Moisture in Flat Roof Joists

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Moisture in Flat Roof Joists

Bakker, J.

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PREFACE

The study of problems and failures of roofing is a recent project of the Construction Section of the Division of Building Research. Field investigations are being made to determine the extent and the nature of roofing failures. While many roofing failures can be traced to poor workrnanship in application, others are due to the presence of moisture in the roofing system from inside buildings. This is usually because of im- proper or inadequate design of the system.

The work of other investigators in this field is always of great interest and can frequently be of con- siderable help in the Division's investigations. The following article by J. Bakker deals with the problem of moisture in the joist spaces of flat roofs and dis- cusses the insulation and ventilation of roof spaces.

This translation was prepared by

Mr. D.A.

Sinclair

of the Translations Section of the National Research Council, to whom we are most grateful.

Ottawa

N

.B. Hutcheon

NATIONAL RESEARCH COUNCIL OF CANADA Technical Trans l a t i o n 1036 T i t l e : Moisture i n f l a t roof j o i s t s ( ~ o c h t I n platbalklagen) Author: J . Bakker Reference2 Bouw, 41: 1290-1292, 1961

MOISTURE I N FLAT ROOF JOISTS

Mastic, z i n c and s i m i l a r r o o f c o v e r i n g m a t e r i a l s a r e b o t h

waterproof and vapourproof. Without adequate v e n t i l a t i o n t h i s f a c t s o o n e r o r l a t e r r e s u l t s I n r o t t i n g of t h e wooden r o o f s t r u c t u r e , whlch i s s t i l l f u r t h e r aggravated whenever t h e r o o f i n g board (which i s n o t s e a l e d on t h e u n d e r s i d e ! ) i s a f f e c t e d and l e a k s occur. T h i s t a k e s l o n g e r o v e r rooms which have v e r y low r e l a t i v e humidity t h a n o v e r k i t c h e n s , bathrooms, bedrooms and i n h a b i t e d rooms i n which

laundry

i s

o f t e n d r i e d , rooms t h a t a r e i n c o m p l e t e l y v e n t i l a t e d , and o v e r b a k e r i e s , h a i r d r e s s e r s 1 shops and similar e s t a b l i s h m e n t s ( s e e F i g . 1, Roof of a bakery which had t o be r e p l a c e d a f t e r a mere

5

y e a r s )

.

The problem cannot be s o l v e d , however, merely by good v e n t i l a - t i o n , I n t e r i o r condensation i n t h e s t r u c t u r e can r e s u l t from

i n a d e q u a t e i n s t a l l a t i o n . Moreover, s i n c e e x c e s s i v e v e n t i l a t i o n can r e s u l t i n much l o s s of h e a t from t h e rooms under t h e r o o f , which i s

d e t r i m e n t a l t o f u e l econony, i t i s obvious t h a t t h e problem i s n o t

a simple one.

Let u s assume t h a t v e n t i l a t i o n i s n e c e s s a r y . It can be brought about by wind o r by n a t u r a l d r a u g h t . The l a t t e r method i s t h e b e s t , b e i n g more uniform. The v e n t i l a t i o n should be k e p t t o a minimum b u t must s t i l l be s u f f i c i e n t s o t h a t t h e w a t e r vapour p a s s i n g

through t h e c e i l i n g i s completely removed. The r e q u i r e d i n t e n s i t y of v e n t i l a t i o n t h u s depends on t h e n a t u r e of t h e rooms below and on t h e p r e s e n c e o r absence of a vapour b a r r i e r between t h e c e i l i n g and t h e v e n t i l a t e d s p a c e . That i s t o s a y , i f t h e p r o d u c t i o n of m o i s t u r e exceeds t h e amount t h a t c a n be absorbed and removed by t h e v e n t i l a - t i o n a i r , t h e n what remains i n t h e s t r u c t u r e above t h e v e n t i l a t e d s p a c e i s condensed, w i t h a l l t h e consequences r e s u l t i n g t h e r e f r o m .

