Technical - Concrete Block & Panels

Habitable / Grade 3

What are my options?

Paragraph 5 of BS8102 states:

“Modular systems, such as twin wall and insulated concrete formwork (ICF) construction should be deemed inherently high risk."

Paragraph 11.1 of BS8102 states:

"Composite and modular structures, such as precast and twin wall, and certainly masonry, beam and block and ICF, have increased risk of failure and are potentially difficult to repair"

Remediation can be extensive and costly.

As a requirement of BS8102, you should consider 2 forms of waterproofing when the risk of a leak is high, which it often is with this form of construction.

It is also a requirement of BS8102 that you consider 2 forms of waterproofing if the consequence of a leak is not acceptable, which of course it wouldn’t be in your new basement.

If you have a high water table you must not build your basement out of blockwork or Stepoc. You must build your new basement out of reinforced concrete in order to comply with BS8102.



If you are insisting on building your basement structure using block concrete or ICF, you will need an external system. You need to be aware that it is a precedence set in law that it cannot be done defect free. Read about the Outwing Case for more information.

If you are applying an external sheet membrance to a structure built using block of Stepoc, which is porous, you're almost certain to suffer leaks or flooding as some point. Therefore, you cannot install a sheet membrance system defect free.

If you cannot drain the outside of the basement from a point below the internal slab to a soakaway or suitable drainage then you will have to install pumped drainage and keep it maintained.

A pumped system in a permanently high water table is not practical. You must have  2 forms of waterproofing to keep the inside dry and to comply with British Standards, legal precedence and all other codes of practice.

If you have to rely on any external drainage, it has to be serviceable so must incorporate inspection/rod chambers.

You will have to have a cavity drain internally as it is the only system that can be guaranteed on a basement constructed from block work. Any other system will not comply with British Standards.

The options for a second form of waterproofing which you do have to have are external coatings, sheet membranes, concrete additives or cement tanking.


Basement Design

If you are building a basement for accommodation or recreation, you are required to achieve a Grade 3 Habitable basement to comply with BS8102/22, the British Standard for earth retaining structures.

In order to comply with British Standard’s, if the risk of a leak is high, such as in a high water table or building out of Block, ICF or Modular, or if building a basement where the consequence of a leak is unacceptable, then you should have 2 forms of waterproofing. External waterproofing is difficult and sometimes impossible in piled construction.

External waterproofing cannot be installed in inclement weather and can effect programmes for weeks on end in poor weather.

Find out more about External Waterproofing

Base of Blockwork Retaining wall 2 WM

Get in touch with our technical team today!

Concrete Additives & Coatings

Concrete additive in a concrete block cavity cannot and will not work.

It is not possible to effectively fill the voids or to achieve the necessary compaction. Specifiers of concrete block systems will claim that a well grouted block wall will be watertight. It isn’t.

Any guarantees for additives will be of no value to the client in the event of a leak and are limited to the value of the materials.


Guarantees given for external waterproofing on its own are never valid. The internal cavity drain system is the only system that has meaningful guarantees.

Cement and Epoxy coatings do not work on concrete block construction as it is prone to some movement.



Sheet Membranes & External Systems

Why do i need an external system?

The cavity drain system can deal with significant amounts of water, however water draining through new construction can pick up free lime in the form of Calcium Carbonate from the ground, or Calcium Hydroxide from the construction materials which can block channels and foul pumps.

The external system is necessary to prevent or at least reduce water ingress, but cannot be guaranteed or serviced as it will be covered when back filled. It will definitely reduce water ingress if an effective drainage system is installed, and therefore protect channels and pumps if pumps are installed.


Sheet membranes are the most suitable for concrete block basements, but have to be installed with a cavity drain system inside as sheet membranes cannot be installed defect free.

With the system we specify, we accept design liability and the system will cost you far less than the cost of repairing a leak.


Guarantees given for external waterproofing on its own are never valid. They are limited to the cost of the materials used and exclude damage by others, which is impossible to prove.

The internal system is the only system that has meaningful guarantee.

Find out more about Internal Waterproofing

Ground Gas

MacLennan Radon gas design detail
MacLennan Radon gas design detail

Before building a basement ground gas has to be taken into account.

If testing proves that Radon, CO2 or Methane are present, then the basement has to be designed so that the gas cannot enter the basement.

This can be achieved with the waterproofing system designed by MacLennan.


Deck & Roof Waterproofing

If the basement is not completely covered by the house above, as is often the case, then you will have a deck or roof are to waterproof.

The deck waterproofing should always be carried out by the same waterproofing specialists who waterproof the basement as the detailing between the two is critical and split responsibility should be avoided at all costs.

Decks and terminations at top of wall are all subject to BS8102 /22 and should be installed by the basement waterproofing company to comply.

Deck waterproofing has to be seamless elastomeric coatings and never single ply membranes and felt which have a history of failure due to the numerous laps, joins and seams.

Finishes on decks include green roofing and hard landscaping or even driveways and parking areas.

Due to the large number of products with different properties available on the market,  the deck/roof waterproofing should be specified by the waterproofing specialist and the work guaranteed and insured for design, supply and installation by the same company.


The Waterproofing Company

The minimum requirement for a waterproofing specialist is:

  • Design, supply and install all waterproofing systems. No split responsibility.
  • Take full responsibility for waterproofing Basement walls, floor and roof.
  • Accept design liability and have Professional Indemnity insurance for waterproofing of 10 million (unlike any manufacturer).
  • Give insurance backed warranty of up to 10 years that covers supply and installation.
  • Comply with all relevant health and safety requirements.
  • Do not use sub-contractors for specialist work, only their own fully trained staff.
  • Must be able to provide references for numerous similar projects.
  • Be able to accommodate project time constraints.

Unfortunately local authority building inspectors only ask for BBA certification for materials and systems. This does not provide any guarantee that the work will be carried out correctly or give any protection to the client.

Waterproofing is wholly reliant on expert design by the waterproofing contractor and installation by very experienced technicians. Waterproofing is so often carried out by ground workers and builders taking advice from manufacturers sales personnel, and this is the main reason for failure.

In the event of a claim. A manufacturer warranty will not cover the installation




Concrete Block - Technical Download Area

Included in our Technical Download Area:

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Data Sheets

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