Unlike a lot of cellars, pavement vaults only have their back wall built into the ground but they also have ground over their roof. Consequently most water ingress occurs as percolation of rainwater through the brickwork at a high level, usually the arched roof itself. The structures have very little in the way of foundations and do not have the weight of the house pressing down on them so they move differentially to the house and are also subject to a lot of vibration from traffic. Any attempt at using a rigid bonded coating to hold water back is doomed to failure due to this differential movement, vibration and the inherently weak tensile strength of the brickwork from which they are most usually 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 as compared with a regular cellar with all vertical walls that is under a house. The main difference stems from the fact that water ingressing from the arch itself will drip onto the membrane and, if the system is installed correctly, run down on the back surface of the membrane rather than running down the surface of the brick wall behind the membrane as is the case in most other cellar conversions. This truly tests the perfection of each of the sealed fixings that have to be used to hold the membrane in place. Furthermore, the use of a semi-rigid membrane is necessary as is the presence of dimples on the membrane as this example of a drained cavity system is one in which the space between the dimples is truly the means of draining the water down.
Basically this membrane behaves like a big umbrella, shedding water to the sides and thus protecting the space underneath. It is normally fixed to the internal masonry surface with plastic fixings, a bit like over sized drawing pins and the hole where the pike of the pin goes through the membrane is sealed with butyl rope or other material, which will be supplied by the specialist supplier of the system.
When the water reaches the bottom of the membrane it is best to have an under floor channeling system that acts a bit like a guttering system, this will run around the whole perimeter of the vault and discharge into a sump chamber. In many cases a perforated sump chamber will act as a soak-away especially if the ground is absorbant and the water ingress is slight – which it usually is. However the inclusion of an automatic submersible pump in the sump chamber will add reliability in the eventuality of a larger rate or water ingress or the ground becoming waterlogged itself.
To waterproof the floor, another similar membrane can be used which will require a concrete screed or board overlay. This will consume a few inches of headroom which is often at a premium. The use of a plastic floor tile system can be used to provide the waterproofing and floor finish in one go thus saving headroom, but such systems are a little harder to find. If more headroom is required the whole process will have to be preceded with an underpinning operation which will add significantly to the cost, but is non-the less a possibility. Some people consider joining two adjacent vaults together but this is a tricky process and will require the services of a structural engineer.
As regards finishing works, the vault could be lined with plasterboard supported by timber battens, which are in turn fixed into the same fixings that hold the membrane in place. Alternatively the membrane can be rendered over using a special type of plaster such as whitewall renovating plaster. If rendering a mesh will be required to form a key, this can be ribbed expanded metal lath of you can get a membrane with a mesh pre-attached.
If it is a little difficult to visualize the process, there is a three dimensional animated diagram of the vault cellar conversion process on our website.