Video: Roof Physics
2023 Author: Douglas Hoggarth | [email protected]. Last modified: 2023-05-24 11:23
As a building envelope, the roof is exposed to a number of factors closely related to the processes taking place both outside the building and inside it. These factors include, in particular:
- solar radiation;
- temperature variations;
- water vapor contained in the indoor air of the building;
- chemically aggressive substances in the air;
- vital activity of insects and microorganisms;
- mechanical loads.
The function of protecting the building from atmospheric precipitation is assigned to the uppermost element of the roof - the roof. To drain rainwater, the roof surface is sloped. The task of the roof is not to let water into the underlying layers.
Soft roofing materials that form a continuous sealed carpet on the roof surface (roll and mastic materials, polymer membranes) do a good job with this task. When using other materials, precipitation with small roof slopes, especially in adverse weather conditions (rain or snow, accompanied by strong winds) can penetrate under the roof covering. In such cases, an additional waterproofing layer is arranged under the roof, which is the second line of protection against atmospheric precipitation.
An important task is the organization of the drainage system - internal or external.
Snow puts an additional static load on the roof (snow load). It can be quite large, so it must be taken into account when calculating the total load on the roof structure. This load depends on the slope of the roof. In snowy areas, the slope is usually increased so that the snow does not linger on the roof. At the same time, on pitched roofs, it is advisable to install snow-retaining elements that do not allow snow to fall like an avalanche, thereby threatening the health of passers-by, often deforming the building facade and disabling the external drainage system.
One of the significant problems in snowy areas is the formation of ice and icicles on the roofs. Ice often becomes a barrier that prevents water from entering the gutter, water funnel, or simply flowing down. When using non-hermetic roofing (metal roofs, all types of shingles), water can penetrate through the roof, forming leaks. The mechanism of ice formation and methods of combating this phenomenon are discussed in detail in the section Anti-icing systems for roofs.
Wind streams, meeting an obstacle in the form of a building on the way, bypass it, as a result, areas of positive and negative pressure are formed around the building (Fig. 2).
The magnitude of the resulting negative pressure exerting a tearing action on the roof depends on many factors. The most unfavorable in this regard is the wind blowing on the building at an angle of 45 0. The roof plan of the building, which shows the distribution of negative pressure at a wind direction of 450, is shown in Fig. 3.
The tearing force of the wind may be sufficient to damage the roof (blistering, tearing off part of the coverings, etc.). It increases especially when pressure increases inside the building (under the base of the roof) due to air penetration through open doors and windows from the leeward side or through cracks in the structure. In this case, the tearing force of the wind is determined by two components: both negative pressure above the roof and positive pressure inside the building. Therefore, to eliminate the risk of damage to the roof, its base is made as tight as possible (Fig. 4). Additional mechanical fastening of the roofing material to the base is often done.
Parapets are used to reduce negative pressure. However, it should be borne in mind that they can not only decrease, but also increase negative pressure. If the parapets are too low, the negative pressure can be even higher than without them.
Different roofing materials have different sensitivity to solar radiation. For example, solar radiation has practically no effect on ceramic and cement-sand tiles, as well as on metal roofs without polymer coatings applied to them.
Bitumen-based materials are highly sensitive to solar radiation: exposure to ultraviolet radiation accelerates the aging process. Therefore, as a rule, they have an upper protective layer of mineral dressing. To protect modern materials from aging, special additives (modifiers) are introduced into the bitumen composition.
A number of materials, under the influence of ultraviolet radiation, lose their original color (fade) over time. Metal roofs with some types of polymer coatings are especially sensitive to this radiation.
Solar radiant energy, falling on the roof, is partially absorbed by the roofing materials. At the same time, the upper layers of the roof can be significantly heated (sometimes up to 100 ° C), which also affects their behavior. So, for example, materials based on bitumen soften at sufficiently high temperatures and in some cases can slide off sloped roof surfaces. Heat-sensitive and metallic roofing materials with some types of coatings. Therefore, when choosing a roofing material for use in southern regions, you should make sure that it has sufficient heat resistance.
As a building envelope, the roof operates in a rather severe temperature regime, experiencing both spatial and temporal temperature variations. As a rule, its lower surface (ceiling) has a temperature close to that of the room. At the same time, the temperature of the outer surface varies within a fairly wide range - from very significant negative values (on a winter, frosty night) to values close to 100 0С (on a summer, sunny day). The temperature of the outer surface of the roof, at the same time, can be heterogeneous due to the unequal sun illumination of its different parts.
But, as you know, all materials are subject to thermal stretching and compression to one degree or another. Therefore, in order to avoid deformation and destruction, it is very important that the materials working in a single structure have similar coefficients of thermal expansion. To increase the resistance of the roof to thermal loads, a number of technical solutions are also used. In particular, in flat roofs, to limit the effect of horizontal movements and excessive internal stresses, special deformation nodes are laid.
A serious danger to almost all roofing materials (except for metal coatings) is represented by frequent, sometimes daily temperature drops from plus to minus. This tends to occur in areas with mild and humid winters. Therefore, in such climatic zones, it is necessary to pay close attention to such an important characteristic for roofing materials as water absorption. With high water absorption, moisture at positive temperatures penetrates and accumulates in the pores of the material, and at negative temperatures it freezes and, expanding, deforms the very structure of the material. The result is a progressive destruction of the material, leading to the formation of cracks.
