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Last edited 20 Apr 2018
Nothing within the construction industry is totally without risk, and perfection does not exist. However, when a structure is under a water head, that water will find the slightest defect and permeate the structure.
It used to be thought that, where high water tables existed, waterproofing was best applied to the outside of a structure, and it would only be applied internally if there was no other way, such as with existing structures.
However, current thinking is changing. It can be shown that internally applied systems are capable of withstanding significant heads of water, provided the structure to which they are applied is sound. It has the significant benefit that, should problems occur, the system is accessible for remedial work.
By understanding the principle involved in waterproofing a structure, the risk of failure of a system can be reduced to an acceptable level. Further, with a little thought, it is possible to design a system where, should problems occur, remedial work can be carried out without excessive disruption.
 What is dry?
Nothing is absolutely dry. Water, in the form of liquid or vapour, will always exist bound up in building materials as well as in the air. However, a building will be perceived as being dry if the moisture present does not present a problem for its inhabitants or contents.
For example, an underground car park at 75% relative humidity, but which has a few minor damp patches in out-of-the-way corners and condensation on the walls, may be considered to be dry. Whereas, an office, which has no visible dampness at all but has a relative humidity of 75%, may be perceived as being damp, because papers could curl and the occupants would find the atmosphere muggy.
Table 1 of BS 8102:2009 (Code of practice for protection of below ground structures against water from the ground) defines performance levels for the dryness of buildings in four grades, as follows:
|GRADE||BASEMENT USAGE||PERFORMANCE LEVEL|
|1||Car parking; plant rooms (excluding electrical equipment): workshops||Some seepage and damp patches tolerable|
|2||Workshops and plant rooms requiring drier environment; retail storage areas||No water penetration but moisture vapour tolerable|
|3||Ventilated residential and working areas. including offices,restaurants, leisure centres, etc||Dry environment|
|4||Archives and stores requiring controlled environment||Totally dry environment|
Table 1 does not define what it means by 'dry' environment or 'totally dry' environment for Grades 3 and 4, nor how much moisture vapour is tolerable for Grade 2.
However, it would be reasonable to assume that the dryness level required for the Usage Grades could be summarised as follows:
- Grade 1 - Some leaks of water permissible. No requirement for relative humidity control.
- Grade 2 - No leaks permissible. There will seldom be a need for relative humidity control.
- Grades 3 & 4 - No leaks permissible. Relative humidity to be kept to a level appropriate for the usage of the basement.
 Vapour control
BS 8102 implies (it does not actually say so) that in order to achieve a Grade 3 or Grade 4 environment, a waterproofing system that will stop water vapour as well as liquid water must be used. Nothing could be further from the truth.
Where water tables are low, it can be shown that for Grades 1, 2 & 3, the internal vapour pressure is usually higher than the vapour pressure in the ground. In other words, water vapour will tend to move out of the basement, not into it. It is only with Grade 4 that internal vapour pressure will usually be less than that of the ground, and then the difference in pressure is so small that vapour movement will be negligible.
When water tables are high (BS 8102 says that for basements in excess of 4 m deep we must assume water tables to be 1 m below ground level), water vapour will penetrate, but at such a low rate as to be of no significance. For example, it can be shown that a Type B structure, 10 m x 10 m x 3 m, 300 mm thick, under a 10 m head of water and at 15°C internal temperature, will allow vapour penetration at such a rate as to increase relative humidity by 1.5% in 24 hours, assuming that the humidity is not removed during that time period. (Cementitious waterproofing systems allow vapour in at an even lower rate.)
Clearly, since building regulations require that air is removed several times an hour, this vapour penetration is of no consequence.
To achieve an acceptable level of dryness in a basement, two distinct and separate functions need to be carried out:
- Reduce lateral penetration of water to an acceptable level (Usage Grade 1) or stop it altogether. (Usage Grades 2, 3 & 4). This is achieved by instituting the appropriate waterproofing procedures.
- Once tile appropriate heating has been installed, reduce relative humidity to an acceptable level depending on the Usage Grade requirement. This is achieved by instituting the appropriate ventilation, dehumidification, or air conditioning systems.
 Product types
In basement refurbishment or upgrading situations, waterproofing systems will invariably need to be applied internally. It is very rare that walls can be exposed to allow an external positioning of the system and, on the few occasions when they can, it is usually impossible to apply the membrane under the structure. For this reason, only internally applied systems are considered in this paper.
