The eaves gutter and gutter are important parts of the roof, which collect rainwater from the roof and concentrate it at the water outlet. They are the main components of the roof drainage system, and the rainwater is discharged to the ground through the downspout. Insufficient depth of the eaves gutter and gutter is the main cause of poor drainage and roof water accumulation during heavy rainfall. Sufficient attention should be paid to their design.

1. The hazards of insufficient depth of eaves gutter and sky gutter

One of the problems of insufficient depth of eaves gutter gutter is that the discharge section is reduced, and the drainage volume is small at the same flow rate, which cannot meet the drainage requirements of rainstorm; Secondly, the insufficient ditch depth results in the weak water storage capacity of eaves gutter gutter, which cannot play the role of water storage buffer in rainstorm. The above two problems can cause rainwater to spread to the roof during heavy rainfall, resulting in water accumulation on the roof area. If there are certain defects in the roof waterproofing engineering, such as defects in the sealing of the joint of the membrane waterproofing layer, low height of the waterproofing layer end, and short exhaust pipe, these defects will become the cause of roof leakage.

2. The main reason for insufficient depth of eaves gutter and sky gutter

(1) Technical factors

In the roof design of construction projects, for the convenience of design and construction, many projects design the structural plates of eaves gutters and gutters at the same elevation position as the roof structural plates. This makes the roof drainage design inherently inadequate, and eaves gutters and gutters can only be formed by the height difference between the structural layers such as the sloping layer and insulation layer on the roof and the structural plate surface, known as building eaves gutters. Due to the small height difference of the building eaves gutter and the need for a certain longitudinal slope to allow rainwater to flow towards the water outlet, the water distribution line of the eaves gutter is almost level with the roof, causing rainwater to spread towards the roof when the rainfall is slightly higher. This phenomenon is more obvious in rainy areas in the south, because the thin insulation layer of the roof in the south can meet the requirements of building energy conservation design. For example, in Zhejiang Province, the roof insulation layer is mostly made of polystyrene foam boards with a thickness of 40mm~60mm, plus the slope height of the slope making layer at the gutter end of the gutter is 20mm, the thickness of the leveling layer on the insulation layer is 20mm, the waterproof layer and its upper protective layer are about 40mm, minus the thickness of the leveling layer of the gutter and gutter, the total thickness is about 100mm~120mm, which is the depth of the lowest part of the gutter and gutter, that is, the water outlet, as shown in Figure 1. The eaves gutter and sky gutter need to be sloped with a slope of 1%. When the distance between the water distribution line and the water outlet is 10m, the water distribution line is already level with the roof. This design of eaves gutter and sky gutter obviously cannot meet the drainage requirements of the roof, as shown in Figure 2.

Figure 1 Water outlet of building eaves gutter

Figure 2: Poor drainage of building eaves and gutters leading to water accumulation

(2) Design management factors

In project design, the depth of building eaves and gutters involves three professions: architects, structural engineers, and plumbing engineers. The drainage design calculation of the roof is completed by the water supply and drainage designer. The section width of the eaves gutter, the drainage slope at the bottom of the gutter, the water outlet, and the position of the water pipe are determined by the architect. The structure of the eaves gutter is designed by the structural engineer. Making building eaves gutters, architects find them aesthetically pleasing and structural engineers design them simply, so a large number of projects adopt this form, sacrificing the main function of eaves gutters and gutters. Therefore, the design supervisor of the project should coordinate the relationship between the three professions and effectively meet the drainage function requirements.

3. Determination of the depth of eaves gutter and sky gutter

(1) Drainage system design calculation

The determination of the depth of eaves gutters and gutters should be based on meeting the basic requirements of roof drainage. Firstly, the design flow rate of roof rainwater should be calculated and determined according to the rainfall intensity and roof catchment area. Based on the degree of harm caused by overflow when rainwater is not drained in time, the design flow rate of rainwater should be adjusted as the basis for the design of the drainage system. After the architect determines the preliminary design parameters of the eaves gutter, such as the width and depth of the eaves gutter, the position of the water outlet and water distribution line, etc., the drainage capacity of the gutter is calculated based on these parameters, and appropriate adjustments are made according to the cross-sectional shape, depth, and length of the gutter. Compare the adjusted drainage capacity with the designed rainwater flow rate to determine if it can meet the drainage requirements.

(2) Basic requirements for drainage depth

The drainage depth should not only meet the requirements of rainwater design flow and drainage, but also comply with the relevant provisions of the "Technical Code for Roof Engineering" GB 50345. Article 4.2.11 of the code stipulates that "the drainage section of eaves and gutters should be determined by calculation based on the rainwater flow rate of the roof catchment area. The net width of reinforced concrete eaves and gutters should not be less than 300mm, and the minimum depth at the water distribution line should not be less than 100mm; the longitudinal slope inside the gutter should not be less than 1%, and the water drop difference at the bottom of the gutter should not exceed 200mm; the drainage of eaves and gutters should not flow through deformation joints and firewalls. ”This article specifies that the depth of the eaves gutter is controlled according to the depth of the dividing line, and the depth at the dividing line should not be less than 100mm.

(3) Implementation of eaves gutter and gutter depth

When designing roof engineering, the depth of the gutter at the dividing line should be calculated based on the thickness of each structural level above the roof structure layer and the slope thickness of the eaves gutter at the dividing line. If it meets the requirements, the building eaves gutter can be used. When using building eaves gutters and gutters, the overall requirements for the waterproof layer are higher, and sufficient attention should be paid to the design of the waterproof layer.

When the eaves gutter of a building cannot meet the requirements, the bottom structure of the eaves gutter should be treated with a lowering plate. The depth of the lowering plate should be calculated and determined according to the depth requirements of the gutter at the dividing line, and made into a commonly known structural eaves gutter, as shown in Figure 3. The lowering of the bottom of the eaves gutter structural slab requires a consensus reached by architects and structural engineers during the design phase, which should be expressed in the construction drawings through node diagrams.

Figure 3 Structural eaves gutter