Previously reserved primarily for low-cost functional buildings, the flat roof has also enjoyed great popularity in residential and representative buildings since the classic modern era from the 1020s. Its structure is just as diverse as its use. Here we explain the most common flat roof constructions and explain their advantages and disadvantages.
The general structure
Like all other roof shapes, the flat roof must generally meet three basic requirements. These can be provided by one and the same component of the structural design, but can also clearly "stand in the way" of each other.
load capacity
Every flat roof structure must be strong enough to absorb the following loads and to transfer them safely into the supporting components, i.e. wall or columns:
- construction - i.e. the weight of the roof itself
- Precipitation - the weight of rainwater and especially snow
- Traffic loads - Depending on the type of roof use, either craftsmen for revision, or - when used as a roof terrace - people, furniture, plants, etc.
Thermal insulation
For heated buildings, the Energy Saving Ordinance ENEV requires a minimum standard of thermal insulation for the flat roof in order to limit the amount of energy dissipating through the roof surfaces. This is not necessary in unheated buildings, but - depending on the use - a minimum insulation to prevent condensation in accordance with DIN4108 may be necessary.
seal
Finally, the flat roof should of course protect against adverse weather conditions, i.e. wind and precipitation. For this purpose, the structural design must provide a dense level. As a rule, this should be designed in such a way that problems that arise, such as a blocked drain or a defective rain gutter, do not immediately lead to water damage in the construction.
Advantages and disadvantages
Finally, the main advantages and disadvantages of the flat roof should be clearly summarized at this point:
advantages
- Low installation height
- Well usable on top (e.g. B. Roof terrace)
- Restrained to invisible appearance
- Production through horizontal installation of insulation and sealing can be implemented quite easily
- No lost attic space underneath, as the space is closed horizontally towards the top
disadvantage
- Slow water drainage due to no resp. low inclination
- In the event of leaks, water quickly enters the structure
- Internal drainage complex and prone to failure
- No slipping of snow due to the lack of a slope
- No self-cleaning effect, as the slowly draining rainwater has little rinsing effect
Tilt
Even if the flat roof is flat, this does not automatically mean that it must not have a slope at all. On the contrary. If you follow the guidelines of the flat roof guideline when constructing the flat roof, the roof must have one Slope of at least two percent in the direction of the drainage points - roof drains or rain gutters - exhibit. Either the entire roof can be inclined, or only the sealing level is, for example, over a tapered slope insulation on a completely horizontal construction plane with the slope Mistake. This tendency leads to the fact that rainwater is directed to the draining components. The degree of inclination, which appears to be quite high, also takes into account the fact that deviations can occur again and again during execution. If the incline was too low, water sacks would form, even if there were only slight differences in level, in which rainwater would remain permanently and put a strain on the structure. Upwards is the demarcation of the flat roof compared to steeper ones Roof shapes not clearly defined. Usually one speaks from approx. three to five degrees inclination no longer from the real flat roof, but from the flat roof.
INFO: The specified minimum incline does not necessarily have to be adhered to. Real flat roofs with a zero degree incline are always being realized. However, the slope value reflects the experience of flat roof builders and is intended to help avoid moisture damage as far as possible
Classic building materials
In contrast to various other roof shapes, the flat roof is quite limited in terms of the building materials used to create it. This is primarily due to the structural challenges of the minimal inclination, as well as a possible use of the roof area. The structure, insulation and sealing have to adapt to this and to the challenges
react to these peculiarities.Structure
The following supporting structures are established:
- Homogeneous reinforced concrete slab
- Steel or concrete girders with concrete elements suspended between them
- Wooden girders with wooden decking as a load-bearing level
- Steel girders with wood or metal covering (then mostly trapezoidal sheet metal)
insulation
Proven insulation variants are:
- Foamed plastics as classic insulation materials for laying on flat levels, e.g. B. Reinforced concrete slab or wood covering
- Foam glass as a flat covering, see above
- Mineral wool, cellulose insulation and other soft insulation materials for installation in cavities between load-bearing elements
SPECIAL CASE: Sandwich elements - here foamed plastics are used directly with a trapezoidal sheet as a combination of insulation level and flat supporting structure.