There are several generic waterproofing systems, but most of them cannot be applied internally unless a loading coat, such as an internal brick or block wall, is constructed to keep them in place. Whilst this has been quite frequently done in the past, it takes up a lot of space, and is uneconomical compared with systems which do not require a loading coat. With better understanding of fully bonded internal systems, and also the advent of cavity drain membranes, 'loaded' systems are tending to give way to unloaded systems in upgrading situations.
Three systems which do not need loading coats are the cementitious coating, the multi-coat render, and the cavity drain membrane.
 Cementitious coatings
Cementitious coatings are premixed compounds comprising cement, graded aggregate and chemical additives. They are supplied in powdered form to be mixed with water on site and applied by brush, trowel, or spray, to form a coating between 1 mm and 3 mm thick. They can be applied directly to a sound substrate, or they can be applied to a render basecoat previously applied to the substrate. They can be polymer modified to improve adhesion, elasticity and flexibility. They are used for waterproofing basements or water retaining structures by external or internal tanking, and for sealing hairline cracks in concrete structures.
Cementitious coatings have no structural integrity of their own, and rely on the soundness of the substrate to keep them in place. Cementitious coatings can be applied directly to concrete, achieving an economical waterproofing system. On masonry, a render coat will normally need to be applied, except in exceptional circumstances, such as when the substrate is new.
 Multi-coat renders
These are cementitious renders and screeds modified with chemical additives. The modified mortars are applied by conventional rendering or screeding techniques in several layers. The mortars are usually batched on site, with the chemical additive being added to the mixing water. The number of render layers and final thickness are dependent on the conditions likely to be experienced and are specified by the relevant material manufacturer. They are used for waterproofing basements or water retaining structures by external or internal tanking.
Both multi-coat renders and cementitious coatings are capable of being applied in very wet conditions. Where flooding is occurring during application, accelerators and plugging compounds can be used to enable the materials to be applied.
Like cementitious coatings, multi-coat renders rely on the soundness of the substrate to keep them in place and working effectively.
 Cavity drain membranes
These are high density polyethylene (HOPE) sheet materials, which are vacuum-formed to produce domes which, when the sheet is fixed to the walls and/or floor, create a cavity between the sheet and the substrate. Any water leaking through the structure is then diverted through the cavity to preformed drains or sumps. This water is then allowed to drain away naturally, or is pumped away.
There are several reputable and well established companies which manufacture and distribute the above systems, and who have in place the appropriate quality control systems which ensure consistency of the products.
These manufacturers are able to offer a high standard of technical backup, and they should always be approached in the first instance for their product literature and, where appropriate, specific advice. Most of them will also visit site if they are uneasy about a specification.
- In refurbishment situations, it is normal for waterproofing systems to be applied internally.
- Systems which need no internal loading coat are cementitious coatings, multi-coat renders, and cavity drain membranes.
- The manufacturer of the chosen system should always be consulted for their literature and advice.
To design an appropriate waterproof system, the structure needs to be surveyed and assessed to establish the following:
- The intended usage of the basement area.
- The existing form of construction.
- The existing ground conditions, e.g. height of water table, contamination.
- The suitability of the structure to accept the chosen system.
Once the above points have been established, the appropriate waterproofing system can be selected.
This information will originate from the client. It must then be compared with Table 1 of BS 8102, to establish the appropriate performance level.
 The existing form of construction
BS 8102 describes three forms of construction, Types A, B & C.
Type A structures may be constructed out of masonry or concrete. If a waterproofing membrane was included during construction, it may be found internally, but on older structures it is more likely to be found on the outside of the structure, or sandwiched between two skins of masonry or concrete.
[Above: Type A structures - tanked protection]
Type B (structurally integral protection) structures require the structure itself to be constructed as an integral water-resistant shell. They will usually be constructed out of reinforced concrete to an appropriate design code such as BS 8110 or BS 8007, which gives guidance on the grade of concrete to be used and the steel spacing.
[Above: Type B structures - structurally integral protection]
Type C (drained protection) structures incorporate a drained cavity within the basement structure. There is permanent reliance on the cavity to collect groundwater entering through the fabric of the structure, and direct it to drains or a sump for removal by drainage or pumping.
