Flat roofs are typically formed from monolithic or semi-monolithic reinforced concrete slabs by layering several building technical elements above the slab. Typical layers may include sloping concrete, thermal insulation boards or waterproofing membranes, or even floor covering layers to ensure accessibility.
Most flat roofs are single-shelled structures. These are also known as warm roofs as the layers are closely stacked.
A variation of the flat roof is the double-shelled flat roof. Here, the layers are separated by a frame structure into the upper shell (for drainage and where appropriate, walkability) and the lower shell (for thermal insulation). The gap between the two shells provides ventilation, so these roofs are also called cold roofs. While they have advantages, their labour-intensive nature and the difficulties associated with ventilation mean that they are much less common than single-shelled roofs.
External drainage for double-shelled flat roofs typically employs a slope toward the perimeter gutter or drainage line. The slope pattern is similar to that of a shed roof, but with a much smaller slope (a minimum of 2% up to a maximum of 8%). This type of roof is easy to clean and maintain, but builders need to consider protection against the eaves freezing.
For single-slope flat roofs, internal drains are typically designed with an optimal layout depending on the roof’s plan area, incline and geometry.
The main rules for designing a drainage system are:
- Interior drains should be located at the deepest point of the catchment area, at least 50 cm from walls and other structures.
- The maximum catchment area per interior drains should be 150-180 m2.
- The maximum distance along the slope line is 12 m.
- A minimum of 2 interior drains shall be required on each roof, or 1 interior drain and overflow scuppers.
- Adequate slope shall be maintained across the entire roof surface, with crossfalls around upstanding elements to eliminate ponding risk.
- Structures shall not be positioned within roof valleys.
- Slopes shall be a minimum of 2%, but also a minimum of 1% measured in the roof valley. However, slopes greater than 5% are not recommended.
There are two basic solutions to the grade plan: roof planes with uniform slopes or varying slopes.
Uniform slopes result in roof valleys meeting at right angles and varying roof plane elevations along parapet walls. The advantage of a different slope is the formation of parapet wall - roof plane intersections aligned with a single horizontal plane - that is a uniform height of the parapet wall relative to the edges of the roofs. For more complex roofs, a combination of both solutions may be possible.
The slope is usually constructed in two ways: with insulating boards or lightweight concrete. Thermal insulation materials can be of different types, depending on the layering. Some examples include expanded or extruded polystyrene, PIR boards or even high-density mineral wool.
The most common materials for rainproofing are bituminous or plastic membranes, which are forms of substructure insulation. There are various types of waterproofing coatings, although these are used less frequently.
Types of materials:
- Bituminous membranes: modified bituminous thick membranes are the most commonly used materials. They have now largely replaced oxide bituminous thin membranes and weldable thick membranes. Modified bituminous thick membranes come with a variety of coatings or inlays depending on the insulation requirement. They are always laid in two layers, at least: The bottom layer (the base sheet) is usually a modified bituminous thick sheet which is polyethilene-film-laminated or sandblasted. It is fixed either mechanically or welded in place. The top layer (the sealing plate) is a modified bituminous thick plate that is usually flame-welded to the bottom layer. If the top layer also acts as the roof sealing layer (as in conventional roof systems), a mineral-finished UV-resistant sheet should be chosen for this purpose. The thickness of the plates can vary between 4 to 4.5 mm. Some panels can only be fixed by welding, some by nailing too, and there are also self-adhesive panels.
- Plastic membranes: There are plastomeric (thermoplastic) plastic sheets such as PVC or PIB or elastomeric (rubber-like) sheets such as EPDM. These membranes are laid with mechanical fittings (clout nails). Membrane thickness varies between 1.2 and 2.0 mm.
- Waterproofing Coatings: can be polyester-, polyurethane- or bitumen-based and vary in thickness between 2 to 4 mm.
Layer Order:
- One-shelled flat roofs that are built with waterproofing positioned above the thermal insulation are called conventional roofs (warm roofs). In this case, the most common solution is to form the waterproofing slope with tapered insulation boards in the upper layer of the insulation. Most manufacturers offer ready-made tapered elements for this purpose. Special care must be taken to ensure minimum insulation requirements are met around drainage points, which represent the weakest areas of the thermal envelope. High-density rockwool or glass wool boards are the most commonly-used thermal insulation materials for this purpose. In a conventional system, it is imperative to protect the insulation underneath the waterproofing against vapour coming from the direction of the slab. A vapour barrier or vapour-retardant membrane should be applied on top of the slab. Finally, the surface of the tapered insulation system is waterproofed. In the less common approach of using sloped concrete, a lightweight concrete layer is typically cast onto horizontally-placed thermal insulation boards. In such a scenario, expanded polystyrene is the most commonly used thermal insulation, with the lightweight concrete slope layer laid on top. A separation layer must be created on top of the insulation to protect it. Also, it is necessary to lay a vapour control layer under the waterproofing. As previously-explained, it is necessary to apply a vapour barrier / vapour-retardant layer on top of the slab.
- For inverted roofs, the waterproofing is placed under the thermal insulation. The main advantage is that the waterproofing sheets have less exposure to solar radiation and thermal fluctuations and this increases their lifetime. Furthermore, the insulation does not need to be protected from humidity, as the waterproofing also acts as a vapour barrier. Since the thermal insulation is exposed to water in inverted roofs, only closed-cell insulation is an option: extruded polystyrene or PIR foam boards laid in a single layer, typically with shiplap edges. Above the slab, the layering is as follows: the lightweight concrete slab is cast first, then the waterproofing membranes are laid, followed by the thermal insulation boards, and finally, any other protective or other covering layers.
All these types can be designed in an accessible (utilised) or non-accessible (non-utilised) way, depending on their suitability for continuous occupancy.
Paving on accessible roofs first requires the installation of a base or bedding layer. Underneath these, it is common to use a layer of gravel and surface drainage. Green roofs are a special type of accessible flat roofs. They require the use of additional special layers, such as root-resistant sheets to prevent plant roots from breaking through the waterproofing membranes. Topsoil, a drainage layer (drainage sheets), a gravel drainage layer, and membrane separation layers are also required.
As a general rule, non-accessible roofs are usually built using a conventional system, while accessible and green roofs are more likely to be built using an inverted system.