A t t h e same time, a l a y e r of i n s u l a t i o n must be a p p l i e d ,

depending on t h e i n t e n s i t y of t h e v e n t i l a t i o n . A s a l r e a d y s t a t e d , t h e s t r o n g e r t h e v e n t i l a t i o n , t h e g r e a t e r t h e c o o l i n g e f f e c t . It

a n u n v e n t i l a t c d r o o f cons4;ruction!

I n sLuru1:cr t h i s 111ay be p l e a s a n t b u t i n w i n t e r 1.1; i s u n b e a r a b l e . T h e o r e t i c a l l y , i t i s p o s s i b l e t o c o n t r o l t h e v e n t i l a t i o n f l o w by ],leans o f dampers. In p r a c t i c e , horrevcr, l i t t l e i s g a i n e d i n t h i s way e x c e p t i n s p e c i a l c a s e s , e . g . i n u t i l i t y s t r u c t u r c s , e t c . A

b e t t e r s o l u t i o n i s t o i n s u l a t e t h e u n d e r s i d e o f t h e r o o f i n such a way t h a t t h e s t r e n g t h o f t h e i n s u l a t i o n i s no l o n g e r a f a c t o r . The i n s u l a t i o n s h o u l d be v a p o u r - t i g h t , s i n c e :

1. The i n s u l a t i o n v a l u e d e c r e a s e s a s tlie r r l a t e r i a l beconies w e t t e r , and 2 . r r ~ o i s t u r e niust e n t e r t h e v e n t i l a t e d s p a c e between t h e j o i s t s t h r o u g h t h e c e i l i n g , i n which c a s e t h e i n t e n s i t y o f t h e v e n t i l a - t i o n f l o w c a n be l e s s and hence l e s s c o o l i n g w i l l o c c u r . F o l l o w i n g t h i s i n t r o d u c t i o n we s h a l l t r y t o a r r i v e a t c e r t a i n s o l u t i o n s t o p r a c t i c a l d i f f i c u l t i e s . The f o l l o w i n g u n d e s i r a b l e c i r c u m s t a n c e s c a n o c c u r : ( a ) no v e n t i l a t i o n o r i n a d e q u a t e v e n t i l a t i o n ; ( b ) no i n s u l a t i o n o r i n a d e q u a t e i n s u l a t i o n . G e n e r a l l y t h e s e c a s e s o c c u r i n cornbination. ( a )

---

No v e n t i l a t i o n o r i n a d e q u a t e v e n t i l a t i o n _--. V e n t i l a t i o n c a n t a k e p l a c e e i t h e r by wind o r by n a t u r a l d r a u g h t . W e n t h e wind e f f e c t o c c u r s o n l y a t c e r t a i n tirnes, n a t u r a l d r a u g h t

i s p r e f e r a b l e . T h i s a r i s e s i n t h e sariie manner as chimney d r a u g h t , i . e . a s a r e s u l t o f t h e d i f f e r e n c e i n weight of t h e w a r m e r a i r i n t h e s p a c e s t o be v e n t i l a t e d on t h e one hand and t h e c o l d o u t s i d e a i r on t h e o t h e r . A t c e r t a i n tines t h e wind may i n t e n s i f y t h i s d r a u g h t . J u s t a s i n t h e c a s e o f t h e chimney, t h e d i f f e r e n c e i n h e i g h t between i n t a k e and o u t l e t o p e n i n g s a l s o h e l p s t o d e t e r m i n e t h e i n t e n s i t y o f t h e ( t h e r m a l ) d r a u g h t . The intalce o p e n i n g s must t h e r e f o r e be p l a c e d a s low a s p o s s i b l e , p r e f e r a b l y i n t h e c a n t b o a r d o r j e t t y ( s e e FLg. 2 ( 5 ) , 2 ( l c ) and 2 ( a ) . Air. v e n t s a r e p r e f e r r e d a s o u t l e t s on t h e f l a t r o o f ( s e e F i g . 2 ( 1 ) and (11)). The i n t a k e

o p e n i n g s must bc p r o t e c t e d a g a i n s t b l o c k i n g w i t h d i r t and cobwebs; t h e y a r e desigr;ned s o t h a t t t ~ e y d r a i n o f f . Any g r i l l work a p p l i e d s h o u l d be a l a r g e enough 111esh t o f o r e s t a l l p l u g g i n g .

