Negative vs Positive Side Waterproofing
Posted October 29, 2020
Questions often arise over the difference between and requirements of positive and negative side waterproofing.
Positive side waterproofing is the predominant type of waterproofing applied to new constructions. This is the application of a waterproof membrane on the outside of a building – to prevent groundwater from entering the brick or substrate.
The downside to positive side waterproofing is it must be done at the time of the build, not down the track after leaks have been detected. It is also inaccessible after installation for repairs since it is covered by soil – therefore any repairs (however small) require removal of topsoil, landscaping, grass, dirt etc – this is a costly and time-consuming process.
Positive side waterproofing – if done correctly – protects the interior of a building from moisture and protects the structural components – such as the steel within concrete structures – from water infiltration.
Negative side waterproofing is applied to the inside face of the substrate – it is used primarily where water needs to be withheld from entering a space. Negative side waterproofing prevents water from entering an occupied space however it does not prevent it from entering the substrate from the outside of the building. Therefore, the structural integrity of the concrete could still be compromised.
Being fully accessible after installation means that negative side waterproofing is easily repaired and leaks or damage are easy to locate. Negative side waterproofing also removes the need for subslabs and well pointing for foundation waterproofing – which positive side waterproofing requires.
Considerations such as the chemical-content of the ground water, freeze and thaw cycles, interior humidity and more need to be considered when choosing between positive and negative side waterproofing. If ground water contains corrosive materials, or if there is exposure to freeze/thaw cycles or will there be interior humidity limitations? If the answer to any of these questions is yes, positive side waterproofing should be considered.
Most waterproofers agree that application of a positive side waterproofing membrane on a new build is the most effective, however issues do arise when damage occurs to the membrane, ground movement causes leaks etc. Therefore negative side waterproofing is an essential remedial service, preventing costly damage to the interior of buildings.
The installation of any waterproofing system on the negative side (or inside) of a building is exposed to the added risk that the membrane could become disbonded by the infiltration of groundwater from the outside of the substrate – either in vapour or liquid form. Though it is not always economical to do so, a negative side bulkhead wall can be installed to hold the waterproofing in place. This method may prove to be too costly, however should be considered on a case by case basis.
Ideally, all new structures should have a positive side membrane professionally installed, and negative side waterproofing reserved for remedial applications where necessary.