seal
When it comes to sealing, there are ultimately only two common options for flat roofs:
- foil - Plastic film for bonding on flat surfaces, e.g. B. pressure-resistant insulation
- bitumen - bituminous waterproofing membranes that are not glued but welded on by flaming and liquefying the bitumen
Covering
Usually not necessary for the pure function of the flat roof as part of the building envelope, different coverings on the roof can bring various advantages:
- Greening: Delay in the release of rainwater, ecological added value, visual enhancement
- Gravel: mechanical protection of the seal, load as protection against wind suction, UV protection of plastic and bitumen seals
- Pavement: For using the roof surface as a terrace, as a paving slab or wooden grating, it can be produced in the same way as a terrace on level ground
Construction principles
If you now know the essential requirements for a flat roof, as well as the catalog of common components, it is now important to combine both into a functioning overall construction. Despite the number of possible combinations that initially appear to be very high, there are actually only a few general systems according to which a functioning flat roof is usually created:
1. On-roof insulation
With this classic and simple form of the flat roof, the components are simply stacked on top of one another. At the bottom is the supporting structure, i.e. the concrete slab, or the wooden covering on a support level. This is now followed by the insulation layer. This must be so pressure-resistant that it can absorb the following components as well as a possible snow load and, if necessary, withstand use of the roof. The seal is now applied to the insulation layer as a flat substructure. It protects the entire construction, as well as of course the building covered with the flat roof. An individual covering can now be applied to the seal, or the seal can remain as the uppermost functional end of the construction.
1b. Special case of inverted roof
A special form of the flat roof structure just described is the so-called inverted roof. Here, the insulation is not installed under the protective seal, but placed on top of the seal. The seal is accordingly located directly on the load-bearing level. For this roof structure, the insulation layer must be laid without joints and gaps in order to prevent the penetration of heat-conducting rainwater as far as possible.
In addition, the insulation itself must be able to withstand permanent moisture. This construction is usually completed by a protective sheet and a gravel layer to secure the insulation in its position.NOTE: Often used in the 80s and 90s of the last century, this construction can still be found in existing buildings. On the other hand, it is rarely rebuilt due to the technical insulation difficulties.
2. In-roof insulation
If the flat roof is created from a carrier layer with overlying panel materials, this is a good idea the possibility of not laying the insulation on top, but already between the beams bring in. The advantage is obvious with a significantly reduced installation height due to the omitted dimension of the insulation layer. On the other hand, the insulation must be secured against falling out so that a false ceiling with trickle protection film and clamping battens cannot be avoided. In general, in-roof insulation is only suitable for wooden structures, as the wood itself has a certain insulation value. Steel or concrete beams, on the other hand, would create glaring thermal bridges between the individual insulation packages, with all the problems that result from them.
3. Sub-roof insulation
For the sake of completeness, the under-roof insulation should also be mentioned here. Here an existing roof is insulated from the underside. This can be done either by gluing and cladding flat insulation materials, or by providing additional battens between which the insulation is installed. Since the insulation can only be brought up to the walls supporting the supporting structure, it is the worst possible variant and is usually only found in the renovation of old buildings. Because its advantage is clearly that it can be installed without opening the existing roof or having to completely dismantle it.
The edge of the roof
Particular attention is paid to the edge of the roof on flat roofs. Because the flat roof can either be provided with a roof overhang or hidden behind a parapet, i.e. a surrounding masonry. Depending on the desired solution, this places special demands on the roof edge:
With roof overhang
- External drainage via rain gutter on one side or all around
- Roof slope inclined outwards towards the gutter
- Guide the seal over the inlet sheet into the channel
- In the case of covering on waterproofing, water-permeable edge strips, e.g. B. Provide a gravel strip
NOTE: Of course, a flat roof with a roof overhang can also be drained via internal drains. However, these are rarely used because they are more prone to failure than a gutter and also require numerous roof penetrations. The first choice is therefore usually the external gutter, through which the water simply runs out and drips off in the event of a blocked downpipe.
At Attika
- Guide the seal around the parapet upwards
- Sealing height according to DIN at least 15 cm above the upper edge of the covering
- Note the bending radii of the waterproofing in the valley of the roof surface to the parapet and provide an insulating wedge
- Upper sealing closure via clamping profile and overhang sheet
- Align internal drainage via roof drains, slope to point low point
- At least two drains required, alternatively an emergency overflow as a second drain
- Protect the parapet from water on the top, e.g. B. by sheet metal