The roof v e n t i l a t o r s , whiuh may have d i a m e t e r s of 4 t o 10 cm, must be p r o t e c t e d from t h e r a i n by v e n t a . S e v e r a l venta of s m a l l d i a m e t e r a r e p r e f e r a b l e t o a s i n g l e one of l a r g e r d i a m e t e r . "Dead" c o r n e r s i n t h e v e n t i l a t i o n system should be avoided, A number of p o s s i b i l i t i e s a r e i n d i c a t e d i n F i g . 2 ( a ) t o 2 ( h ) .

F i g u r e 2 ( a ) shows t h e manner i n which v e n t i l a t i o n i s a p p l i e d t o a completed s t r u c t u r e . The vent i s p u t i n t h e c e n t r e of t h e roof 80

t h a t no "dead" spaces ( s e e A ) a r i s e and a vent i s i n s t a l l e d between each p a i r of j o i s t s . I f t h e roof has t o be r e p l a c e d , then t h e method a c c o r d i n g t o F i g . 2 ( b ) i s s i m p l e r . Fewer v e n t s a r e needed. I n t h e

j o i s t I n t e r s p a c e c o n t a i n i n g a vent, a s i d e v e n t i l a t i o n h o l e i s pro- vided t o c r e a t e a draught through t h e whole space. The spaces

without v e n t s have draught openings on both s i d e s , s o t h a t regard- l e s s of t h e wind d i r e c t i o n v e n t i l a t i o n i s p o s s i b l e throughout t h e e n t i r e space. The n o t c h e s on t h e top of t h e j o i s t a r e 23 cm deep and 4 crn wide. This c r e a t e s no d i f f i c u l t y from t h e s t r u c t u r a l p o i n t of view provided t h e y a r e made c l o s e t o t h e s u p p o r t s . This i s a l s o f a v o u r a b l e t o t h e draught, because t h e a i r heated from below r i s e s .

A f l a t roof t h a t t e r m i n a t e s a t a r i s i n g w a l l i s v e n t i l a t e d

according t o F i g . 2 ( c ) i f t h e c o n s t r u c t i o n i s a l r e a d y completed, and a c c o r d i n g t o 2 ( d ) where t h e work i s new. I f t h e j o i s t s a r e suspended between two o r more r i s i n g faqades, F i g . 2 ( e ) a p p l i e s . Here a

c e r t a i n d i f f e r e n c e i n h e i g h t between i n t a k e and o u t l e t openings i s

a t t a i n e d . The v e n t s a r e p l a c e d i n s t a g g e r e d a r r a y s o t h a t t h e wind always h a s a f a v o u r a b l e i n f l u e n c e on t h e draught i n h a l f t h e j o i s t s p a c e s . It i s a l s o p o s s i b l e , of course, t o c o n s t r u c t a l l openings a c c o r d i n g t o t h e d e t a i l of 2 ( m ) , e s p e c i a l l y when t h e draught does n o t have t o be t o o s t r o n g .

V e n t i l a t i o n through w a l l c a v i t i e s should be avoided because damp a i r can t h u s come between t h e j o i s t s . The w a l l c a v i t i e s should be s e a l e d o f f a t t h e t o p ( s e e F i g . 2 ( f ) ) .

F i g u r e 2 ( g ) shows a n o t h e r p o s s i b i l i t y f o r new c o n s t r u c t i o n , where a g a i n notches can be used. I f t h e c e i l i n g i s f r e e l y suspended under t h e J o i s t s on c e i l i n g hangers o r on r e i n f o r c e d c o n c r e t e bearns

thci-t. i s a fr>ec: c l ~ o i c e o f p l a c e m e n t of t h c v e n t s ; however v e n t s yhould always be p l a c e d I n t h e o u t e r l n o s t c o r n e r s ( s e c F i g . 2 ( h ) and 4 ) . The I n t a k e c a n be c o n s t r u c t e d I n one o f t h e ways shown i n t h e d e t a i l e d s k e t c h e s .

