Help & Advice

Basement Waterproofing 

Everybody in construction has a different idea about “how to waterproof a basement”. The world seems full of “basement experts” and “waterproofing experts”. So here we try to offer our thoughts on waterproofing in general, and the waterproofing of Polarwall formwork in particular, to achieve a permanently dry basement. 

All these waterproofing methods can (and should) work – but all can (and might) fail. The waterproofing design should be specific for the structure, for the location and for the soil conditions. There is no “one size fits all” approach. 

Waterproofing solutions are classed as either Type A, Type B or Type C. 

Basement Waterproofing Type A – External Membrane – there are several types of external membrane on the market, but all essentially do the same thing and create a waterproof barrier on the outside of the wall. The most common form of external membrane is a bituminous “stick-on” plastic sheet. The main advantages of this system is that it is relatively inexpensive and appears to be quite simple, but there are several disadvantages. Firstly, when things get wet on site (or even when there is high humidity) the membrane can lose adhesion , and it is often fixed incorrectly by the contractor (corners can be a weak area). These membranes are also frequently damaged during the backfilling operation. Another type of external membrane is painted (or sprayed) onto the external surface and this has less room for builder error, but it too can be damaged in the backfill if not properly protected. We would generally recommend the use of a paint-on (or spray-on) membrane, which could be either rubber or plastic based, and which will be put on in two coats, and which then should then be covered by a cavity drainage board which provides a degree of protection from the backfill. 

Basement Waterproofing Type B – Integral Waterproofing (Waterproof Concrete) – A chemical is added to the concrete to make the concrete impervious to any passage of water. This is great in theory but it often fails due to site conditions, batching plant problems or the inexperience of the people placing the concrete. If someone is experienced in the use of waterproof additives then everything should go fine, however it could prove to be a complete waste of money. With any ICF system, the formwork stays in place and covers the concrete face, which means that no-one can be certain as to how well the concrete has been placed and whether there are any voids or honeycombing or cracking. Some specifiers believe that waterproofing additives are a fool proof solution to the waterproofing conundrum, removing all possible site error from the solution. Unfortunately this is most definitely not the case. Most of these product warranties are invalidated by poor workmanship and are usually limited to the amount actually spent on the additive. Consequently, the refund of material costs would not cover anything like the cost of fixing a leaking basement. You should always read and understand the extent of the manufacturer’s warranty. We do not recommend the use of waterproofing additives, and nor do the real waterproofing experts, such as professional structural waterproofing companies. 

There is a British Standard for the design of a waterproof concrete mix – called BS8007 The Design of Concrete Structures for Retaining Aqueous Liquids. The crux of this standard is that if you put enough cement into the concrete with a low water/cement ratio then the concrete will be waterproof by virtue of the high cement content. It is interesting to note that several of the chemical waterproofing manufacturers require that their additives are mixed into a concrete with an even higher cement content and a lower water/cement ratio than is stipulated in BS8007 ! 

We suggest that waterproofing should be concentrated on Type A and Type C systems – but let’s spend just a little extra and stipulate a high-cement concrete with low water/cement ratio (to BS8007) into the concrete mix design. 

Basement Waterproofing Type C – Internal Cavity Drain – The thinking behind this solution is that the exclusion methods (described above in Type A and Type B) will probably fail and that some water will find its way through the basement wall. The fact is that a basement in decent free-draining ground conditions could have its waterproofing applied in the shoddiest way and still stay dry for ever, but if there is a build-up of water against the wall under pressure then the water will find its way through anything that is not 100% perfect in both materials and the way it has been applied. 

The thinking is that this ingress of water should be controlled and taken to a drain, or a sump from where it can be pumped away. Many people think this solution is admitting defeat in the waterproofing battle, but in reality, it is the method recommended and preferred by NHBC, Premier Guarantee and other warranty providers. In this solution any moisture coming through the walls is drained down inside the drainage membrane to a perimeter drain at the foot of the wall. From here it is either directed to a sump for pumping away or to a soakaway. In a pumped system the main danger is failure of the electric pump so it is normal practice to have two pumps in the sump chamber, and possibly a warning system to alert of failure of one of the pumps. 

As well as the internal drain it is always a good idea to include a Perimeter Drain – This is a land drain which runs around the outside of the perimeter of the building just below the level of the concrete raft. The purpose of the drain is to remove water from the external face of the building. The land drain should be covered with a free draining material such as gravel and protected with a geo-textile membrane. The drain should be designed so that it can be cleaned out and de-silted when necessary. We believe that where a perimeter land drain can be used effectively then it should be used. However, such a drain is not a waterproofing solution, but only ever a part of the overall waterproofing design. 

The design considerations of any new build basement should factor in soil type, conditions (including contaminants), the water table position, and the intended use of the structure. The extent of the risks leads to the choice of the waterproofing materials and it’s design. A belt and braces approach needs to be adapted to keep your basement dry.