Basement conversion: how to turn an arched vault under the pavement into a useful living space

Unlike many basements, paved vaults only have the rear wall recessed into the floor, but they also have floor over the ceiling. Consequently, most of the water ingress occurs as rainwater seepage through the masonry at a high level, usually the vaulted ceiling. Structures have very few foundations and do not have the weight of the house pressing on them, so they move differently from the house and are also subject to many vibrations from traffic. Any attempt to use a rigid bonded coating to retain water is doomed to failure due to this differential motion, vibration, and the inherently weak tensile strength of the masonry with which they are generally constructed.

The best method that will provide reliability over time is the drained cavity method, although there are some differences between the details of a drained cavity system in a vault compared to a regular basement with all vertical walls underneath. a house. The main difference lies in the fact that the water entering from the arch itself will drip onto the membrane and, if the system is installed correctly, it will run down the back surface of the membrane instead of running down the wall surface. of bricks behind the membrane. as is the case in most other warehouse conversions. This really tests the perfection of each of the sealed fasteners that must be used to hold the membrane in place. Also, the use of a semi-rigid membrane is necessary as is the presence of dimples in the membrane, as this example of a drained cavity system is one in which the space between the dimples is actually the means to drain the water.

Basically, this membrane behaves like a large umbrella, spilling water on the sides and thus protecting the space below. It is normally attached to the internal surface of the masonry with plastic fasteners, a bit like oversized thumbtacks and the hole where the pin pick goes through the membrane is sealed with butyl rope or other material, which will be supplied by the supplier. specialized. of the system.

When the water reaches the bottom of the membrane it is best to have an underfloor gutter system that acts a bit like a gutter system, this will run around the entire perimeter of the vault and discharge into a sump chamber. In many cases, a perforated sump chamber will act as an absorbent, especially if the soil is absorbent and the entry of water is slight, which it usually is. However, the inclusion of an automatic submersible pump in the sump chamber will add reliability in the event of a higher rate or water ingress or the soil becoming soggy.

To waterproof the floor, another similar membrane can be used that will require a layer of concrete or a layer of board. This will consume a few inches of headroom, which is often at a premium. Using a plastic floor tile system can be used to provide one-time waterproofing and finishing of the floor, saving head space, but these systems are a bit more difficult to find. If more headroom is required, the whole process will have to be preceded by a support operation which will significantly increase the cost, but is no less a possibility. Some people consider joining two adjacent vaults, but this is a complicated process and will require the services of a structural engineer.

As for the finishing work, the vault could be clad with plasterboard supported by wooden slats, which in turn are fixed in the same fixings that hold the membrane. Alternatively, the membrane can be covered with a special type of plaster, such as white wall renovation plaster. If you need to render a mesh to form a key, this can be a ribbed expanded metal slat or you can get a membrane with a pre-installed mesh.

If the process is a bit difficult to visualize, there is an animated three-dimensional diagram of the vault conversion process on our website.