If o n l y a s ~ ~ l a l l amount o f v e n t i l a t i o n i s needed, i t Is t h e n sometimes enough t o p r o v l d e o p e n i n g s o n l y i n t h e c a n t b o a r d s . It i s t h e n recommended t h a t a n u p p e r and a l o w e r o p e n i n g be p r o v i d e d I n e a c h j o i s t s p a c e i n a l t e r n a t i n g arrangement w i t h a n eye t o t h e wind d i r e c t i o n ( s e e F i g . 2 ( k ) ) . If' t h e v e n t i l a t i o n i s a d e q u a t e . n i o i s t u r c s p o t s i n t h e c e i l i n g w i l l g e n e r a l l y d r y up o f t h e i r ovm a c c o r d . I f w a t e r r u n s a l o n g t h e w a l l , t h e r e i s a p o s s l b i l i t y t h a t t h e c o n d i t i o n o f t h e m a s t i c h a s d e t e r i o r a t e d and t h e r o t t i n g p r o c e s s w i l l p r o b a b l y be advanced. It i s t h e n n e c e s s a r y t o r a i s e t h e l e a d c o p i n g o f t h e z i n c f l a n g e and check t h e c o n d i t i o n o f t h e m a s t i c . The l e a d c o p i n g may a l s o be t o r n , which may l e a d t o l e a k s i n t h e r o o f , o r i t c a n l i e t o o f l a t on t h e m a s t i c r i s e , r e s u l t i n g i n c a p i l l a r y a c t i o n and a flow o f w a t e r o v e r t h e r i s e . If t h e p r o c e s s h a s a l r e a d y gone t h i s f a r , i t i s t h e n u s u a l l y n e c e s s a r y t o r e p l a c e a l l o r sorne o f t h e j o i s t s a s w e l l a s t h e f l o o r . A t t h e same t i m e t h i s a f f o r d s a p o s s i b i l i t y o f i n s t a l l i n g b e t t e r o r s i m p l e r v e n t i l a t i o n a n d , where n e c e s s a r y , i n s u l a t i o n . The r e p a i r s c a n be c a r r i e d o u t e i t h e r from below 01. fron: above, d e p e n d i n g on t h e

e x t e n t t o which t h e c e i l i n g , f l o o r and r o o f c o v e r i n g need r e p l a c i n g . Holes i n e x i s t i n g r o o f s t r u c t u r e s where t h e j o i s t s a r e c l o s e t o g e t h e r a r e made w i t h a s o - c a l l e d " a n g l e d r i l l " , w i t h which h o l e s c a n be d r i l l e d a t zn a n g l e o f 9 0 ° . If one o r more j o i s t h e a d s a r e damaged a r e p a i r I n t h e rnanner o f F i g .

5 ( a )

t o 5 ( c ) i s p o s s i b l e . The method o f F i g . 5 ( a ) can be c a r r i e d o u t e i t h e r frola below o r from above. Reinforcement o f t h e e n t i r e j o i s t by clamping o v e r I t s f u l l l e n g t h i s p o s s i b l e I n t h e sarlie way. The method according t o

F i g . 5 ( b ) i s p o s s i b l e o n l y w i t h a lowered c e i l i n g o r a v i s i b l e J o i s t , and 5 ( c ) c a n be c a r r i e d o u t from below o n l y .

I n t h e c a s e o f c o n t i n u o u s J o i s t s p a s s i n g o v e r s e v e r a l p o i n t s o f s u p p o r t i t may be p o s s i b l e t o form a s o - c a l l e d d r a u g h t j o i s t

zccoydirlg l;o PI?;. 5 ( ~ ) 1i.f 1;hc Si;.a:~t is ciai;lal.,cd o v e r one bay ( s e e

d i a g r ~ ~ ~ i l of Pri.:?;.

₅

_{( d ) )}

.

