Slope making of flat roof

"Combination of prevention and drainage" is one of the basic principles of roof engineering design. The flat roof is a waterproof fortification method with prevention as the main and drainage as the auxiliary. Therefore, a watertight overall waterproof layer must be formed on the roof to prevent rainwater from entering the room from the roof. At the same time, drainage is also essential, which is an important means to ensure and continue the function of the roof.

1. why do flat roofs need slope making

Slope making is a means of roof drainage. Rainwater flows from the roof to eaves gutters and gutters under the action of gravity, and enters the ground drainage system through downspouts. Rainwater can be quickly drained away, which can reduce the burden of roof waterproof layer and the possibility of roof leakage. Unreasonable roof slope making will lead to poor drainage and ponding in case of rainstorm. If the ponding height exceeds the end of the waterproof layer, even if the waterproof layer does not leak, rainwater will bypass the waterproof layer from the upper end of the end and seep under the waterproof layer, resulting in water leakage.

If the drainage slope is not enough, local ponding will form in low-lying areas. The lap joint of the coiled material waterproof layer is also an area that is easy to cause local ponding. Ponding creates favorable conditions for mold propagation, and the waterproof layer is easy to be corroded by mold. In addition, water accumulated in rainy days and dry gradually in sunny days. This local alternation of dry and wet will cause cracks on the surface of the water accumulated part and accelerate the aging of the waterproof layer.

2. slope making form of flat roof and its advantages and disadvantages

Flat roof slope making includes material slope making and structure slope making.

(1) Material slope making, also known as building slope making, is to use light materials to gradually increase the thickness from the gutter to the ridge on the horizontal roof structural plate to make the drainage slope. At present, commonly used slope making materials include slag, expanded perlite, vermiculite, ceramsite concrete, foam concrete, cinder concrete, coke slag concrete, etc., which are selected and used according to local materials.

(2.1.1) advantages of material slope making

The structural ceiling seen in the room on the top floor is horizontal, and the visual effect meets the normal needs of people. It can be used without ceiling treatment. The structural plate is a horizontal panel with basically uniform elevation, and the architectural and structural design is relatively simple.

(2.1.2) disadvantages of material slope making

The slope making materials are all lightweight porous materials with high water absorption. Cement is used as the cementation material, and water is added for mixing during construction. The excess water after cement hydration will accumulate in the slope making layer. In rainy days after the completion of the slope making layer, the water in the pores will increase sharply, and the process of water evaporation is very slow, resulting in the water being covered by other upper layers and nested in the slope making layer. In the process of use, the water will gradually vaporize, which is easy to make the waterproof layer bubble. These shortcomings are very harmful, and even affect the use function of the roof.

When using materials for slope making, in order to reduce the roof load and facilitate use, materials with light weight and low water absorption shall be selected. When the water content in the slope making layer is very high due to rain or other reasons during construction, steam exhaust structure shall be considered. The formed slope making layer shall have a certain bearing capacity and will not produce excessive deformation under the action of construction and service load.

(2) Structural slope making

Structural slope making is to make the surface of the roof structural layer into a certain slope, and form the slope required for drainage after leveling. When the depth is small, it can be realized by adjusting the thickness of the roof structural plate. When the depth is large, it should be realized by adjusting the section height of the beam along the slope making direction. The thickness of the structural plate is the same.

Structural slope making has the advantages of light roof load, simple construction, easy slope control, labor and material saving, low cost, etc. because there is no slope making material, the problem of large water absorption of slope making material and swelling and cracking of waterproof layer caused by water vaporization is avoided.

From the structural point of view, the advantages of structural slope making are obvious. Therefore, article 4.3.1 of the technical code for roof engineering GB 50345-2012 clearly states that "structural slope making should be adopted for concrete structures, and the slope should not be less than 3%". The description of the article also specifically states that "as long as the requirements for the levelness of the ceiling are not high or the building function allows, structural slope making should be selected first for general industrial plants and public buildings". According to Article 5.2.2 of Zhejiang local standard technical specification for building waterproofing engineering DB 33/t 1147-2018, "structural slope making shall be adopted for grade I waterproof roofs, and structural slope making shall be adopted for grade II waterproof roofs". This standard is a recommended standard, but once this standard is quoted in the design, the roof design shall be carried out according to this clause.

In addition to the levelness of the ceiling, the greatest difficulty in structural slope finding is that it increases the difficulty of design and construction. Because of the need of structural slope making, the elevations of different parts of the roof are different. If the plane shape of the roof is relatively regular, the structural slope making is relatively easy to achieve. If the shape of the roof is complex, the elevation calculation of each point of the roof is relatively complex; Moreover, in order to unify the height of the ridge and cornice, the drainage slopes of different slopes will be inconsistent, making the design and construction more difficult. This is the fundamental reason why the structural slope finding cannot be implemented. If the designer can carry out the structural slope making design of flat roof like the slope roof design, the structural slope making can be better realized.