[Above: Type C structures - drained protection]
 Existing ground conditions
Ideally, a thorough soil survey (to BS 5930) should be commissioned for the site where the basement exists, if an existing survey is not already available. All too often, this stage is omitted because of cost. However, on larger projects or where difficult conditions are known to be present, a soil survey could save a lot of money and hassle.
If this information is not available, then it should be assumed that the water table will be high. (See BS 8102, Section 2, Paragraph 3.4.b.)
Armed with the above information and after completing an inspection of the property itself, a decision needs to be made as to whether the structure can accept an internally applied waterproofing system.
The following points need to be considered:
- If there are high water tables, will the substrate be sound enough to resist the induced tensile and bending stresses?
- Further, will the structure be able to resist the uplift forces induced by a high water table?
- If an internal cementitious system is to be applied, is the substrate sound enough to prevent the system from debonding?
- Is there any form of contamination within the structure or the groundwater which could affect the long term durability of the system?
If there are doubts about any of the above, then a structural engineer should be consulted.
 Selecting a generic system
- If the structure is an externally tanked Type A form of construction and there is moisture ingress, then serious consideration needs to be given to applying a completely new waterproofing system internally.
- If the structure is an internally tanked Type A form of construction and there is moisture ingress, then the point of ingress needs to be established. A decision will need to be made as to whether or not the defects can be repaired. If not, then either the existing system must be removed and replaced, or a cavity drain membrane can be considered, effectively turning a Type A form of construction into a Type C.
- If the structure is a Type B form of construction but there is moisture ingress, then consideration can be given to carrying out localised repairs. However, sometimes the leaks will act as pressure release points and sealing them can cause hydrostatic pressure to build up externally, resulting in moisture ingress elsewhere. If this occurs, then at some point consideration will have to be given to applying either an internal waterproofing system, (turning the structure into a Type A form of construction) or a cavity drain membrane (turning it into a Type C form of construction).
- If the structure is a Type C form of construction but there is moisture ingress, then an investigation needs to be undertaken to establish why it is leaking. It could be that the cavities and/or drains have become blocked, and simply unblocking them may rectify the situation. If not, then consideration needs to be given to removing the existing cavity system and replacing it with another internally applied system, although there may be several other options available, depending on the form of cavity construction.
Having fully assessed the structure, a decision needs to be made as to whether a cementitious coating, a multi-coat render, or a cavity drain membrane is to be applied. Any one of the above systems can be applied equally effectively. However, the following points should be considered before final selection:
- Concrete, and brickwork/blockwork with a suitable surface, can have a cementitious coating applied directly without the need of additional renders. This can result in significant savings.
- Where speed is important and noise and disruption must be kept to a minimum, cavity drain membranes are effective.
- Cavity drain membranes act as a vapour check, so care needs to be exercised to ensure that interstitial condensation does not cause problems.
In addition to the above, where water tables are high the following points need careful consideration:
- Cementitious systems, because they are fully bonded to the internal face, will cause the substrate to go into tension when water tables rise. If the substrate cannot accept the induced tension, then consideration needs to be given to applying a cavity drain membrane. Alternatively, the substrate can be structurally lined with concrete or shotcrete, incorporating a membrane on the internal surface.
- Problems can arise where the groundwater carries high levels of aggressive contaminants, such as sulfates. In these circumstances, an external membrane needs to be applied but, as discussed, this is not usually practical. A convenient way around the problem would be to provide an internal structural lining, which sandwiches a waterproof membrane between the old structure and the new lining. A common way of applying the membrane is by 'reverse tanking', as described in the Waterproofing Basement Design Guide.
If there is any doubt about the suitability of a particular system, the manufacturer of that system should be contacted for advice.
- Establish the usage of the basement from the client, and establish the performance level required from Table 1 of BS 8102.
- Establish the form of construction - Type A, B or C from BS 8102.
- Check ground conditions. If the level of water table cannot be established, assume it to be 1m below ground level, or one quarter of the depth of the basement, whichever is the lower figure.
- Assess the structure's ability to accept the system under consideration. Will the substrate go into tension if water tables rise? Will the substrate accept this tension?
- Check to see there are no aggressive contaminants within the soil.
- Finally decide on the generic system. Will it work? Can a more economical solution be found?