( b ) No irisulacliorl --- o r inaciequatc i n s u l a t i o n

I n n~osi; c a s e s no i n s u l a t i o n o r p ~ ~ a . c t l i c a l l y none i s a p p l i e d . Idhere p r e s e n t , the i n s u l a t i o n usua1l.y c o n s i s t s o f s l l e e t s a p p l i e d on t o p of t h e cleck u n d e r n e a t h tile coverfn:; o r b a t s of g l a s s o r m i n e r a l W O O ] on o r between t h c j o i s t s , o r i n s u l a t i n g rrlaterial s p r e a d

l o o s e oil t h e c e i l i n g .

If t h e i n s u l a t i o n i s a p p l i e d t o the deck t h i s i s f a v o u r a b l e I.'\ 1 1 . t h e v e n t i l a t i o n . The a i r tenlqoer7ature i s t h e r e b y k e p t h i g h e r and i . i ~ < : d r a u g h t i s iniproved. !:ioreover, t h e vvarmer a i r can a b s o r b and c a r r y away rr~ore moisture. I f t h e d r a u g h t Is n o t s t r o n g t h i s i n s u l a t i o n

w i l l be a d e q u a t e i n leilost c a s e s . I n t h e p r e s e n c e of s t r o n g d r a u g h t s , e s p e c i a l l y i n w i n t e r , holvevcr, t h e c e i l i r l g rilay be c o o l e d and s p o t s of rrloisturc, and i n t h e s e v e r e s t c a s e s ( k i t c h e n s , bathroorns, e t c . , e t c . ) even d r o p s inay form. I f t h e r e i s no v e n t i l a t i o n , however, t h e i n s u l a t i o n may t h e n promote a b s o r p t i o n o f vapour by t h e f l a t r o o f c o n s t r u c t i o n , which w i l l t h e n h a s t e n i t s d e t e r i o r a t i o n . I f t h e r e i s

no vapour b a r r i e r between t h e i n s u l a t i o n and t h e deck t h e i n s u l a t i n g l a y e r w i l l a l s o becorne wet and w i l l no l o n g e r f u l f i l l i t s f u n c t i o n . Glass wool and o t h e r * b a t s , as wc11. as l o o s e l y s p r e a d n . : a t e r i a l s u c h a s p e e t moss, s a w d u s t , c l i n k e r i n s u l a t i n g , e . ~ ; . between t h e j o i s t s t a r e s a i d t o g i v e b e t t e r s a t i s f a c t i o n b o t h w i t h and w i t h o u t v e n t i l a - t i o n . I.Iov,rever, i f t h e r-oonls belov.1 produce a g r e a t d e a l o f vapour t h e i n s u l a t i o n beco~ries m o i s t and no l o n g e r f u l f i l l s i t s f u n c t i o n .

'then no v e n t i l a t i o n i s used t h e i n s u l a - t i o n i s u s e l e s s i n t h i s c a s e a l s o .

A good i n s u l a t i o n can be a p p l i e d i n a c c o r d a n c e w i t h F i g . 5 ( f ) and 5 ( g ) when t h e c e i l i n g h a s t o be r e p l a c e d . An ili~provement can be

o b t a i n e d a l s o by t h e a p p l i c a t i o n of a vapour b a r r i e r , e . g . aluminiwrl f o i l between t h e c e i l i n g and t l - l c i n s u l . a t i o n ( s e e F i g . 5 ( h ) ) . lJct i n s u l ~ t i n g m a t e r i a l ~riusi; of c o u r s c be y e p l a c e d o r d r i e d ( m o i s t u r e t h a t d i f f u s e s t h ~ . o u g h jo i s t s i s Laken away by t h e v e n t i l a t i o n ) .

It;

i s t h e n a i ~ r l p l e r to irlcruase t h e v e n t i l a t i o n a t t h e sarlie time, s o t h a t t h e i n s u l a t i n g 111aterial d r i e s , and t h e n t o apply a v a p o u r - t i g h t l a y e r of i n s u l a t i o n . T h i s can be i n t h e form of s l a b s of foam

r r ~ a t e r i a l w i t h t i g h t c e l l s , o r of board, e t c

.