The roof should not only be resistant to significant temperature variations, but also reliably protect the interior of the building from them, protecting it from cold in winter and from heat in summer. The role of the thermal barrier in the roof structure belongs to the thermal insulator layer. In order for the thermal insulation material to perform its function, it must be as dry as possible. With an increase in humidity by only 5%, the thermal insulation capacity of the material is almost halved.
Water vapor is constantly generated in the interior of the building as a result of human activities (cooking, washing, bathing, washing floors, etc.). Humidity is especially high in newly built or renovated buildings. In the process of diffusion and convective transfer, water vapor rises and, cooling down to a temperature below the dew point, condenses in the under-roof space (Fig. 5). The amount of generated moisture is the higher, the greater the difference in temperature outside and inside the building, therefore, in winter, moisture accumulates quite intensively in the under-roof space.
Moisture has a negative effect on both wood and metal roof structures. In excess, it begins to drain into the interior, forming leaks on the ceiling. The most unpleasant consequences are the accumulation of moisture in the heat-insulating material, which, as already mentioned, sharply reduces its heat-insulating properties.
A significant barrier to the penetration of steam into the under-roof space is a special film with low vapor permeability, which is placed in the roof structure directly under the thermal insulation. However, no vapor barrier material is able to completely exclude the flow of steam from the inside of the building into the under-roof space. Therefore, in order for the roof not to lose its heat-insulating ability from year to year, it is necessary that all the moisture that accumulates in the heat-insulating material in winter should go outside in the summer.
This task is being solved by constructive measures. In particular, for flat roofs, it is recommended not continuous, but partial gluing of roofing materials to the base.
Special ventilation gaps are arranged in pitched roofs (Fig. 6). As a rule, there are two of them - the upper gap and the lower one. Through the upper gap (between the roofing and the waterproofing), atmospheric moisture trapped under the roofing is removed. Thanks to ventilation, wooden structures (counter-lattice and lathing) are constantly ventilated, which ensures their durability. Moisture is removed through the lower ventilation gap that penetrates into the insulation from the interior. High-quality arrangement of vapor barrier from the side of the interior and the presence of a sufficient lower ventilation gap, exclude waterlogging of the roof structure.
Note that when breathable membranes are used as waterproofing materials, there is no need for a lower ventilation gap.
To ensure good air circulation, many companies that produce roofing materials for pitched roofs, as a rule, offer a number of ventilation elements as additional elements: overhang aerators, ridge aerators, ventilation grilles, and for tiled roofs - special ventilation tiles.
The most reliable protection against water vapor is especially needed in roofs over rooms with high humidity: swimming pools, museums, computer rooms, hospitals, some industrial premises, etc. Steam protection must also be given special attention when building in areas with extremely cold climates, even with normal indoor humidity. When analyzing the environmental conditions and temperature and humidity conditions inside the premises, one can make assumptions about the possibility of moisture condensation and its accumulation, and, using various combinations of roof components, try to prevent these phenomena.
Chemically aggressive substances in the air
As a rule, in large cities or near large enterprises in the atmosphere there is a fairly high concentration of chemically aggressive substances, for example, hydrogen sulfide and carbon dioxide. Therefore, for all structural elements of roofs, and especially for roofs in such areas, it is necessary to use materials resistant to chemicals present in the air.
Vital activity of insects and microorganisms
Various insects and microorganisms can cause significant damage to the roof structure, especially to wooden elements. High humidity is a particularly favorable environment for their life. To protect wooden structures, special impregnations are used that protect the material from microorganisms.
The roof structure must resist mechanical loads, both permanent (static) - from the fill and installation elements, and temporary - snow, from the movement of people and equipment, etc. Loads associated with possible movements between the roof and building units are also temporary.
So, in order for the roof to reliably perform its functions and be resistant to various kinds of influences (listed above), it is necessary: firstly, it is enough to correctly calculate the bearing part; secondly, find the best design option; and finally, thirdly, to ensure the optimal combination of materials of construction.
It follows from all that has been said that the following main layers may be present in the roof structure (Fig. 7):
- roofing material, on which an additional layer is applied if necessary (dressing, ballast, etc.);
- waterproofing layer (on sloping roofs) - additionally insulates the inner layers of the roof from atmospheric moisture penetration;
- thermal insulation - provides a fairly stable air temperature in the premises;
- vapor barrier - prevents the penetration of water vapor from the inside of the building into the roof structure;
The structure of the roof must be provided with measures for free air circulation (ventilation).
The need for certain layers and their location depend on the type of building and the effects to which it will be exposed. When choosing, you must also take into account the technical characteristics of the materials used: coefficients of thermal expansion and compression; ultimate tensile, compressive and shear strength; characteristics of vapor permeability and moisture absorption; aging characteristics, incl. increased fragility and loss of thermal resistance; elasticity; fire resistance. The importance of all of the above listed technical characteristics is determined by each specific case.