Lightweight Bathrooms with Increased Durability

Traditionally bathrooms and other wet rooms in Denmark are made solely from inorganic materials, e.g. masonry, concrete or lightweight concrete. The advantage of these materials is primarily that they are not vulnerable to water and besides some of the materials are watertight in themselves. Experience has shown that wet-room constructions made from inorganic materials and with a watertight - or in some cases just water repellent – surface function satisfactorily. For renovation purposes, new bathrooms are often constructed by casting a concrete slab on the old wooden floor joists as shown in Figure 1.
However, when apartment buildings are renovated and new bathrooms are installed, it is not always possible to construct the floor of cast concrete due to the load-bearing capacity of the structures. The reason for this is that foundations or the wooden beams in the floor cannot support the extra load of the concrete which is about 60-80 mm thick. In these cases, lightweight floors of organic materials might be applicable, but generally subfloors of organic materials are not approved for apartment buildings by municipalities in Denmark. A working group under The Building Damage Fund for Building Renewal concluded that a new lightweight structure was needed and that it could consist of a twin floor with a waterproof casting compound in-between two watertight membranes, cf. Figure 2. It was a prerequisite that the bathroom could be supported by the building’s load-bearing system, i.e. where heavyweight bathrooms are not applicable due to insufficient load-bearing capacity of foundations or wooden beams in the floor. The whole point of the twin floor is that if the upper membrane fails, i.e. water penetrates; there is another watertight layer to ensure the watertightness of the entire construction. In this way the risk of mould growth or dry rot in the construction is minimised.
A twin-floor bathroom construction is a potential solution when old buildings are renovated and new bathrooms established e.g. on old floor joists. Therefore, the two constructions described in Figure 2 were tested in the laboratory to see if the twin-floor solution would work. The two structures were tested for watertightness around penetrations and other details in wet room floors. The results from these tests showed no signs of water penetrating into the structure. Hereafter, one of each type of bathroom was constructed in an old apartment building. The materials used to construct the bathrooms were all well-known to the workmen, and the challenging part was the new combination with two watertight membranes. Especially the structure described in Figure 2b with a PVC covering was well received. Bathrooms are perhaps the most expensive room to construct in buildings and with the twin floor a supplement to the heavyweight bathrooms now exist. Especially in old buildings with weak foundations and/or floors with reduced load-bearing capacity, the twin floor benefits from the reduced weight. The main advantages of the twin floor are the reduced weight, the reduced risk of water penetration and the anticipated long service life. Besides, the task of carrying concrete up to apartments – normally in old buildings without lifts - will be reduced significantly. The construction time is usually short as special compounds are used for the casting (in contrast to concrete where tiles cannot be installed for several weeks). Furthermore it is anticipated that the extra cost of the twin floor is less than the cost of extra reinforcement of the existing construction where this is necessary due to inadequate load-bearing capacity.