u c t u r e s a s long as logs and labour were f r e e l y available. Extreme airtightness was
seldom required, even in h o u s e s , s i n c e f i r e p l a c e s burning wood fuel always r e q u i r e d an adequate supply of air. Log construction could n o t compete when improved c l e a n l i n e s s , appearance, form, comfort and economy
i n t h e use of wood were wanted. It lacked the adaptability o f frame construction which m u s t now b e e x p l o i t e d even f u r t h e r to meet today's
h i g h s t a n d a r d s f o r insulating v a l u e and airtightness t h a t a r e necessary
because o f t h e need to conserve e n e r g y .
REFERENCES
( I ) S t r i c k l a n d , S., 'Twenty-Seven Years i n Canada West,
"
Edmonton,Hurtig, 1970.
( 2 ) RrrnIop, W,, '"'Tiger DurlEop'sUpperC;lnzldn," Toronto, McClcllanJ
and Stewart Ltd. , 1967.
( 3 ) Traill, C . P . , ' T h e Backwoods of Canada : Sclcctions," T o r o n t o ,
blcCleIland and Stewart L t d . , 1966.
(4) IVeslager, C . A
.,
"The Log Cabin in America," New J e r s e y ,Rutgers U n i v e r s i t y P r e s s , 1969.
(5) A s s o c i a t e Committee on t h e National Building Code, "Residential Standards, Canada, " Ottawa, National Research Council of Canada,
NRC 15563.
(6) Stephenson, D .G
. ,
"Determining t h e O p t i m u m Therma 1 Resistance for Walls and Roofs,"
Ottawa, B u i l d i n g Research Note 10, DBR/NRC.
Tamura, G.T. a n d Wilson, A . G . , I F A i r Leakage and P r e s s u r e Measure- m e n t s an Two Occupied Houses," ASHRAE Transactions, V o l . 70,
pp
.
110-119, 1964. (DBR Research Paper No, 207, NRC 7758).
Fishenden, M . W . , "The Domestic Gratc," Technical Paper No. 13, F u e l Research Board, DSIR, Hh.ISO, 1925,
American S o c i e t y o f Heating, R e f r i g e r a t i n g and Air-Conditioning E n g i n e e r s , lfASHRRE lIandbook and Prodrrct Directory 1975
Equipment," New York.
I b i d
.
Fishenden, h! .Id., Op
.
C i t.
(8).
F o r e s t P r o d u c t s Laboratories D i v i s i o n , Mi n i s t r } . of Resources and Development, 'Tanadian I\'oods : T h e i r Properties and Uses,
Ottawa, King's Printer, 1951.
Hutcheon, N .B
.
and J e n k i n s ,J
. H . , "Some Basic C h a r a c t e r i s t i c s ofWood," Canadian Building Digest 85, DBRJNRC, Ottawa, Canada,
J a n u a r y 1967.
R i t c h i e , T . , "Log and Timber Structures o f t h e Ottawa Area,"
P r o c . Conference on Log Structures, October 1977. See Figure 5 for a photograph o f a stovewood wall.
Baker, M.C., "Designing Wood Roofs to P r e v e n t D e c a y , ' V a n a d t a n R u i l d i n g D i g e s t 112, DBR/NRC, Ottawa, Canada, April 1969.
S h e n ~ o o d , A . , "Building in the North," Bulletin o f t h e
Association f o r Preservation Technology, Vol. VI, No. 3 , 1974, p p . 1 - 2 5 (DBR Technical Paper No. 4 9 2 , NRCC 14550).
FSackie, R . A . , "Building with Logs," [Second E d i t i o n ) , P r i n c e George, B . C . , 1 9 7 2 .
I b i d .
H u t c h e o n , N.B. and J e n k i n s , $ . [ I . , 'Some Implications o f t h e P r o p e e i e s of Ii'ood, l T Canadian Building D i g e s t 56, DBR/NRC,
O t t a w a , Canada, February 1967.
Rempel, 3.
I.,
"'Building with \food," Urliversity o f T o r o n t o P r e s s , 1967.TABLE I
RESISTANCE
VALUESFOR VARIOUS
WALLCONSTRUCTIONS
Element o r Wall
Indoor plus o u t d o o r s u r f a c e films Air space, 314
in,
orover
Single-glazed window Double-glazed window 8-in. s o l i d brick wall 8-in. c o n c r e t e b l o c k wall
Typica 1 wood-f rame w a l l , uninsula t e d Wood-frame wall, R 11 i n s u l a t i o n ,
a d j u s t e d for framing
Requirement f o r walls in 1977 Residential
s t a n d a r d s 10,000 degree d a y s , heating not electrical
Walls o f s o l i d wood, corrected to 10% moisture, f a r average
Figure 1. Typical r a d i a l cracking
20
JAN
FEB MAR
APR
MAY
JUN
JUL
AUG
SEP OCT NOV
DEC
16
12
8
4
Figure 2 . Moisture content
