Basements waterproofing

The educational building could not have been more than 25 years old. The outer walls are of brick and block cavity work, with intermediate floors of concrete beam construction.

Figure 1: This building had not been built that long, but was already about to be altered quite substantially.

Background

The sports wing had a number of shortcomings at this school – the lack of ceiling height for use as a fully-fledged gymnasium was the most pressing. The alternatives were to raise the ceiling or lower the floor – or both!

It was decided to lower the floor. Walls previously above ground now need to perform as below-ground walls.

The first task would be to remove the existing suspended concrete floor, which would involve cutting concrete beams where they were built into perimeter cavity walling.

The brick and block cavity walls would be strengthened to take additional lateral loads. The method used was pure cunning! Access holes were cut in the inner cavity wall skin, and concrete pumped in – changing the wall to a solid wall with increased strength.

Lateral damp penetration would be excluded by applying an internal multi coat render system.

Figure 2: The project is well advanced, with a new concrete slab in situ, and the perimeter walls already waterproofed to around 1500mm height.

Figure 3: A clever technique for maintaining continuity of waterproofing at a wall intersection.

Figure 4: The classic damage sandwich of raised ground levels. Yard-dpc-yard-dpc, and so on.

An opening has been cut through the internal blockwork, so waterproofing just carries on through!

The bulge in the waterproofing (figure 3) is where the new waterproofing carries over the slight projection of the old cut-off floor beams. The render is reinforced with a patent ‘combiflex’ strip at this position. Combiflex is also used to strengthen rendering at the wall/floor junction.

When this building was built, the external yard levels would have been much lower than they are today. It is not just old Georgian, Victorian and Edwardian houses that suffer from raised land putting the wall base at risk from damp damage. Any building can be threatened.

The new gymnasium floor level is six or seven brick courses (i.e. 450-525mm) below existing yard level along this elevation. But before the floor level was lowered internally the original physical dpc would have been two courses below yard level. This may or may not have been causing a low-level damp penetration problem.

Looking at the lowered door opening reveal, there looks to be a good height of new waterproofing protection, as the new waterproofing extends easily a full metre above the external finished yard level. For added protection, the contractor has installed a chemical injection dpc, using a gunned silicone cream, to provide in my opinion a support measure to help control dampness. The internal waterproof render coats will be the mainstay.

In order to produce an attractive interior finish, the rendering internally would need to be covered by panelling. It was decided to form a decorative dado detail, which would require expert fixing methods.