FLAT CONCRETE ROOF DESIGN
IN RELATION TO THERMAL EFFECTS
Reprinted from the Building Research Station Digest by Permission
THIS article discusses the main simple symmetrical roof, move- white surface paint treatments, nor cement
points which should be taken ments will be outward from the washes, should be used because thy are likely to into account when designing flat centre. but i the building abuts damage asphalt or bitumen felt. concrete roofs, so as to reduce ther- mal expansion and contraction and keep a building cool in summer, and conserve heat and avoid condensa- tion in winter. Problems of struc- tural strength and of moisture exclu- sion are not discussed.
Thermal Expansion and Contraction
A concrete slab, whether solid or buut up from pre-cast units, expands as a whole when it and contracts warms and cools. The amount or the movement within the range of tem-
in Britain peratures found
from winter to summer is sufficient, if the slab is not properly protected, to cause very troublesome cracking in a building of any appreciable size. For example, a roof 50 it. long can under-
go a movement of about 1 3 inch.
against another so that the roof can cny move in one direction, cracking
Mortar screeds can be fairly effec- will be localised at the unrestrained tive, but in practice some of them end, and will accordingly be more dirty rather readily and lose their severe there. Other simple geometri- efficiency. cal effects can be deduced from the diagrams of Fig. 3. Lift shafts and stair wells, and other projections above a roof, will tend to fix certain points, and so will areas which are shaded from sun for long periods in the day. Expansion then takes place outward from these fixed points.
Such treatments as these will re- duce heat gain from the sun, but weather through radiation or convec- cannot prevent heat loss in cold
tion. For this purpose some form of insulation is obviously required and the Table above includes three, in. fibreboard, 1 in. cork and 2 in. cork. Expansion occurs when the slab is The figures show that treatments of warmed by the summer sun, and con- this kind are effective in protecting traction results from the loss of heat the slab from solar heat, while in to cold winter skies, especially at addition they will prevent heat losses night. Protection against both effects from the inside of the building. In- is desirable, though the former is sulation has also other advantages. In
by reducing heat commonly the more important in this winter,
losses country. Some form of protection through the roof, it will keep the slab can serve both purposes reasonably itsel warm, and this will mean less The Station has examined a condensation on its underside. Also, well. variety of protective treatments, and like all heat insulation, it pays for Table I gives the most significant. itself eventually in fuel savings. The temperatures shown in the Table were measured on the upper surface of a 4-inch concrete slab.
In most cases the types of cracking and other defects caused by thermal movements can be ready recognised. They show themselves most obviously indoors because plaster finishes are well suited to show them up. They are almost entirely restricted to the top storey of bulidings, and, where the cause of cracking is in doubt, this fact will help to identify it. Another Maximum Temperatures Bencath Various Flat means of identification is the charac-
downward. Roof Coverings on Average Sunny Day teristic opening and closing of the cracks on hot days.
in England
Table 1
One important precaution in use must be noted. When insulation is put immediately beneath the water- proof layer, the latter will tend to get very hot because heat which it ab- sorbs from the sun cannot escape
Under these conditions bitumen or asphalt may be damaged, or so softened as to be easily injured For this
The cracking takes slightly diffe- (max. shade temperature approximately 800 F.) by people walking on it.
rent forms in framed and unframed buildings. In unframed buildings the whole roof slab will tend to bow up- ward slightly owing to the higher temperature of the upper surface.
Roof Covering
3 layers of felt-gritted
3 4 in. asphalt-1 in. mortar
screed
and at the same time the slab will 3.4 in. asphalt sanded finish spread outward. The two movements together tend to cause local cracking at the tops of walls and partitions, rather like rips when the walls are in line with the expansion movement, but otherwise with a simple type of horizontal break.
In framed buildings the effects are gencrally more disfiguring. The root arches as before, but in panels. Beams, for instance, tend to bow up- wards and take part of the wall with them, as in Fig. 1. Walls and parti- tions, which very often are panels between columns, are distorted as a whole, those in line with the roof movement showing a somewhat dia-
fracture running gonal
downward 1rom the top corner where the pres- sure is applied, as in Fig. 2. and those across the line of movement a more simple horizontal break as in un- framed buildings, but often farther down the wall, because the wall is bent by being connected to bending
columns.
The distribution of the cracking will depend a great deal on how the roof as a whole is restrained. In a
1
3 layers of felt-gritted-1 2
in. mortar screed
Max.
<F
reason some measure to keep the sur- face reasonably cool by reflection treatments, e.g. limewash or white Temp marble chippings, will be necessary. The precautions to be taken in ap- plying the waterproof covering over various insulating materials are dis- cussed in British Standard Codes of Practice, C.P. 143 and 144.
110
110
95
93
2 in. fibreboard-3 4 in. as-
phalt-sanded finish
87
3 layers of felt-sanded with
white grit
3 layers of felt-gritted-1
in. mortar screed
asphalt-white
3 4 in.
chippings
spar
3 4 in. asphalt-2 in. layer
gravel
1 in. cork-3 4 in. asphalt-
sanded finish
3 4 in.
asphalt-limewash
2 in. cork-3 4 in, asphalt-
sanded finish
гг.
Screeds of porous concrete (for in- stance, with foamed slag aggregates) and layers of hollow clay blocks have been used under the waterproofing layer to protect slabs from expansion. 85 The Station has been informed of in- stances where the clay blocks have 85 themselves cxpanded and given trou- ble by displacing parapets, but no 80 similar trouble with the porous con- crete screeds has been reported. 79 Trouble may be occurring, of course, without the Cause being observed. 78 Until more information is available, 75 it will be best to incorporate expan-
sich joints in the screed.
Thermal Insulation Treatments
73
A number of the treatments studied should be specially noted. Outstand- ing is limewash, which is obviously well suited for application to existing buildings which were not properly protected when they were originally built. The limewash should be pre- pared according to the method given at the end of this note. No other
47
The protection of the roof slab by insulation treatments has so many practical merits that it requires dis- cussion in more detail. Something must be said about the amount of in- sulation which is worth while, and about the desirability of putting some of it beneath the slab in some circum- stances.