Waterproofing systems

Cement renders can be applied to the internal faces of walls or as screeds onto floor substrates. The renders act as a barrier to wetness or dampness from the substrate.

Cementitious renders used for waterproofing typically incorporate waterproofing agents that are added in prescribed quantity to the mixing water.

How successful a multi-coat render system is in below-ground waterproofing will depend on a number of critical factors:

• Is the substrate suitable for application of a cementitious render system?
• Is the substrate correctly prepared for render application?
• Are the render materials of suitable type and quality?
• Are the render materials mixed appropriately?
• Are the render coats applied correctly?
• Are renders correctly detailed at junctions, intersections, service entries?
• Are the renders cured suitably?
• Is the integrity of rendering compromised by any subsequent construction or fixing activity?
• Is the performance of the render system compromised by application of unsuitable building finishes/paints?

You will appreciate that achieving reliable waterproofing requires considerable attention to design, detail, application and usage.

Multi-coat render systems are not designed to withstand structural movement. For example, Sika Ltd clearly state in their technical literature:

'Sika-1 waterproofing system is a rigid membrane and will not accommodate movement in a structure to which it is applied.'

So only specify this type of waterproofing when you are sure the building will not be subject to any movement cracking that could compromise its integrity. If you see cracks in basement walls during your pre-design inspection, you need to assess the cause of movement, and whether a multi-coat render system of waterproofing could be designed to accommodate it.

The BRE Good Repair Guide supports multi-coat cementitious render systems for waterproofing: 'correctly mixed render or compound, properly applied to a stable background, should last for many years'. Note the words: stable background.

There will be situations when a substrate is too friable to permit good bonding of cementitious renders or coating, but might be capable of being deep-drilled and plugged to support a cavity membrane.

Since around 1995, Sika Ltd, one of the leading suppliers and designers of waterproof render systems, has supplied mixed aggregates, pre-bagged and colour-coded. It was discovered that even pre-bagged and graded aggregates from mainstream builders' merchants were not always graded suitably for use in below-ground waterproofing where strict quality control of materials used is necessary.

The pre-bagging of aggregates in a controlled factory setting removes risk common on the building site. Sika Ltd also run training courses for recommended contractors to help maintain standards on site. Below-ground corner beads can compromise effectiveness of waterproofing if they corrode or affect continuation and thickness of coats at an external corner or return. Check returns with a metal detector: if metal is flagged up, you could have a potential waterproofing weakness at that position, if a corner bead is in-situ and most particularly any kind of bead that could corrode or deteriorate in damp conditions.

Pre-bagged aggregates are visible stacked to the left side, so there is a good chance this waterproofing application will be successful. Make sure those carrying out multi-coat rendering application know the correct specification for the materials and know how to apply them. Always refer back to the product manufacturer for specification information.

Typical render application for Sika 1

Key stages for floors:

1. Hack off existing finishes.
2. Fill holes, apply fast-acting sealers to stop water ingress.
3. Wet the substrate.
4. Apply splattered spritz coat.
5. Apply second render coat.
6. Apply finishing render coat.

The spritz coat is not 'laid on' using a trowel, but spattered on using a throwing action. This is not a standard plastering technique.

Fixings

A well-executed waterproof render system can fail if there is lack of care in designing and installing fixings.

One method is to affix a softwood batten to the render finish, into which fixings could be screwed or nailed. Sometimes it is possible to use struts wedged between wall and floor to produce the necessary rigidity to hold components in place. Heavy fixings may require the formation of deep pockets cut into the substrate. In all cases, the manufacturer of the render system must be consulted.

Skirtings and other lightweight linings can simply be glued to the render system.

It is important that only adhesives recommended by the manufacturer are used.

You could build an inner wall to solve the fixing problem. The BRE in Understanding Dampness (figure 6.52) shows how this can be carried out. Building an additional inner wall takes up space, which could be limited, and is an additional cost. But should the below-ground space be intended for use, for example, as a gym – requiring heavy equipment to be fixed off a wall or floor – then it may benefit from an inner wall and perhaps an additional loading floor over the multi-coat waterproof screed.

Considerable care is needed in fixing joinery, particularly skirting boards. Standard nail or screw fixings can defeat the waterproofing – most specialist contractors use strong contact adhesives for joinery fixing.

