Frequently Asked Questions - Basement Conversion

FAQ: Basement/Cellar Conversion & New-Build Residential Waterproofing.

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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.

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What about the ceilings?

In a residential basement conversion there is typically a requirement to form a fire break between basement and ground floor levels. We would also consider what a space is being used for both above and below, in that for example where you have timber floor finishes above, it is likely that noise (particularly impact noise from footsteps) will transfer to the space below which can be obtrusive in bedrooms for example.

What about soundproofing?

Trace have installed soundproofing systems internal of our waterproofing, using both proprietary branded systems in addition to forming systems using everyday building materials (independent studs (room within a room), mineral wool, soundblock plasterboard & caulk sealants) constructed to achieve soundproofing via the principles of decoupling, acoustic mass and reduction of potential air passage.

I think my basement has already had some work done to it, so are there any implications with regard to converting it?

What is the history of the basement, and are there any existing guarantees in respect of the basement, remedial DPC (damp proof courses) or timbers, because if so, the proposed works may have implications on them, calling for the guarantor to be consulted. The interaction between the proposed waterproofing system, the structure and external ground conditions should always be considered.

What requirements may be imposed by the local planning and building control departments? What if the property is listed?

Whilst Building Control Approval is not necessarily a requirement, this depending on the scope and intent of a given project, if there is any question it is always best to consult the local authority, particularly if the building is listed, because while requirements can vary greatly from areas to area, in the event of later sale of the property, having proof of Building Control approval and compliance with building regulations, can greatly simplify the sale and ensures that you get maximum return for your investment into the basement space.

Trace Basement Systems are a registered Local Authority Building Control partner with High Peak Building Control; see more information on this here.

If a property is listed or within a conservation area, the Conservation authorities usually prefer that a Type C drained protection system be installed because cavity drainage membranes can be taken off in the future, therefore making it ‘reversible’, leaving the original substrate largely unspoiled. Cases commonly occur where original stone flags are taken up and then put back as the finish over the cavity drainage DPM (damp proof membrane), and where there are strict preclusions on the amount of work in basements that flood, listed floors are sometimes only disturbed to place drainage within them.

Can we take down or alter walls?

Are they structural, i.e. supporting floors over or walls at the higher levels, calling for the use of RSJ’s/Universal Beams or other structural measures? A Structural Engineer may be required for such alterations and RSJ’s may sit lower than the floors over, limiting head height below. There is also a requirement with this work to consult with your Local Authority Building Control Dept.

If timber lintels are present over doors, should they be replaced with concrete, or be treated before being closed in behind a vapour barrier?

Treatment is a viable solution if the lintels are sound, but if in questionable condition or need to be raised, concrete lintels are preferable.

What about existing stairs?

If there are existing stairs that will remain, most commonly with stone or brick treaders & risers, either suspended over a below stairs cupboard or with solid fill below, they can stay in place and can be treated appropriately by Trace Basement Systems.

Changes in finished floor level may have an implication on the height of the bottom step. There are various answers to this consideration where additional steps can be added or a landing formed. Detailing around this would form part of our design for the space and waterproofing.

Do we need drainage channels?

Further to the proliferation of basement conversion providers and the associated price based competition which has occurred, we know of situations in which (in our own opinion), corners are cut with a view to reducing costs and therefore price. One such measure (in drained protection waterproofing), is the installation of dimpled cavity drainage membrane, without the inclusion of drainage channels beneath.

While a functional waterproofing system can be formed with this specification, in many cases it is not then possible to clean out and maintain such systems, firstly limiting compliance of a design with British Standard 8102 and secondly increasing risk or reducing the functional lifespan of the system.

It might take five or ten years to cause a problem, but many of us occupy our properties for much longer, and it is not uncommon for Trace to receive calls from members of the public, citing issues in properties protected by guarantees, provided by companies which they cannot now contact because they are no longer in business. Indeed a degree of our work is on addressing failed waterproofing (drained protection) systems, with a recent enquiry on this (at the time of writing), being a waterproofing/conversion system in Sheffield, where the homeowner now wants a reputable firm to address issues with basement conversion work undertaken only four years ago.

Perhaps the most extreme case of failure we have seen associated with this design (no drainage channels), involved a system which clogged and failed within approximately two years of completion. The installing contractor no longer traded but was contactable, and seeing our quotation for approx. £15k of remedial waterproofing work, stated that the design was approved, then refuting liability.

The sad fact is that while some membrane manufacturers state that this design is acceptable (using common sense as to how it is employed), in this case it did not do the homeowner any favours.

Can the basement be opened up to ground floor?

Yes but with proviso’s, namely in that such alterations would need to comply with the building regulations, primarily in respect of means of escape in the event of fire.

What about means of escape?

If there are light wells or an external stair-well they can be used, or external wells may need to be formed to provide a ‘means of escape’ direct from the basement. As an alternative a protected escape route can be formed in some cases through the property above.

Will my existing boiler and electrical consumer unit handle the additional living space and its requirements for power and heating?

Where basements are converted to habitable space, there is a requirement to include heating (environmental controls) so that vapour introduced by occupation does not result in problems of condensation. Electrical sockets are obviously necessary in a finished space.

