Detailed Design of Flat Roofs for Canada

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Technical Note (National Research Council of Canada. Division of Building Research), 1959-03-01

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Detailed Design of Flat Roofs for Canada Legget, R. F.

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It is common practice in modern Canadian residential

block construction to utilize flat roof designs. Canadian

housing practice does not include many large apartment blocks such as are found in Europe, but in larger Canadian cities the number of two- or three-storeyed apartment blocks is steadily

increasing. It is for such buildings that flat roof designs

are almost universal.

NATIONAL RESEARCH COUNCIL OF CANADA

No.

277

FOR INTERNAL USE

APPROVED BY R.F.L.

M.I!. March 1959

NOTE

CHECKED BY

DIVISION OF BUILDING RESEARCH

'fIE

CJHI

N ][ CAlL

NOT FOR PUBLICATION

PREPARED BY R. F •Legget

PREPARED FOR Circulation regarding special

technical inquiry

SUBJECT Detailed Design of Flat Roofs for Canada

In Ottawa two apartment buildings have recently been ｣ｯｮｾｴｲｵ｣ｴ･､ with roofs as indicated on Section No.1 on the

attached drawing. Five more buildings were erected by the same

owner and by the same contractor ｾｩｴｨ roof details as shown on

Section No.2. Section No.1 is a type of construction that has

been recommended by both Central Mortgage and Housing Corporation and the Division of Building Research, but ·it has been found that most contractors are opposed to it.

During the past two winters the two buildings constructed with their roofs conforming to Detail No.1 have

caused continuous trouble. The top floor ceiling gets wet at

every thaw; water accumulates in the lighting fixtures; and

in some instances water has dripped down over the heads of doors

and has been collected in pails. The other type of construction

as shown in Detail 2 appears to have performed satisfactorily. The buildings which have caused trouble have been

inspected by representatives of C.M.H.C. and D.B.R. The roofs

appeared to have been constructed exactly as designed. Adequate

vents had been installed, walls in the roof space had been built ｾＮ

so that proper venting was possible; all vertical shafts were

sealed from the roof air spaces. Nothing, therefore, ｡ｰｰ･｡ｲｾ､

to be wrong with the construction but despite this a serious condensation problem had occurred.

2

-It is thought that condensation on the underside of the roof slab may have been caused by outside air entering the space and the water vapour condensing on to the underside

of the slab. It is possible that the uninsulated concrete

slab with cold air on each face gets so cold during the night that in the morning frost would form on the underside of the slab if warmer and more humid air moves in before the slab has had the chance to warm up.

If this process were repeated for several days in a row, during very cold nights with warm days, heavy frosting

could accumulate. If this did occur then large quantities of

water would be released when the temperature rose to above

freezing and would fallon the insulation and damage the ceiling in just such a way as has been observed.

It therefore appears that the detailed design shown in Section 1 may have to be amended if this explanation is

correct. This note has been prepared and is. circulated for

comment in order to see if other building research authorities in cold climates have encountered similar problems with

, . - - - 20 YR BONDED ROOFING

r - - - -

Y2"

2

Y;'.

VAPOUR BARRIER

12" BAR JOI STS

ｾＭｾＬＮＭＮＮＬＬｌＭＭＭＭＭＭＫＭＭ f" TENTEST ON I"X 2" FURRING

PLASTER

ｾＭＭＭＭＭＭＭ BUILT UP 20YR BONDED ROOFING

, - - - 2ｹｾＧ CONC. SLABS

12" BAR JOISTS

ＧｐｲｾｾＤＡｾｾｾｾＲｾｾｾ｟ all X 16" VENTS AT 10'-0" O.C.

ｾｮＭｴＧ＼ＢＢＱＢＧｬＢＢｦｴＮＮＮＬＮＮ r---+- 2" FIBREGLASS BATTS

ｲｮｾｾｾｾｾｾｾｾＭ VISQUEEN VAPOUR BARRIER

IJI.,.---

ｾＭＭＭＭＭＭＭＭＭ I" TENTEST BOARD

ＭｌｌＮＮｌＮｾ ..L\L=-..L..Llt:ItlIi:l::,...--- I" X 2II FURR ING

' - - - 12" MASONRY

ACCORDING TO BUILDER ALL SHAFT DUCTS ETC. ARE CARRIED CONTINUOUSLY PAST ROOF SLAB. NO OPENINGS INTO CEILING SPACE. CONCRETE BLOCKS ON INTERIOR WALLS HAVE BEEN

SO PLACED ｂｅｔｗｅｾｎ JOISTS TO PERMIT CONT INUOUS