Penetration by services

The trick is to minimise the number of points where pipes or cables penetrate through waterproofing. Consider rerouting services, so penetration of a waterproofing system is unnecessary. Patent methods and materials are available from manufacturers to seal existing or new service entries, and the advised detailing must always be followed.

Curing

Once the render has been applied, careful curing is essential. Often too little attention is paid to careful 'curing' of renders, either from lack of awareness or lack of care. Typical symptoms of unsatisfactory curing are cracks in the renders, which can compromise performance.

Curing of cement renders does not happen quickly. Maximum strength is achieved after around 28 days. Renders must not be  allowed to dry out too quickly – that is, before curing has been fully achieved. This means that in hot and windy weather renders should be occasionally sprayed with water, or covered by plastic sheeting, etc. Sika advise that in hot, dry or windy conditions, covering or spraying the render may need to continue for up to 7 days. If water evaporates out of the render mix too rapidly, the render might not achieve maximum strength and performance.

You may be called upon to check a waterproof system showing cracks. Note that in some systems, surface 'micro-crazing' in the top layer might not be significant enough to compromise waterproofing performance. It may be that the top layer serves as a protection for the main waterproofing layers. Micro-crazing must not be misinterpreted as curing shrinkage, drying shrinkage or substrate bond failure.

We need to control cracking. If too much water is used in the mix, the render can suffer cracking from drying shrinkage. Cracking may be reduced by enabling the render to properly cure, using a water-reducing agent in the render mix, making sure the render is well bonded to a sound substrate, using sands with a low water demand, and limiting any 'dubbing out' to 10mm maximum thickness. Cracks, if they do form, fall more or less into 2 classifications: surface cracks that can simply be filled, and cracks that could reduce the strength or waterproofing quality of the render.

As a general rule, cracks greater in width than 0.25mm can enable passage of moisture and would not be acceptable in waterproofing renders. Cracks are more significant if they affect more than 1 render coat at the same position. The greater the hydrostatic pressure, the more likely it is that a crack will enable the passage of water. The implication here is that, if you find cracks that for the moment do not allow water to seep through, in the future, if water pressure increases, the same cracking could fail to keep water out. Waterproofing performance always needs to be assessed over its expected design life.

Multi-coat renders are what they say they are – multi-coats. If the crack under investigation is limited to the top skim plaster coat, it may be nothing more than superficial – a crack to fill before painting. But if the crack is found to continue through successive layers of waterproofing, things become progressively more serious. A crack that is found to extend through all the render coats is in effect a complete puncturing of the waterproofing. So to assess cracking, first check that the crack is limited to a surface skim coat crack. Permission and site conditions allowing, you can then investigate to greater depth.

Decoration

Paints should not be applied until the surface is dry.

If a multi-coat waterproofing system has been finished by a skim coat of plaster, for example 'multi-finish', you might find it useful to inspect the bagged plaster, as the applied finish will not be dry until it lightens to the powder colour. A 'multi-finish' skim coat may not be ideal in a basement or ground floor application – or indeed anywhere where it could be affected by moisture – as such a finish is gypsum-based and not very tolerant of damp. It also contains sulphates, and unsightly efflorescence can form on the plaster surface from drying out. Consult the render mix manufacturers regarding appropriate finish as there may be patent plaster preparations available.

An earth-retaining wall behind a tanking system will never dry out. As waterproof render systems are themselves vapour permeable, you must not allow the decorative finish to act as a vapour check. This means that you should use vapour permeable paints on tanking plasters – certainly a paint that is no less vapour permeable than the tanking itself. Only use paint recommended by the contractor and checked out in consultation with the paint manufacturer. Also, agree the timing of the paint application with the waterproofing contractor – or again, the contractor might argue that it was applied too early. Better still, commission the waterproofing contractor to decorate. Assuming that defects can happen, never give the contractor the opportunity to blame the paint.

There must have been thousands of disputes where chemical injection damp-coursing above ground has failed, but the damp-proofer has escaped liability when associated plastering was carried out by others. Damp-proofing, if needed, depends more than anything else on the standard of internal remedial plastering which acts to mask damp behind. (We can never assume that chemical injected dpcs actually stop damp.)

Avoid using multiple contractors for specialist remediation of damp. As for above-ground damp-proofing, multiple contractors could mean multiple blame – so always make sure there is a single point of responsibility from a main contractor for work carried out.