If the existing boiler or electrical consumer units are not up to the task, this can be a reasonable expense and can vary greatly depending on the standard of fittings required. Therefore it is important that such aspects be investigated.

Basements are generally thermally efficient because of the insulating effect of the retained ground. They may also provide cool living space during hot weather.

Can you manage all of the required work? Are your workers directly employed?

Trace have great experience in structural waterproofing both in residential basement conversion and commercial projects working with Architects, Engineers, Contractors of all types on all sorts of properties. Where homeowners require a turn-key package for a residential conversion, we can typically arrange all aspects, including plumbing, electrics, underpinning and Building Control Applications.

What will be the working access into the basement?

The ideal is an entrance without the need to disturb the ground floor accommodation during the main period of work, for example through a light-well (existing or constructed as a means of escape, and to bring natural light into the property), or via an external stair well.

What will be the contractor parking arrangements?

If off road great, but if in paid parking places, costs will reflect such.

Any other likely influence on basement project costs?

  •     Can skips be positioned off road or close to the property on the road?
  •     Will there be any work time restrictions?
  •     Can the workmen use the house WC facilities with appropriate care. If not, portable hire charges would occur, which because of health and safety requirements.
  •     Are power and water available?
  •     Is there existing external access to the basement to limit disturbance to the living accommodation over?

How do I know that you know what you’re doing?

Ask us for references and speak to past clients. Read the information on our website, look at our history and come to us with your questions.

I have heard about contractors doing work, giving guarantees and then disappearing, so how can I protect against this?

This is a very important subject and consideration, since much of what you are paying for when having a basement conversion (or structural waterproofing work generally) undertaken, is that in the event of a future issue, the installing contractor will come back and remedy under the terms of their guarantee.

We would recommend that you do your homework and check out the finances of the company that you want to use, whether Trace Basement Systems or one of our competitors.

Checking that a firm carries professional indemnity and product liability insurance is another safeguard. If a firm suffers a large claim and they aren’t protected, they may be forced out of business, which then affects every single client holding a guarantee provided by that firm. If however they are protected by appropriate insurances, they should remain in business to uphold the guarantees.

Perhaps the most effective means by which you can protect yourself, is by checking that the company you are using offers GPI guarantee insurance, which protects you in the event of the contractor ceasing to trade.

You can read more about Trace client protection, here.

GPI – Sounds Great! When do I get my guarantees?

Guarantees are supplied when work is paid for in full.

GPI – Is it worth the cost?

Well, it will be one of the most important pieces of paper in the event of a sale, and more importantly, notwithstanding that this web site represents the most proven, able and best qualified contractors in the UK, if your chosen contractor should ever cease trading and you need to claim to GPI, the one off payment required will have been well worthwhile.

Furthermore it’s a one off payment to provide ten or twenty years cover, not something you have to pay for on an annual basis.

Are there any long term costs?

See our page on system maintenance here.

What other benefits will I gain?

Clients typically report that where we have converted basements in their home, the living space above the basement becomes warmer because of the improvement of the environment below.

In respect of heating costs at basement level, with the insulation provided by the ground surrounding the structure, in addition to the inclusion of insulation (applicable where spaces are converted to habitable living space), they typically make for efficient spaces.

Research undertaken by the Basement Information Centre (see our involvement with TBIC here), concluded that properties including basements are typically 10% more energy efficient than those without.

Additionally basements can make for extremely comfortable bedrooms during hot weather because the retained ground maintains a relatively steady temperature below a given depth, which protects against the thermal gain that typically warms living space above ground.

See our page on sustainability here.

I’ve paid for work to be undertaken, there are problems and I need advice.

Speak to use and we can advise, see our pages on ‘free advice’, and consultancy services.
What is the “Outwing vs Weatherald” case, and what are its implications for the homeowner?

Outwing was a legal case where a property was “tanked” externally using a bonded sheet membrane, which failed, allowing water penetration. It resulted in a court case, where the contractor Outwing, was found not to be liable, because the problem was caused by design failure, not workmanship. In brief, the Court held that for a tanking system to work when water pressure came to bear, it would have to be defect free, which means it would need to be perfect, which is neither reasonable nor realistic.

I think the water problem in my basement might be caused by issues with my drains, does that matter?

It may have consequence in that water penetration into a basement or cellar, which results from issues, associated with failed drainage or what may be considered ‘a domestic plumbing leak’, may be covered to varying degrees by your buildings insurance. In such a circumstance any existing but failed waterproofing system would usually only be replaced on a like for like basis, with anything greater needing to be argued for.

We have certainly undertaken waterproofing projects in residential basements, where issues have occurred resulting in the destruction of internal finishes (see this case study), where the insurers paid for the strip out and reinstatement of fixtures, fittings and finishes, while the clients paid for the addition of the waterproofing system (this basement was converted by previous owners without including any waterproofing).

Note that it is difficult to categorically identify the source of the water penetrating into a given basement space, and in the event of drains being questioned, you would need to employ a drainage specialist to undertake a survey (we do not provide such a service).

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