, with a n alurniniurri f o i l underneath. I n t h c l a t t e r c a s e , however, a p l a s t e r c e i l i n g 1 3

a p p l i e d ( s e e F i g .

5(

j ) and ~ ( I c ) ) . The t h i n l a y e r of p l a s t e r i n F i g . 5 ( k ) cannot a b s o r b much ~rloisturc?; good ventilatliori of a roon~

t h a t produces much nloisture ( n o t t o b c c o i ~ f u s e d w i t h t h e v e r ~ t i l a t i o r ~ between t h e j o i s t s ) through a s k y l i g h t o r g r i l l i s t h u s t o be rcconlri~ended. I f t h e i n s u l a t i o n i s a l r e a d y a p p l i e d between t h e c e i l - i n g and a i r s p a c e s and t h e c o o l i n g e f f e c t i s s t i l l t o o g r e a t , a ( s l i g h t ) r e d u c t i o n of a s t r o n g v e n t i l a t i o n can b r i n g r e s u l t s i n c e r t a i n c a s e s . It i s c l e a r t h a t t h e method t o be a p p l i e d w i l l be dcterrnined by whether roof decking o r c e i l i n g s have t o be r e p l a c e d .

Other methods of i n s u l a t i o n a r e a l s o p o s s i b l e . It i s n o t p o s s i b l e t o d i s c u s s a l l of t h e s e h e r e . However, i f what h a s been

s a i d h e r e i s borne i n mind i n t h e c o n s i d e r a t i o n of t h e s e c a s e s , then i t w i l l g e n e r a l l y be p o s s i b l e f o r t h e i n d i v i d u a l t o deterrnine f o r himself t h e measures t o be t a k e n . I n s p e c i a l c a s e s i t i s a d v i s a b l e t o c a l l i n a n i n s u l a t i o n e x p e r t , f o r example t h e T . N . O .

F i g . 1

R e p a i r i n g a mastic roof c o v e r i n g which was a bare f i v e y e a r s o l d . Much of t h e j o i s t i n g and t h e

roof deck of t h i s bakery had t o be r e p l a c e d .

j$==R=F

a in e x c s f i n g Structure e Ln e x t s t t n q or n e w s t r u c t L L ~ S g Ln a new structure

Z7

-F

b In a new s t r u ~ t u r e . , . fin a new s t r u c t u r e . , i n an e g i s t i n s s t r u c - t u r e ~ 4 t h c e i l i n g han9er.s F i g . 2 V e n t i l a t i o n of f l a t roof j o i s t s ( s e e t e x t f o r d e s c r i p t i o n )

Ventilation grates in walls around the flat roof of Fig. 1. The structure was obviously

not correctly built

Ventilation vents on a flat roof wlth a freely suspended ceiling

a d o u b l e c h m p s

b wooden k e y

m I e c . = =

c an anqle cron as _{a key}

L:.

Lii

A

I I I I I I IE _-mu

d

plan- e r e p a i r o f c a f i t i n u o u s - ~~. beam f v c n t r l a t c o n between masttc roverrnf ---

I nsdaimg board, cork, Uerakltth or U e /,he- alumrnrum l b r l Laths -- the 1 7 k ' e r , plaster c e ~ l l n ~ 1' g n e w c e t l ~ n q c o n s i r u c t r o r r

L:14 k.3

c o r k slab o r t h c a l w n ~ n r u m foil- exrst;nq rna-rk~c c o v ~ r ~ n q r n s i l n g CLat r o o f deck exrsttng Laths- - eristtng c e l l i n s - - t n ~ u l a t i n g board or- alvm@nourn l 0 . 1 ~ - eh,

1

l a t h s - 1 ' new p l s s t c r c t . l r ~ ~ , F i g .

5

Repairing o f d e t e r i o r a t e d f l a t roof j o i s t heads ( a t o d i n c l . ) and methods o f a p p l y i n g I n s u l a t i o n t o e x i s t i n g f l a t roof J o i s t c o n s t r u c t i o n s ( f o r d e s c r i p t i o n , s e e t e x t )