Technical -
Insulated Concrete Formwork

Habitable / Grade 3

What are my options?

ICF is now deemed in BS8102 / 22 to be inherently high risk. This is due to the huge number of joints and joins and the problems with concrete filling been permanently behind formwork with no opportunity for quality control and repair of defects.

It is also mentioned in Paragraph 11 of BS8102/22 that it is very difficult to repair leaks in ICF construction.

It is a requirement of the British Standards that waterproofing is designed so as it can be repaired in the event of a leak. If you have to build a basement using ICF , you have to be aware that it is against the advice of BS8102 and that you must have a cavity drain system for your basement waterproofing, as it is the only system that can be repaired after completion of the work including finishes, and therefore guaranteed for 10 years. No other form of waterproofing is suitable for ICF because of all of the joins.

This page will set out your options for combining systems. At the bottom of the page you will find links to complying with BS8102 ,  third Party warranties and the types of waterproofing.

ICF construction is very porous. The cavity drain system can deal with significant amounts of water, however water draining through new construction will pick up free lime which can block channels and foul pumps. The external system is necessary to prevent water ingress but cannot be guaranteed or serviced as it relies on the structure not moving, it will be covered when back filled and is often damaged by follow on trades.

BS8102 say's  that you should consider 2 forms of waterproofing when the risk of a leak is high, which it always is with ICF construction due to joins and inconsistent filling with concrete.

2 forms of waterproofing are to be used if the consequence of a leak is not acceptable, which it never is.

Guarantees given for external waterproofing on its own are never valid because they come with so many caviates and only guarantee materials. The internal cavity drain system is the only system that has meaningful guarantees. Therefore, you must have 2 forms of waterproofing to keep the inside dry and to comply with British Standards and all other codes of practice and to get guarantees that actually give protection to the property owner you have to have a cavity drain system.



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

Concrete additive in an ICF cavity cannot and will not work. It is not possible to effectively fill the voids without defects, honeycombing or cracks greater than 0.3 mm or to achieve the necessary compaction. MacLennan work on failed waterproof concrete basements every weekputting right leaks which were supposed to be under guarantee by a additives companty or manufacturer.

Coatings Any form of coating is going to be far too thin and will not cope with the multiple joins in the ICF.

Sheet membranes We would only ever recommend sheet membranes adhered to the outside. Sheet membranes cover the huge number of joins that cannot be effectively waterproofed by any other method.

However, We would never install just external membranes. You must have a cavity drain system, to comply with standards and to get meaningful guarantees.





Basement Design

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

To comply with British Standard’s, if the risk of a leak is high such as in a high water table or if building a basement where the consequence of a leak is unacceptable then you should have 2 forms of waterproofing.

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


Get in touch with our technical team today!

Ground Gas

MacLennan Radon gas design detail
MacLennan Radon gas design detail

You must check for Gas before designing your basement.

If testing proves that Radon, CO2 or Methane are present, the waterproofing system needs to be designed as a Gas system as well.

The combined  system can be designed and installed 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.

In the last few years, deck waterproofing has moved towards seamless elastomeric coatings away from 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




Insulated Concrete Formwork - Technical Download Area

Included in our Technical Download Area:

Specification sheets

Data Sheets

3D Details

Product Details

Safe Contractorx
Full 4th May 2010 06 22 42 Gpi Logo E1443710209909x
Safe 4 Sitex

Get in touch!

Ready to start a project or require more information about us?
Call us about your project or problem and we will be happy to help.

Call us today on:

0330 3200 240
Or email us here