Building roof

1. Roof function

The roof is a component of the external protection system of a building. It generally refers to the surface of the building roof, and on a flat surface, it refers to the area enclosed by parapets, mountain walls, or eaves. The roof should have the following functions.

1.1 Windproof and Rain Protection

The most basic function of a roof is to provide shelter from wind and rain, prevent wind, frost, rain, and snow from entering the interior. Therefore, the roof must be waterproofed, which is the main measure to ensure its basic function. Rainwater should be smoothly drained from the roof to reduce the difficulty of waterproofing, so roof drainage is an important supplement to roof waterproofing.

1.2 Thermal insulation to reduce energy consumption

Modern architecture requires providing comfortable living and working environments, reducing the impact of outdoor environments on indoor temperature and humidity, which is also a basic functional requirement for roofs. The roof is directly exposed to heat and rain. In summer, the surface temperature of the roof can reach over 70℃when exposed to the sun. In winter, when the roof is covered in snow, the surface temperature is very low, and in cold northern regions, it can drop below minus 30℃. Modern buildings have HVAC systems that regulate temperature and humidity, so improving the insulation requirements of the roof and ensuring the effectiveness of the roof insulation function can increase the thermal resistance of the roof and reduce the energy consumption of the HVAC system, which is one of the main measures for energy conservation.

1.3 Protecting the structure and extending its service life to provide shelter from wind and rain

In addition to waterproofing and thermal insulation, the roof also serves as a protective function for the roof structure. Whether it is a reinforced concrete structure, wooden structure, or steel structure, it can cause serious damage under the erosion of fluid leakage water. Reinforced concrete structures will accelerate the carbonation and alkali aggregate reaction of concrete, shorten the service life of concrete, and the corrosion and expansion of steel bars will cause concrete cracking; Wood structural components undergo decay; Corrosion occurs in steel structural components. The ultimate result will shorten the service life of the component and reduce its load-bearing capacity. The waterproof and insulation functions of the roof can provide excellent protection for the structure.

1.4 Usage and beautification functions

Nowadays, more and more roofs provide auxiliary functions for buildings, such as being used as rooftop parking lots, rooftop gardens, rooftop sports venues, etc.

In addition, modern architecture has increasingly high requirements for aesthetics, and the roof, as the fifth facade of the building, also has requirements for beautifying the building and environment. The roofs of low rise or multi story buildings are mostly exposed to the view of high-rise building users, with higher requirements for aesthetics.

2. The structural hierarchy of the roof

In roofs with reinforced concrete structural plates, in order to achieve the basic functions of the roof, there will be waterproof and insulation layers on the roof structural layer. For drainage purposes, non structural sloping roofs need to be equipped with sloping layers; In addition, there will be auxiliary structural layers such as leveling layer, protective layer, isolation layer, vapor barrier layer, etc; There will be a surface layer when using the roof, and a planting layer when planting the roof. The waterproof layer and insulation layer are the basic functional requirements, and the content is very rich. They need to be separately introduced in detail. The following only introduces the functions and methods of other structural layers.

Find the slope layer

2.1 Finding Slope Layer

The combination of prevention and drainage is one of the basic principles of roof waterproofing, and smooth drainage is a basic requirement for waterproofing. This requires the roof to have a certain slope, and structural slope should be prioritized when sloping the roof, which can reduce the number of structural layers and facilitate the realization of roof functions. Roofs with complex planar shapes and top floor interiors without suspended ceilings that cannot be structurally sloped can be sloped using lightweight materials. Lightweight materials are generally made of locally inexpensive and easily available materials, such as slag, cement expanded perlite, cement expanded vermiculite, ceramic aggregate concrete, aerated concrete, etc. However, these materials have a high water absorption rate. The mixing water and rainwater retained during construction can accumulate in the sloping layer, which can easily cause bubbles in the waterproof layer and require additional exhaust structures to solve.

leveling layer

2.2 Leveling layer

The purpose of setting the leveling layer is to ensure the construction and engineering quality of the waterproof layer or insulation layer, and it is not a necessary structural layer for the roof. If it can be set or not, it is better not to set it. For example, when making a flexible waterproof layer on a cast-in-place reinforced concrete roof panel, it is advisable to directly tamp and smooth it. When laying the board shaped insulation material with mortar, the leveling layer can also be omitted.

