Basements and External Tanking Repair.

So the subject of external tanking repair came up at work today in relation to a property we are advising on, and I thought that it would make for an interesting post.

Over the years I’ve dealt with many waterproofing systems where the original system fails, then someone attempts a repair which then also fails and then we are called in at the point that they definitely need to fix it.

This includes external barrier tanking systems which in principle are designed with the intention of providing a physical barrier to exclude water.  Often peoples first thought when it comes to new-build waterproofing is that we/they must totally block water out, and keep the structure dry.

This can result in the specification and installation of externally applied barrier tanking systems, where total and complete reliance is placed on the tanking.  These external systems are subsequently ‘buried’, and when problems do occur, it is not always practical to excavate and access the system again, however large holes are sometimes excavated and tanking systems are patched, or even replaced wholesale (at least to the walls), and then in time they fail again.

The following are some examples of this exact scenario, which I recalled when considering this today:

This was one of a number of townhouse style dwellings constructed into a split level site, being three storey (above ground) to the front and two at the rear, therefore with a lower ground floor / semi-basement level, retaining ground to the rear.  The structure involved a reinforced masonry retaining wall, and as per the photos below you can see where it leaked, and the inner leaf block and the concrete core where a block has been removed.

Opening up undertaken internally prior to our attendance, note the tell-tale dehumidifier.
Opening up undertaken internally prior to our attendance, note the tell-tale dehumidifier.
Reinforced masonry with inner block removed.
Reinforced masonry with inner block removed.

The original waterproofing was an external adhesive bitumen sheet membrane system; however this failed at an early stage before the properties were finished and occupied, and so they were able to get a digger/excavator around to the rear of each property, allowing the retained earth/backfill material to be removed.  The bitumen membrane was then stripped and replaced with a bentonite clay membrane, before the backfill was replaced.

It took several years but then the bentonite system failed, and this was when we were asked to advise.  We subsequently designed and installed remedial cavity drainage waterproofing systems within three of the properties.

This was another residential property, detached and constructed into a sloping site with a garage and finished stairwell areas all with ground retaining elements.  The garage and stairwell suffered groundwater penetration and a contractor subsequently excavated around the exterior, replacing the existing external tanking system with a new adhesive bitumen sheet membrane system.

Backfill after the initial repair (replacement external bitumen sheet membrane).
Backfill after the initial repair (replacement external bitumen sheet membrane).
Saturated garage floor construction.
Saturated garage floor construction.
Deterioration of low level plasterboard & finishes within the habitable stairwell area.
Deterioration of low level plasterboard & finishes within the habitable stairwell area.

The replacement external tanking also failed in a short time frame and we remedied, again using internal cavity drainage waterproofing.

Example no. 3 is a self-build type large detached structure, with the typical reinforced concrete raft, reinforced masonry retaining wall spec., with external adhesive bitumen sheet membrane.

For whatever reason a means of escape had not been included in the original design and Local Authority Building Control called for a retrofit escape/light-well to be added.

This meant cutting and splicing to the existing bitumen tanking membrane and of course this subsequently caused a problem of water penetration. The ground was excavated in that locality and tanking repairs were attempted; however this did not resolve the issue.

Groundwater penetration at the junction of the added escape well and the existing structure, after an external tanking repair had been implemented.
Groundwater penetration at the junction of the added escape well and the existing structure, after an external tanking repair had been implemented.
Water collecting within the screed at low level.
Water collecting within the screed at low level.

When groundwater continued to penetrate we designed and installed a remedial waterproofing system using internal cavity drainage.

So there you have it, external repairs at least in my experience do not always provide the required outcome.

In fact when external tanking systems do fail, the internal penetration is usually at low level (wall floor junction), and issues often occur years after the original installation when homes are finished and occupied.  At this point it is not particularly practical to dig down circa three metres through driveways and gardens, moving tons of earth, to undertake a repair with intrinsic risk.

The risk comes from difficulty in knowing the exact point of the defect in the tanking membrane – since water can track within the depth of some types of structures and/or internal wall/floor build-ups, this meaning that the point that you see water internally might not match up with the position of the defect externally.

You can also not ever fully replace an external tanking system, since you cannot access the tanking below the foundations. You are not linking to a pristine material as they have often been buried for years, and combined with working in a muddy wet hole, it is not surprising that perfect barriers to exclude water are not then formed.

If you do go to the expense of excavating, then try to repair the tanking before back-filling etc., in the event that it subsequently fails again in time, what do you do?  Spend again on excavation for something that has already been trialled and failed?  Most people cut their losses and seek a right first time solution.

Where right first time solutions are required (with provision of guarantee), the typical repair is to employ internal cavity drainage, in fact in many cases external repairs are not even attempted because of the risk that such investment yields no benefit and this is money which could be better spent on a lower risk cavity drainage based solution.  While internal cavity drainage might be higher initial cost, it isn’t the potential false economy that an external repair might be.

Having said all that, I have successfully remedied issues with an external approach but it entirely depends on the circumstances and the ability to DESIGN an appropriate solution.

J

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