Basements waterproofing

Figure 1: A rather attractive Islington residence – said to be the oldest house in Islington. All may well look presentable above ground – but below …?

Figure 2: Standing water – not that you would wish to stand anywhere in this basement without wellington boots.

The hydrostatic pressure can be alleviated by improving land drainage around the building – as an external support measure. The more lateral damp penetration can be reduced, the greater the chance of success for any waterproofing installed. It will simply have less water load to deal with.

For this building, water ingress under pressure would not be an ‘event’ for a waterproofing system to cope with, but a day-in, day-out continuous occurrence. Maybe a little too much for pumps and sumps to cope with.

There was also a height issue. The ceiling as existing from solid floor to ceiling was a scant 1.9 metres. A cavity-drained floor could take up 125mm of thickness (i.e. from the cavity membrane – 20mm, plus screed of 65mm, plus allowance for falls in the screed – much needed to drain the floor towards perimeter drains). So height counted against a cavity-drained floor system in this particular case.

The original waterproofing must have been installed by less than fully conscientious waterproofers. It comprised a waterproof slurry, dry-lined with plaster-boarding, fixed by adhesives. Needless to say plaster- boards and adhesives offered very little resistance to the dampness that had defeated the slurry-coating defence. There had been no linkage of any sort between the less than substantial floor slab and the wall waterproofing, and no perimeter drainage at all. The brick substrate had not provided a good enough key for the slurry coating, and there was no evidence of brush hammering (a common method to key-up masonry). The current contractor feels that on the evidence of substrates as they appear now, brush hammering would have probably caused more damage than the rather friable substrates could tolerate, in any case. Much water was found collecting under the slab. Some of the concrete reinforcement was found to be damaged by pitting. A new concrete slab would be needed, independent from the perimeter walls. Some limited differential movement may be accommodated by the wall/floor junction of new waterproofing.

Figure 3: No need to take samples for moisture content checking by carbide meter in this basement. The masonry is visibly saturated. You can appreciate the task of creating a dry habitable space as you cast your eye over the mixture of loose and friable brick and stonework either side of the exit steps to the garden area.

Figure 4: As we hack off some of the wall finishes we uncover the story of previous attempts to waterproof this space. Under the finish plaster is a grey- coloured slurry coat that did not keep the water out. Preparation is the key. This render coat must now be removed and the exposed substrates made good as necessary, to receive new waterproofing coatings or fixings.

Figure 5: Note the position pointed out well below the water table, where water actually spouts into the basement. Such an entry point can be dammed very rapidly by applying fast action powders.

Remedy selection

If we refer to the table at the end of 'Types of systems', we could scan down the key selection criteria of a waterproofing system. At Islington, as can be seen from the rather dramatic pictures above, we were faced with pretty evident hydrostatic pressure. How would the various potential waterproofing systems cope? Not only is there a very definite water entry, but the substrate looks rather unpredictable, to say the least, and we need to effect substantial brickwork repairs to achieve the desired standard of substrate on which waterproofing may be fixed or applied.

Now let us weigh up the relative merits of a cavity-drained waterproofing system versus a multi-coat render system. These are two of the favoured systems on offer from the selected contractor.

Looking first at cavity-drained waterproofing:

• good fixing positions are needed at minimum spacings;
• the system is vulnerable to hydrostatic pressure if pumps fail; and
• it can suffer problems if drain routes block from silting (under-floor membranes, etc.).

On the plus side, the air gap should dissipate hydrostatic pressure before it can build up to impose on the membrane system.

Multi-coat render systems are designed to withstand hydrostatic pressure, and will do so as long as the renders can be well bonded to their substrate, are mixed and applied carefully using suitable materials, and, importantly, a really reliable substrate needs to be achieved by good attention to preparation.

No doubt either system potentially could succeed – but which has the least risk of failure? The contractor needs to be convinced of the method about to be used, or costly and commercially damaging reparations could follow. The contractor decided to carry out some destructive testing to clarify the floor slab construction.

In order to decide on the best waterproofing approach, the contractor needed first to know the strength and stability of the existing solid floor. So the floor was core-drilled, and found to comprise a 50mm concrete slab, topped by a 100mm cement and sand screed. The slab was reinforced by steel bars, and a plastic damp-proof membrane was found underneath the slab. Membranes under slabs can sometimes be damaged quite early on in their life, e.g. the membrane can be accidentally damaged during concreting, or may be subsequently punctured by sharp aggregates or hardcore underneath.

The hydrostatic pressure was such that within the first day after drilling the inspection core, 25mm of water had collected over the floor slab, having seeped up via the test hole and from perimeter water weeping along the chimney breast brick coursings.

The sheer amount of water continually entering the basement space was thought by the waterproofing consultant to be too much for a sump and pump to cope with.

There was also a concern that the water collecting or draining under cavity floor membranes could be contaminated, as there were known to be underground drains in the vicinity. The waterproofing consultant began to veer away from a cavity-drained waterproofing system – contaminated water could not be tolerable under drained floors of a habitable space.

Looking at the exposed masonry underground at Islington, you would not be able to immediately apply cavity drain membranes with any confidence. First, quite an intensive preparation would be required, as the existing failed slurry coat should be removed down to a sounder substrate. De-bonded finishes cannot remain in situ, or you risk accumulation of debris in the air gap. Like any cavity wall, the gap serves a function, and has to be maintained. The exposed brickwork would almost certainly be uneven, and probably damaged by removal of previous attempts to waterproof, so at the very least the uneven and damaged substrate would need to be ‘dubbed out’, using a 3:1 sand and cement or 1:1:6 cement:lime:sand mix, to produce an acceptably flat finish for accepting the membrane sheets. This of course would add to the cost of a cavity membrane installation.

In this case the contractor opted for a thorough preparation, and the application of a multi-coat render waterproofing system. Remember too that cavity- drained systems have not been used in the UK for much more than ten years, so we are still learning about their installation and in-use performance. Waterproofed renders have a longer track record, although failures have not been that uncommon – but probably arising more from unsatisfactory application and substandard materials than inherent shortcomings of the method. Some old buildings have suffered foundation subsidence, which a rigid waterproofing has not been able to cope with, and so multi- coat render waterproofing has cracked and failed. Some cracking, at the wall/floor junction could now be designed out by installing patent reinforcing strips at the abutment.

Remember too that there can be hybrid solutions. You could, for example, apply cementitious renders to walls below ground, and add for good measure an inner drained membrane system. Sumps can be waterproof rendered and pumps incorporated as a belt-and-braces or further line of defence against water ingress. As mentioned in the main chapter, it is quite common to render walls but install a cavity-drained floor system. It is often the walls that present the difficult shapes and configuration for the relatively rigid cavity membrane sheets, but the floor will usually be flat (or perhaps at a slight incline!).

The contractor had definitely decided to go down the multi-coat render route to waterproof the basement (see the virtual pond in figure 2). For this contract, pre-bagged aggregates would be used, and manufacturers’ specifications strictly adhered to, so there would be a very good chance of the render system succeeding. Nothing underground is perfect, but the finished space was attractively finished and furnished – and thankfully dry!