When a leveling layer is required, it is advisable to install a C20 fine aggregate concrete leveling layer on a soft or cracking prone base layer, and if necessary, add steel mesh to improve overall integrity. Cement mortar leveling layer is generally set on other base layers, with a mix ratio of cement: sand=1:2.5~1:3, or M15 pre mixed ground mortar is used.

protective layer

2.3 Protective layer

The purpose of the protective layer is to extend the service life of the waterproof layer and insulation layer, reduce the erosion of waterproof materials and insulation materials by media such as ultraviolet rays, ozone, acid and alkali, and prevent external forces from damaging the waterproof layer and insulation layer. Reinforced concrete or block materials should be used as a protective layer for the upper roof. For non accessible roofs, light colored coatings, aluminum foil, mineral aggregates, cement mortar, etc. can be used as protective layers according to the type of waterproof layer. Block materials or fine stone concrete can be used for better protection effect.

2.4 Isolation layer

The isolation layer is set up to prevent the rigid protective layer from damaging the waterproof layer during construction and use. Therefore, when setting up block materials, cement mortar, and concrete protective layers on the flexible waterproof layer, an isolation layer should be placed between the two. The isolation layer generally uses low-priced oil felt, geotextile, plastic film, etc., and can also use low strength grade mortar.

vapour barrier

2.5 Vapor Barrier Layer

The purpose of setting up a vapor barrier is to prevent indoor moisture and condensation water from the roof structure from entering the moisture absorbing insulation layer and reducing the insulation effect. Therefore, a vapor barrier should be installed on the roofs of buildings that often produce a large amount of water vapor indoors, such as public bathrooms and large kitchens; In extremely cold and frigid regions, a vapor barrier should be installed when condensation may occur between the roof insulation layer and the structural layer. The vapor barrier layer can be made of waterproof rolls or waterproof coatings with good water and air tightness

2.6 Use of surface layer

There are different surface layer methods for using roofs according to their purposes. When used as a parking lot, the surface layer should be reinforced with reinforced concrete with a thickness of about 80mm. When used only for personal use, floor tiles, outdoor wooden flooring, etc. can be laid as needed. Plastic can also be laid on a reinforced concrete protective layer with a thickness of about 80mm for use as a track or stadium. It can also be designed as a rooftop garden, which not only beautifies the environment but also provides a place for people to rest.

plant layer

2.7 Planting Layer

A rooftop garden must have flowers and plants, so there must be a planting layer. In fact, planting roofs is a system, and not just having a planting layer is enough. In addition to the slope layer, insulation layer, leveling layer, waterproof layer, protective layer and other layers, planting roofs also need to have water storage and drainage layers, filter layers, planting soil layers and vegetation layers.

The function of the water storage and drainage layer is to discharge most of the rainwater when it is abundant, avoiding the planting layer from being soaked in water and causing plant root rot; At the same time, it has a certain water storage capacity, which can provide some of the water needed for plant survival during continuous sunny days; The water storage and drainage layer generally adopts mesh interwoven or concave convex water storage and drainage plates; There are also graded crushed stones or ceramic particles with a particle size of 10mm to 25mm, and pebbles with a particle size of 25mm to 40mm, laid with a thickness of not less than 100mm. These methods have almost no water storage capacity and are only used as drainage layers.

The function of the filter layer is to prevent the planting medium from being washed away by rainwater. Generally, polyester non-woven fabric with a unit area mass of not less than 200g/㎡is used.

The planting soil layers include field soil, improved soil, and inorganic planting soil. The thickness of the planting soil is determined according to the type of plant. For planting lawns and vegetation, it is 100mm to 200mm, for planting shrubs, it is 300mm to 600mm, and for planting trees, it is not less than 600mm.

Rooftop vegetation should be planted with lawns, ground covers, and short shrubs, and should not be planted with tall, fast-growing trees or plants with well-developed roots.

Planting roofs must have root blocking measures to prevent plant roots from puncturing, such as setting up a root puncture resistant waterproof layer under the water storage and drainage layer, or setting up a separate root blocking layer to prevent plant roots from penetrating the waterproof layer and causing leakage.

The above are common structural levels in roofs, which are not necessarily present in every roof. Some levels, such as leveling layers, may also appear multiple times in a roof. When designing a roof, designers should choose and combine it based on factors such as the nature of the building, its functional use, roof form, and climate conditions to form a reasonable roof construction method.