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Structural waterproofing

Structural waterproofing to ensure below ground structures are not unduly affected by water ingress can be a complex procedure. There’s a wide range of considerations including;

  • Each structure's unique design and build type,
  • Knowledge of the local environment, water tables, flooding history, ground gases etc
  • Extensive knowledge and understanding of the technical specifications of various waterproofing methods

It can’t be emphasised enough that structural waterproofing (especially below ground level) is a highly specialised operation and should be undertaken by professional contractors and waterproofing designers who have the requisite levels of skill, understanding and experience.

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The importance of waterproofing design

Unfortunately failures in structural waterproofing resulting from inadequate specifications or poor workmanship are usually very costly to put right. Waterproofing structures that are below ground is a highly specialised operation. Competent and experienced contractors and designers should be engaged as they should have the requisite levels of knowledge of the three different waterproofing systems (Type A, B and C) along with the skill and understanding to make sure that the job is completed correctly and that your basement project is a success. 

The BS8102 requirements for structural waterproofing

Since its inception in 1990, BS 8102 has been instrumental in raising the standard in ensuring that underground structures are provide an environment suitable to its intended use. The revision in 2009 raised the bar even higher to the point it is considered a worldwide leading document that many other nations have looked to adopt as the accepted expectations when waterproofing below ground structures.  

The latest version was published in March 2022 and looks to build on the solid foundations set out by the previous editions of this strong and robust document. Some of the changes look to include a wider consideration for sources of water and a revision to the grades of waterproofing. This guidance document is aimed at providing a brief overview of the 2022 changes to the Code but should not be used in place of the document itself. All those undertaking waterproofing work should familiarise themselves with the new Code of Practice.

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Download the Summary of Changes to BS8102 2022

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Regulations you need to consider for residential homes

When it comes to structural waterproofing within basements and below ground structures, there are rules, regulations and considerations that all applicable to new structures that all building, surveying and property professionals need to take into account. Below is some regulations & considerations you may want to check on before considering any waterproofing design and structural waterproofing works.   If you are unsure regarding any of the regulations, we would strongly suggest you consult a building control body who will provide advice when the regulations apply.

+ When do Building Regulations become applicable?

If adding a room for residential purposes, then the building regulations will become applicable. Exceptions may apply depending on the use of the building for example plant rooms not visited by people. If unsure you should always consult a building control body who will provide advice when the regulations apply.

Responsibility for ensuring that the work complies with all applicable requirements of the building regulations falls to those that carry out the building work, this includes the designers, installers and the building owners.

+ Approved Document B - Fire safety in Dwelling houses

Was last revised in 2019 and covers precautionary measures necessary to provide safety from fires for building occupants, persons in the vicinity of buildings, and firefighters and.
The effect that this document will have on basement conversions will be dependent on the intended use. Basements being designed to habitable grade should have one of the following.

  • a. An emergency escape window or external door providing escape from the basement” to ground floor level ensuring any such opening well may need to be guarded to stop people falling into the well.
  • b. “A protected stairway leading from the basement to a final exit.”

In addition, Approved Document B may influence the choice of internal linings to prevent internal fire spread. Particularly the ceiling areas within the basement may need to be upgraded to be fire resistant.

In some circumstances a basement should be separated from the rest of the house using an FD20 fire door. Mains operated and interlinked smoke detectors may be required to the ground floor hallway and first floor landing. If the basement becomes a kitchen a heat detector should be provided to the kitchen.

+ Approved Document H- Drainage and Waste Disposal

This document will be applicable in relation to any drainage of Type C Systems but also any land drains that maybe used within the waterproofing system.

+ Approved Document L1b - Conservation of Fuel and Power in existing dwellings

This document sets out the requirements of the building fabric to prevent heat loss and may influence the requirements of insulation to the ceiling, walls and floors within the basement conversion. In addition, for grade three environments where appropriate ventilation, de-humidification or air conditioning are required, this document sets out the requirements to prevent energy loss. Most insulation manufacturers provide U-value calculations to show compliance with current Building Regulations / Standards and best practice. These can be supported with a detailed report.

For the purposes of any structural waterproofing projects below ground below (such as basements), these areas should be fitted with lighting that is energy efficient lighting.

+ Approved Document F - Means of Ventilation

Basement, cellars and below ground spaces by their very nature will have reduced rates of air exchange and in basements (that are habitable), greater emphasis is placed on the need for the provision of adequate ventilation. For basements with a large permanent opening, ventilation should be classed as part of the whole dwelling. However, properties with an isolated basement may require special consideration and might need to be considered a separate entity to the remainder of the property.