 Application methods
Application methods from different manufacturers will vary, so it is most important that the appropriate literature is obtained for a specific system. The general principle to be remembered is that the finished waterproof coating must form a continuous, unbroken membrane over the entire area being treated.
Furthermore, it is most important that the finished coating forms a continuous envelope completely encompassing the area affected by damp. Where internal structures such as stairs or partition walls exist, they must be either included in the waterproofing if appropriate, or the system must be taken behind and under them, where practical.
Wherever possible, services which need to enter the basement should be brought in at as high a level as possible, preferably from above ground, and then surface mounted internally where it is brought down to the basement. However, with existing basements, one is frequently faced with existing service penetrations which cannot be moved. These services can be difficult to seal, and guidance must be sought from the manufacturer of the chosen waterproofing system. When done in accordance with manufacturers’ recommendations, they can usually be sealed without excessive difficulty.
As with services, where fixings have to penetrate the waterproofing system, the manufacturer must be approached for their recommendations. Fixings through fully bonded cementitious systems are not a problem provided they are done correctly. Manufacturers of cavity drain systems usually have their own proprietary fixing systems.
After lateral penetration of moisture has been stopped in a basement, it may need to be decorated. Cementitious tanking systems are vapour permeable. Because vapour movement is usually from within the basement out towards the ground, this is an advantage. However, there will be occasions when the vapour movement is reversed, moving from the ground into the basement.
It is most important therefore, to ensure that any decorative covering over the tanking system is sufficiently vapour permeable to prevent a build-up of humidity. If it is not, interstitial condensation could occur within the plaster layer supporting the decoration and result in bubbling and peeling of the covering.
Most people appreciate that gloss paints should never be used over a tanking system, but it is commonly thought that water based emulsion paints are acceptable. It is true that at one time, emulsion paints were sufficiently vapour permeable to not cause a problem. However, advanced paint technology has resulted in much higher binder/pigment ratios, which are needed when producing higher quality paints such as the Vinyl Silks and Vinyl Matts. The higher the binder/pigment ratio, the higher the vapour resistance of the paint, and problems can sometimes be experienced when these paints are used and they should be avoided in basements.
Trade Matt Emulsion paints have a low binder/pigment ratio, and are designed to be used over new plaster. They have a high vapour permeability, which means that residual moisture in new plaster can escape, and they are excellent for use in basements.
Mineral paints, which combine with the surface of mineral substances such as plaster or render, also have a very high vapour permeability and are excellent in basements if a higher quality decoration is required.
Since all paints are vapour permeable to a degree, specifiers of paints in basements are faced with the problem 'Is this particular paint acceptable in a basement?' Only the paint manufacturer is qualified to provide a definite answer to the question, and they should be consulted wherever there is doubt. However, this creates a new problem. Most paint manufacturers do not know what vapour permeability is required, as there are no standards to which they can refer. Furthermore, the problem is further aggravated by the fact that different proprietary systems will tolerate differing degrees of vapour resistance.
The problem can be overcome by asking the paint manufacturer 'Would this paint be suitable for use over new, damp plaster?' If the answer is a definite yes, then it is reasonable to assume that the paint will be acceptable in the basement situation.
With cavity drain membranes, the same principles regarding vapour permeable decoration apply unless environmental conditions are kept reasonably constant, in which case decorations with a higher vapour resistance can be tolerated.
 Problem solving
With care, an effective waterproofing system is achievable. However, with the best will in the world, defects in the structure are sometimes not visible or may have been overlooked, and workmanship is not always as it should be.
Because internally applied systems are usually reasonably accessible, defects can usually be remedied without having to resort to major structural upheaval. The most common problems which occur with cementitious systems are:
The most common problems which occur with cavity drain systems are:
When problems do occur, in the first instance they should be discussed with the applicator. However, they do not always have sufficient background knowledge about the systems to be able to resolve all problems, and so the technical department of the manufacturer should be contacted for further assistance.
This was originally published in the BWPDA Convention Proceedings 1997. You can explore the PCA archive here.
 Related articles on Designing Buildings Wiki
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- Basement Excavation (Restriction of Permitted Development) Bill.
- Basement v cellar.
- Basements in buildings.
- Building an extension.
- Damp in buildings.
- Damp proof membrane.
- Damp proofing.
- Planning (Subterranean Development) Bill.
- Reservoir construction.
- Shotcrete technology.
- Structural waterproofing consultant.
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