Written by Delta Membrane Systems Limited.

One of the most interesting questions in the waterproofing sector is “What is positive side” or “What is negative side” waterproofing. It is a great question and one that gets us interaction hopping from the get go.

Let us firstly look at the types of waterproofing. There are many different approaches to structural waterproofing. The construction methods will in part contribute to the specification of types of waterproofing systems (positive or negative side) and may also determine the overall structural waterproofing strategy.

Structural waterproofing falls into 3 categories:

Type A – Barrier Protection
Type B – Structurally Integral Protection
Type C – Drained Protection

With 3 grades:

Grade 1 – Some water seepage and damp are tolerable depending
on the intended use
Grade 2 – No water penetration is acceptable
Grade 3 – No dampness or water penetration is acceptable

From these categories we can offer designs based on:

•Positive side waterproofing
•Negative side waterproofing
•Integral waterproofing

Integral waterproofing (Type B Structurally Integral Protection) is comprised of admixtures that are incorporated into the concrete mixture during the initial pour and are not usually seen as a “positive” or “negative” side waterproofing solution.

On some occasions, warranty providers may ask for two (2) forms of waterproofing, meaning both the positive and negative sides are offered protection.

What is Positive side waterproofing?

Positive side applied waterproofing is an external waterproofing system. A waterproofing system installed to the exterior face of the below ground structure. A suitably designed positive system can protect the structure and the interior against the effects of moisture and pressurised water.

A positive side applied system takes the water pressure in conjunction with the structure to which it is applied to effectively create a permanent consistent barrier.

What is negative side waterproofing?

Negative side applied waterproofing is applied to the internal surfaces of the structure, usually when there is no access or ability to either get access to the access to the external surfaces or where continuity is not possible. An internal negative side system is often reliant on the ability of those surfaces in accept the waterproofing treatment.

A general rule of thumb is that an internally applied system is as good as the sum of the integrity of the structure and the preparation undertaken. Negative side waterproofing in some instances has the capability of dealing with 13 bars of pressure.

Both positive and negative waterproofing systems have pro’s and con’s – For example, a piled structure cannot be waterproofed on the positive side and it is also difficult to waterproof structures where access is limited. Internal waterproofing for example, can be applied to piled structures because access is not a problem and continuity is not a problem.

Full site investigations are important when mitigating and reducing risk in waterproofing design. The results will have a bearing not only on the design but on the waterproofing system itself (positive (external)/negative (internal).

Whilst findings of a site investigation are often seen as conclusive, it needs to be remembered that this is just a ‘snapshot in time’. It should be assumed that water will come to bear against the full height of the below ground structure at some time in its life.

The system designed may be acceptable at the time of construction meeting the ‘snapshot in time’ investigations, however this system may not be adequate for environmental changes of the future.

The decision to use positive side or negative side waterproofing can be made after exploring:

•Changes in water tables
•Changes in hydrostatic pressure
•Thermal changes
•Dynamic Movement
•Heave
•Subsidence
•Clay soils
•Changes in environment
•Stress
•Changes in ground drainage
•Structural stability