Further guidance on distinguishing between atmospheric moisture and ground water issues in basements can be found in the: PCA Code of Practice for Waterproofing of Existing Underground Structures and the discussion paper Considering Ventilation and Air Management in Basements as part of an Overall Waterproofing Strategy.

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+ Approved Document E - Resistance to the Passage of Sound

Walls between domestic dwellings are required to have a certain resistance to the passage of sound. This Approved Document sets out the acceptable limits and may also be applicable for deciding the make up between floors of the same building.

+ Approved Document P- Electrical Safety

This Document will be applicable for connecting pumping systems and any subsequent electrical wiring. These works should always be undertaken by suitably qualified persons.

Note: Power points, cables, light switches, pipes and any other services must be remounted in front of the membrane in type C systems.

Some key questions for a successful project

Although hidden away, structural waterproofing can be particularly sensitive to failures, largely due to poor workmanship, the inappropriate use of materials or bad design.  As a result, a range of factors need to be carefully considered when undertaking such works, to ensure a positive outcome.  Below are some ‘key questions’ for anyone considering taking on a structural waterproofing project.

+ What consideration needs to be given to a waterproofing designer?

A design team includes an experienced waterproofing specialist, who should be engaged at the earliest possible stage.

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+ What are some of the key considerations in terms of ground conditions and site evaluation?

It Is important that the geology and hydrogeology, as well as the external drainage options and groundwater conditions of a site, are carefully considered. The type of soil and the water table also need to be evaluated, as well as any ground gas concerns.

+ Are there different levels of dryness to consider?

There are four grades to consider, 1a, 1b, 2 and 3. Examples of a grade 1 structures include a car park and plant room without electrical equipment, where it’s considered tolerable to have some seepage and damp areas. No water penetration is acceptable for grade 2 structures, and for damp areas that are tolerable, ventilation might be required. No water penetration is acceptable again for grade 3, and this performance level also specifies ventilation, dehumidification or air conditioning as necessary appropriate to the intended use. If you are looking to use the new basement room as habitable space then you will need to achieve grade 3.

+ What are the waterproofing options available?

There are three waterproofing systems, Type A, B and C. However in reality for most basement conversions you will be limited to certain type A and type C systems.

Type A, usually referred to as ‘tanked protection,’ is a structure with no integral protection against water penetration. It relies totally on a waterproofing membrane to keep water out. Type A forms of waterproofing may be applied internally or to the outside of the structure or, in some cases, sandwiched between two skins of masonry or concrete.

Type B are structures built with a water-resistant shell, usually constructed out of reinforced concrete to an appropriate design code, which gives guidance in the grade of concrete to be used and spacing of the reinforcing steel. Special additives may also be used.

Type C or ‘drained cavity’ systems rely on a drained cavity within the basement structure. There is a permanent reliance on the cavity to collect groundwater that enters through the fabric of the structure. The drainage system directs the water to a drain or sump, where it can be removed from the building by gravity or pumping.

Types of waterproofing - more info >>

+ What are the considerations regarding defects and remedial measures?

First and foremost, the construction teams applying, installing and building the underground space must be fully aware of the critical nature of what they are doing, the importance of accuracy and the implications of any defects and errors.

Contingency planning for any localised defects should also be included as part of the overall water-resisting design for the structure. If feasible repair is not possible, then it may be required to look at what can be done in respect of the risk posed by groundwater.

+ Will I need planning permission?

In most cases, planning permission is unlikely to be needed when converting an existing basement or cellar. However Before progressing with a basement conversion, you should check with your local authority whether planning permission is required.

Additional technical documents you can view

For those interested, there is a variety of structural waterproofing related 'Codes of Practice', Technical Documents, Guidance Notes via our Technical Document Library.  Simply click on the button below to view the library.  Documents of interest include:

  • Code of Practice for Waterproofing of Existing Below Ground Structures
  • Insulation in Basements guidance note
  • Best Practice Guidance: Continuity of Waterproofing Systems
  • Best Practice Guidance: Podium Decks and Buried Roofs

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Free structural waterproofing related CPD training

Want to learn more about structural waterproofing

For those interested in learning more about structural waterproofing and BS8102 requirements; there is a variety of PCA training options for surveying professionals as well as technical/trade professionals. 

Use the search tool below to find available waterproofing related training courses or simply go to our training & qualifications section.  Alternatively, if you want to chat to someone, contact our training team on 01480 400 000 or contact them online.

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