FAQ Basement Conversion

How much will it cost to convert my existing basement?

This is often the first question asked, but it is often difficult to answer because of the many factors associated with this work, and the wide variety of property in which it is undertaken. If you are interested in obtaining a better indication of probable costs, contact us via email answering the following questions:

•    What sort of property is it
•    What is the approximate size of the basement
•    What do you want to use the space for and what is it used for at present
•    What are the walls, floors and ceilings consist of (brick/stone/concrete)
•    What are the existing and proposed plumbing and electrical services ?
•    What is the current floor to ceiling height?
•    What access routes are available in and out of the property
•    What is the history of the space in respect of any water penetration, is it damp/wet or not?

What is BS8102?

British Standard 8102:2009 is the ‘Code of practice for protection of below ground structures against water from the ground’. In essence it is the primary design guide for structural waterproofing in the UK, with the recent update being released November 2009. If you are producing a waterproofing design, or advisors are designing on your behalf, this should take account of the considerations listed within this document. If you have further questions in respect of BS8102, please feel free to contact us.

What is ‘Tanking’?

Tanking, or ‘Type A barrier protection’ (BS8102 definition) is a specific form of waterproofing, although it is a common term applied to all forms of waterproofing. In the true sense of the word, tanking means blocking water out of a structure by employing a barrier product which is installed internally, sandwiched within, or applied external of a structure. Tanking is still widely employed in new construction, but has declined in popularity in recent years where retrofit protection is required for existing basements.

What about waterproof concrete?

This form of waterproofing construction is only an option on newbuild or retrofit basement construction. Defined as ‘Type B structural integral protection’ within BS8102, this form of waterproofing shares a degree of similarity with ‘tanking’ in that water is blocked out of the structure, however it differs substantially in that there is no specific ‘barrier’ product included. It is the structure which acts to block out water.

There are various forms of Type B structure, including precast concrete panels, plain concrete with varied quantities of steel reinforcement (to limit cracking and therefore prevent/reduce water penetration), and systems which also involve the introduction of additives into the mix with the aim of reducing permeability.

Our experience is that this form of waterproofing is the immediate choice of some designers and specifiers, and while it can form low risk solutions where due consideration is given to the design and factors affecting a given basement scheme, we also have experience of extreme disaster scenarios where this form of waterproofing was employed. As with any of the systems, it is really the analysis and subsequent development of a sound waterproofing rationale which is important; or in other words, waterproof concrete is not simply the answer to everything.

What is ‘Drained Protection’?

Often referred to somewhat incorrectly as ‘tanking’, this is a form of protection which is provided by managing groundwater penetration, using an internal drainage system which typically comprises drainage channels and sump pump systems which are employed to collect and remove water, while vapour barrier drainage membranes may be included over to isolate the internal basement environment from damp/wet wall and floor substrates. Drained protection Can provide low risk solutions when all necessary considerations applicable to a given scheme are duly considered.

Note: more simplistically it basically involves lining the walls and floors with dimpled cavity drainage membranes (like the back of an egg box but smaller), with the walls then dry lined and plastered and the floors finished in screeds or floating timber. Hidden below this is a drainage system that will either drain out at low ground if in a hillside position, or be linked to fixed floor drains or a sump station.

How dry does my basement need to be exactly?

This depends on what the basement will be used for, in addition to what you propose to place or store within the space. If you can answer these questions, we can determine how dry it needs to be and can then design appropriate waterproofing measures to achieve this.

What are Environmental Grades/Grades of Protection?

While this might not be the first question that springs to mind, it relates to the ‘how dry’ question.

Essentially, BS8102 refers to three ‘grades of protection’, detailing the performance level of each grade, then providing examples of usage, for example:

Grade = 3

Performance = no water penetration acceptable, ventilation/air conditioning appropriate to the intended use.

Example usage = ventilated residential and commercial areas including offices, restaurants etc.

If you’re employing a waterproofing or conversion specialist and they are not quoting the appropriate BS8102 environmental grade provided (or rather guaranteed) by their installation, you should be asking why not, because it serves to clarify exactly what you are, and are not getting for your investment.

My basement is not very wet, so do I really need a great deal of work?

This also relates to what you want to use the space for, but may also be influenced by levels of risk which may be acceptable to you the client.

To explain this, we must first consider that: BS8102 (2009) states that ‘even when site investigation indicates dry conditions, the risk of some waterlogging in the future should be assumed’.

The majority of water affecting basement spaces is simply water introduced into the ground via rainfall, however; we have seen many examples of other causation, the classic example being burst water mains. In essence, just because it is dry now, it does not mean that it will be dry forever more!

In respect of usage, it may be acceptable to take risks (i.e. little spend on waterproofing – partial systems) with a very basic basement space, but if you are spending thousands of pounds and investing time into converting/finishing a basement space, introducing materials such as plasterboard which are particularly sensitive to dampness, it does not make sense to skimp on waterproofing measures because the space will likely deteriorate in time, even if believed to be dry at present.

