NEW BUILD CASE STUDY 1 - Private Dwelling House

The Reasons For Using Internal Cavity Drainage and Water Control

As a company, Trace Basement Systems^TM, advise on many cases of flooded basements in both new build and existing buildings, and whilst this does not always involve standing water to a great depth, any amount of water in a usable basement is unacceptable and can cause great damage and stress. This is becoming more common because of the increased rainfall over the last few years in the UK, which is forecast to continue.

In undertaking our own work, and as you will see throughout our web site, we emphasize that the safest way to deal with water from the ground is to work with it by internally controlling, diverting and draining or pumping it away, this instead of attempting to 'hold back the dam' by the direct applied, external or internal, 'tanking' methods, that unfortunately form the current mindset in the UK.

In the past before the 'modern' tanking materials, such as adhesive membranes, painted products and cement additives, were available, our forefather used drainage, even going back thousands of years. We often excavate floors in old houses, finding drainage channels below formed of bricks or tiles to prove the point. One property about to be dealt with is a Manor House, the oldest part of which dates back 900 years and started life as Knights Templar Chapter House, and guess what was present under the stone flag floors? Yes, INTERNAL drainage channels formed of cut stone, draining out to low ground. What did they know that we don't!

Even where they used older 'tanking' products in the past, such as pitch, they will have incorporated drainage, downgrading the role of the 'tanking' to that of a vapour barrier.

Whilst modern 'tanking' in new build is usually backed up by land drains externally, our experience is that there are many pitfalls in doing so.

Unless vertical access pipes are included to allow periodical jetting out, such land drains are wholly unserviceable and  uncontrollable.

Even if such service pipes are included and kept clean, and even if they are correctly wrapped in graded stone and a geotextile  mat, the mat and stone may clog, and allow ground water pressure to build up against the external or internal 'tanking', which if  not perfectly applied, will inevitably fail.

In BS8102:1990, Protection of Structures Against Water From the Ground, amongst many factors it specifically states that one  must consider the consequences of less than adequate workmanship, the method and feasibility of repair in the event of failure, and the importance of removing ground water pressure before it comes to bear. 

What then if the tanking is applied less than  perfectly, particularly if external, how do you correct it, and how would one deal with a blocked land drain. The only answer to date has either been to excavate and repair externally, or to undertake secondary tanking internally, all at great expense and always in hope with fingers crossed that failure will not occur again when ground water next comes to bear.

Phil Hewitt of Phil Hewitt Associates, the pre-eminent expert in diagnosing waterproofing failures in the UK, and who runs The  School of Waterproofing, has shaken up the waterproofing industry by proving in the Outwing v Weatherald legal case that tanking on it's own without effective drainage, cannot be relied on, with the Judge taking into account, amongst others, the factors mentioned above, ruling that both the drainage and tanking medium form the overall system, rather than solely the tanking. The specifying designer of the system, that involved an adhesive sheet membrane and an external land drain had allowed the drain to be installed above the level of the floor, and when water subsequently penetrated internally, unsuccessfully argued that the tanking on it's own should have been able to withstand water pressure. The Judge ruled against this, specifically stating that because of the guidance in BS8102:1990 in respect of taking account of less than adequate workmanship, it is too much to expect a bonded tanking membrane to be perfectly applied. From personal experience this applies to any form of water blocking 'tanking'.

External waterproofing and associated land drains are commonly applied and laid by ground workers, who may or may not be adequately skilled in installation.

In instances,  the designer may solely rely on the 'tanking' material, they allow the land drain to be raised above dpm level, so when water comes to bear, it finds any small defect that occurred as a result of 'less than adequate workmanship'

For all the above reasons, when designing a safe waterproofing system, the emphasis must be on:

(ii)    Removal of ground water pressure before it comes to bear.

(ii)    Catering for less than adequate workmanship.

(iii)   Accessibility and feasibility of repair in the event of failure.

As an instance of how things can and do go wrong, in the week of preparing this addition to our web site, we were called to a 35 by 25m metre two year old storage warehouse filled from floor to eaves with packing cases. It's floor is nearly three metres below ground and water periodically stands upon the floor. THE LAND DRAIN WAS FORMED UPON THE TOP OF THE KICKER, ABOVE THE LEVEL OF THE DPM!

Needless to say, we advocate safer forms of waterproofing, and in the case of new build, can show an example in the photographs below.

Whilst a land drain was installed externally in isolation, it was sacrificial and not relied on because our drains are internal.

The Engineer catered for the structural integrity, whilst we took care of the waterproofing.

An existing house was demolished, the basement location excavated, the slab formed without any form of dpm included, but  incorporating a sump enclosure, with ground retaining erected.

An existing house was demolished, the basement location excavated, the slab formed without any form of dpm included, but incorporating a sump enclosure, with ground retaining erected.

Once the ground floor beams were in place, Newton 500 cavity drainage membrane and treated timber battens were applied to the walls, with Basement Systems^TM WaterGuard^TM drainage channels laid around the periphery internally.

We supplied a wide profile polyethylene dpm top form the dpc, linking it to the basement membrane, with a cavity tray formed. Even the basement windows were fully encased within the system.

The meshed membrane, Newton 500 clear meshed, was applied in the light well and linked to that internally, so that the internal and 'external' system were one. This was done before the free standing wall in the well was constructed, with sealed plugs fitted to accept wall ties.

A Basement Systems^TM UltraSump^TM was installed and linked to drainage channels. This has a primary mains powered and battery backup pump, and is state of the art technology from America. The channels include access ports to prove they remain clear. 

A lower screed (not shown) was laid up to the shutter timber seen against the drainage channels, the timber removed, stone infilled in the space, with Delta MS500 cavity drainage membrane (not shown) laid as the dpm, with a screed over. The walls were plasterboard, with the two air gaps in the system, one on the wet side of the wall membrane and one on the dry side, giving good thermal values. Furthermore fixings can be made within the plasterboard and battens without fear of breaching the waterproofing.

Whereas a 'tanking' system might work, this method does work and complies with BS8102:1990 better than any other form.

The light well drainage is linked to the internal system!

The sump is hidden below a kitchen unit, is alarmed and will be serviced every year. We have since changed the sump lid for a new one, that seen having become soiled during the construction process.

FINISH

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