We often see designs from those new to cavity drain waterproofing that show a 20mm stud floor membrane laid above a level raft or slab. Relying on just the membrane is a dangerous method of the use of cavity drained membranes and was the reason for many failures back in the early days of this form of waterproofing in the mid to late nineties, before the implementation of drainage channels. The membrane is not capable of resisting water pressure, indeed even a few millimetres of water pressure is enough to see water pass through the seals at the joints in the membrane, leading to failure.
Issues with Free Lime and BS8102
Another issue with this design is that the water is standing over a new raft or slab that has a significant amount of free lime available. The water draws the free lime out if the concrete leading to significant limescale deposits, often in enough volume to fully block the 20mm cavity. Once failure has occurred, the only remedial action is rip up and start again and so the design falls foul of the requirement within BS 8102:2009 (Code of Practice for The Protection of Below Ground Structures From Water From the Ground) for the designer to take into account the form and feasibility of remedial works. To be blunt, laying a drainage membrane over a flat substrate is a very poor design that is fraught with issues and should not be considered in any circumstances.
Advantages of the Basedrain Drainage Channel
Newton introduced the Basedrain drainage channel in 2000. The channel is placed either below (in the case of a wall kicker joint) or above (in the case of a construction joint to the raft or slab) the joints in the structure and so intercept the water as it enters the structure. The drainage channels deliver the water directly to the sump, not moving the water over the expanse of raft or slab as is the case with a full floor drainage membrane.
The channel network is accessible and so maintainable. It can be inspected and cleaned if necessary, meaning that even critical remedial works can be done without disruption. The channel is 50mm deep and the floor membrane always placed above it. This gives a total maximum floor drainage void of 70mm against a maximum of 20mm for just the floor membrane. Water within the 50mm deep channel forms its own hydraulic gradient meaning that the water moves freely within the channels and to the sumps. The resulting system is much safer and is maintainable.
A spacer is required to keep the floor membrane above the height of the 50mm deep drainage channel. This spacer can be of screed, concrete or of closed cell flooring grade insulation. The closed cell nature of the insulation means that the cell structure does not allow for movement of moisture from cell to cell; water cannot move from the surface into the main body of the insulation. These insulation boards are designed to be used on the outside of the structure, where it is permanently wet, for the whole of the building life expectancy.
Alternatively, the drainage channel can be cited within a 50mm deep recess. In this way the membrane is again elevated to a position above the drainage channel.
In all cases, Newton 106 Lime Inhibitor should be applied to all concrete surfaces to prevent the leaching out of free lime.
BS 8102 Recommends A Maintainable Solution
We also often see designs that show a 20mm stud floor membrane laid above a raft or slab that has been profiled to create slopes to the sumps or to floor gullies. This is not wrong, there are instances when we specify our 20mm Newton 520 in this way, but it is more difficult and costly than other methods available and is not maintainable, and so falls foul of the requirement within BS 8102:2009 for the designer to take into account the form and feasibility of remedial works. If there is a failure, the whole of the floor membrane needs to come up as the whole of the floor membrane is the drainage conduit. I used the word ‘slope’ instead of ‘fall’ as the concrete is not formed to falls but is actually formed to rising slopes. This is because the concrete cannot be lowered in thickness (for structural reasons and to ensure the steel is fully covered by a minimum thickness of concrete) and so is actually thickened to form the slope.
This thickening of the raft has the following consequences that are often overlooked but can add huge cost to the project:
- Where the raft is thickened to form the slopes, the gradient has to be taken out within the floor finishes, normally the screed. Once again, a minimum thickness is required and so the screed is thickened to take out the slope. The extra concrete and screed required over a large structure can be enormous. For example, if to get the slope required, 1:50 is recommended, the slope in the raft or slab rises by 100mm, the screed also needs to be thickened by 100mm to take out the slope. This equates to an extra 50mm depth of concrete and 50mm of extra screed. The cost of the materials and labour to add this extra product is very expensive.
- The extra 100mm of depth required to form the slope and then take it out has to be dug out the ground. The cost of removing an extra 100mm of ground and disposal of that extra soil and the trucks required to move it to the disposal point is very expensive.
- Forming modern concrete to make slopes is not as easy as it sounds. Modern concrete is usually pumped and is relatively low in viscosity – almost to a point where it will self level. Creating rises in liquid concrete is not easy; ask any ground worker. The result is often a mess of attempted falls that in truth become troughs that instead of allowing water to flow, capture water in large puddles, often deeper than the floor membrane, resulting in membrane failure. Below is a drawing which shows our typical designs for our Newton 500 Cavity Drain Membrane System using our preferred method of draining i.e. perimeter basedrain.
Below is a drawing the most common way of collecting and evacuating the water i.e. a sump and pump and also a drawing showing an inspection port which allows the drainage of the system to be maintainable and therefore conforms to the requirements of BS 8102.
Considerations for New-Build Structures
BS 8102:2009 (Code of Practice for The Protection of Below Ground Structures From Water From The Ground) defines 3 types of structure and waterproofing i.e.
- Type A – Structure has no integral protection against water-tanked protection
- Type B – The structure itself is constructed to be integrally waterproof and the primary resistance against water.
- Type C – It is accepted that water could enter the building and an internal cavity is provided to depressurise and manage the water.
With regard to new-build structures we will often suggest that you to consider using a combined waterproofing approach using
- internal cavity drain membrane (Type C)
- an external system (Type A) or
- an integral waterproof structure (Type B)
Furthermore BS 8102 recommends that consideration should be given to the use of combined protection (i.e. Type A and Type B, Type A and Type C or Type B and Type C) where in a single system:
- the likelihood of leaking is high
- the consequences of leakage is unacceptable
- additional vapour checks are necessary for a system where unacceptable water vapour transmission could otherwise occur.
When we are designing for habitable space we will generally require a completely dry internal environment defined as a ‘Grade 3’ by the BS 8102. Whichever combination of waterproofing is chosen to achieve the Grade 3 environment we would always recommend that one of the forms of waterproofing is an internal cavity drain membrane system. The choice of the other system is largely dictated by the type of structure.
Newton Recommendations For The Safest Design
In our mind, the safest design is obviously a combination of Types A,B and C but usually two forms is enough and therefore depending on the type of construction we will usually recommend Type A + Type C, or Type B + Type C. Combining Type A + B is not as safe in our view.
Newton sell waterproofing to all Types and combinations and we are always pleased to help with designs for your projects.
The Advantages of Using a Newton Specialist Basement Contractor
We will always recommend that the system be installed by one of our registered contractors. If they are involved in the design work early enough and then undertake the work they will ultimately be responsible for both the design and the installation which means that they will give meaningful, insured installation guarantees for the work they undertake and with most of our contractors having suitable PI allowing you to delegate the full design liability for the waterproofing aspect of the project to them. They are approved installers of a number of waterproofing products and as such will, without bias, put forward a design using the system or systems which they feel will be the most suitable, trouble free and effective in the given circumstances. BS8102 calls for a waterproofing specialist to consult as part of the design team and this can be a Newton Specialist Basement Contractor. They are aware that it is usually not possible to nominate the waterproofing specialist in a competitive tender situation but still would appreciate the opportunity to put forward their design and costs at an early stage.
For any further information please e-mail firstname.lastname@example.org, or Request List of Newton Specialist Basement Contractors
Author: Toby Champion, Newton Waterproofing Systems