IT ASPHALT WITH
· UNDERLAY
-1' SCREED
ROOF SLAB -PLASTER
CORNICE LINE
FIG. 1.
DIRECTION OF ROOF MOVEMENT
BENDING
OF COLUMN
FIG. 2
FIG. 3.
X
X
Fig. 1- Effect of arching of steel in beam Fig. 2- Diagonal cracking of the walls as
the result of roof movement. Fig. 3 Expansion effects in different
building plan arrangements.
The amount of insulation afforded by an element of structure is assessed by the numerical value of the ther- mal transmittance (U, B.Th.U. ft.2 hr. F.). The smaller the value of U. the better the insulation and the smaller the heat loss. The U value recommended for roofs of houses and flats in Post-War Building Study No. 19 (Heating and Ventilation of Dwell- ings. published by H.M. Stationery Office) is 0.20-0.30, and for schools 0.30 (Post-War Building Study No. 27, Heating and Ventilation of Schools. published by H.M. Stationery Office). For other types of buildings it is generally necessary to estimate the economic amount of insulation at the design stage, taking into account any additional cost involved, the saving of fuel and the costs of heating plant.
The U value of different constructions ment can still be embarrassing, and can be computed readily with the aid provision for expansion joints should of the handbook "The Computation be made in order to take up the
movement without of Heat Requirements for Buildings,'
damage to the published by the Institute of Heating structure. Intervals of about 100 ft. and Ventilating Engineers (price 2s. between joints seem to be found net). The insulation provided by some typical combinations of mate- rials for roof structures is shown in Table II, and will serve as a rough guide.
Table II
Asphalt on 6 in. concrete, pla-
stered
U
0.52
Asphalt within. fibreboard
0.30
with 1 in. wood wool
0.28
**
17
with 2 in. foamed concrete screed
slag
0.32
Asphalt on 6 in. hollow concrete
units
are
+++
TER
+++
0.47
From what has been said earlier it is obvious that there are good rea- sons for putting thermal insulation generally above the slab, but there occasions when part, at least, can usefully go below it. For instance, in buildings which are heated inter- mittently it will help for quick warm, ing up and also reduce the risk of temporary condensation. Sometimes also there are constructional advan-
But no
tages to be gained from using insulat- ing materials such as wood wool for permanent shuttering, and occasional- ly sound-absorbing materials with some value for heat insulation will be wanted beneath the slab. matter how much insulation is put beneath it, some should generally go above. It helps to avoid condensa- tion by keeping the slab warm, and it is a good way of reducing thermal movements.
Expansion Joints
When all reasonable precautions have been taken, roof movements. and the differences between the roof movements and the thermal move ments of the walls and other con- nected parts, will be minimised but not entirely avoided. Some move- ment must always occur due to changes in air temperature. In very long buildngs the amount of move-
suitable.
Expansion joints are sometimes be inserted also in the external walls They should put in the roof only.
and should extend inward at least part way through the building to enable the stresses of expansion to be distributed without causing ob- vious cracking. The best course is probably to cut right through the building down to foundation level.
Buildings in the Tropics
The treatments described wili prevent the temperature of a con- crete roof slab from rising much above maximum shade air-tempera- ture during the day. In temperate climates this is usually sufficient.
In the drier tropics, where there is а marked day-night temperature used during the day, to select a form range, it is desirable, for buildings of construction which will warm up slowly: the underside of the roof slab will then be warmest when the gen- The eral air temperature is falling. extent of this time lag depends on the mass of the roof-the heavier the roof, the longer being the lag. With a 4-in. solid concrete slab, for in- stance, the lag is about 24 hours: with an 8-in. hollow block slab, about
5 hours; and with an 8-in. solid slab, about 6 hours. It should also be
noted that the heavier the slab the smaller is the variation of tempera-
ture on its underside.
In the more humid troples, where the day-night temperature range is slight, the mass of a roof is less im- portant, provided that the tempera- ture of the underside of the slab is prevented from rising above shade temperature. To ensure this, a white surface treatment, or some form of sun-shade, is essential.
For rooms which are used only at night, the roof should be capable of cooling down as rapidly as possible. A light structure is then appropriate
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HONGKONG
BUILDING CENTRE
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