Tell me more about partial systems:

Some forms of waterproofing construction (primarily drained protection), can be installed as a partial system which means treating some areas while leaving others. The implication of this is that you get partial protection and partial guarantee, because we can only guarantee the areas in a basement which we treat. It is important where proposing such solutions that clients understand what they are, and are not getting!

Despite the fact that partial solutions carry an element of risk, we can provide designs which allow the scope of waterproofing to be extended at a future time if ever required, while limiting disturbance of the initially installed scope.

Such solutions are not recommended where proposing to fully convert a space, then introducing complete dry linings, because if you do not protect materials such as plasterboard from potential dampness, and dampness occurs, then although savings are made on omitting the waterproofing initially, in total clients would pay for: fit out (i.e. plasterboard, skim and decoration), then strip out when it gets damp, then retrofitting waterproofing, then re-fit out. In other words it is not worth the risk.

We also have experience of quoting against competitors specifying protection including low level chemical injection damp proof courses on internal walls, and believe this to be a half measure (in some cases), included as a means to compete on price and not on quality.

Additionally, note that in the case of cementitious tanking, there is risk in treating partial areas because blocking water out of one area, can simply act to push water on to untreated areas.

The existing head height isn’t sufficient, can the floor be excavated? What about underpinning?

To determine whether excavation is possible, it is important to determine the depth of existing wall footings, because if headroom is low but close to acceptable, and if the footings are relatively deep, the floor can usually be lowered enough to achieve reasonable head height without expensive underpinning.

It should be noted that where residential basement conversion is undertaken there is a requirement to comply with part L (conservation of fuel and power) of the building regulations, which means including insulation within the floor construction, which obviously has an implication in respect of finished floor to ceiling height.

It is helpful for the client to arrange for a number of exploratory holes to be dug out adjacent to the walls before surveyors visit, including at external, internal and party walls. If the footings are shallow, then underpinning may be a requirement. Trace work with several specialist underpinning contractors and can provide this service where necessary.

Alternatively if there is adequate headroom for the average person, say 2.10m or more, and subject to the existing floors being sufficiently sound to lay a cavity drainage membrane over, and subject to falls, drainage channels can be cut into the floor and a drained protection system formed over, sometimes losing only 25mm (1”) overall headroom if 3mm cavity drainage membrane and 22mm floating timber is used (and assuming there is no requirement for insulation with the proposed usage).

Screed or Floating Timber?

Screeds are essentially concrete layers formed over a structural floor slab, these being formed flat and level ready to accept finishes (while the structural concrete beneath is typically not flat and level). Floating timber floors are formed by installing tongue and groove boards over our waterproofing membranes and/or insulation as appropriate.

Screeds take greater height so are less commonly employed in the conversion of existing basements, however are more suitable where certain floor finishes are required i.e. large ceramic tiles. Floating timber provides greater comfort but with a degree of potential compression/deflection as with any timber floor.

Can we have under-floor heating?

Under-floor heating is extremely popular in new-build basements, and is commonly formed in screeds over waterproofing membranes installed by Trace. It is also feasible beneath floating timber floors, albeit less common.

Can we increase the footprint and size of a basement by extending into voids or cavities?

This is certainly possible and we have undertaken residential conversion projects where basement space (a home music studio) was created within what was originally a sub-floor void. To an extent anything is possible, given sufficient investment, this including retrofit basement construction (i.e. creating a basement beneath a property where none exists at present), through to construction of a basement beneath the garden (we have one such project on site at the time of writing).

While subjective opinion only, we believe that a great deal of waterproofing is being installed using very basic (and therefore low cost) specifications, which may not stand the test of time. We know from providing CPD talks on waterproofing to Local Authority Building Control departments, that much of the waterproofing/conversion projects being undertaken are by non-specialist general builders diversifying into basement conversion under pressure during the recession. Price competition, while great for the consumer in some respects, is not beneficial when it impacts on quality.

Will the works impinge on party walls, bringing liabilities under the Party Wall Act 1996 and calling for a recognized Party Wall Surveyor?

When a drained protection system is installed at a party wall, the party basement actually benefits because the system relieves potential ground water pressure. This is not the case with direct applied ‘tanking’ systems because they act to block water out, causing increased pressure of ground water on party basements.

The requirement to serve party wall notice, would depend on the work being undertaken, in that underpinning or structural alteration for example would require this, whereas the application of drainage membranes with minimal fixings, may not.

Although drilling holes in a party wall to make small fixings is generally exempt from the Party Wall Act, it is good practice to let neighbours know what is occurring. Also, if a party wall is of limited depth, such as a ‘half brick thick’, notwithstanding the exemption relating to making fixings, any works on the wall may easily cause vibration on the other side of the wall and even result in the face blasting off. In all cases care must always be taken.

Trace Basement Systems employ (full-time) a member of the Royal Institute of Chartered Surveyors, and can advise appropriately on a given basement scenario.