Large New Build Basement

Basement Design & Build tips – External Waterproofing


External Waterproofing

Some basement build & external waterproofing tips we think you should know.

External waterproofing is a barrier installed under and around a basement to prevent the ingress of water, it is normally either a bitumen, bentonite clay or plastic type material.

Most of the materials are completely waterproof, however, they all have to be joined and lapped. It is the detailing of joins and laps which is impossible to complete defect free.

This is largely due to building site conditions as they are by nature dirty , muddy and in the UK frequently wet.

All waterproofing systems have to be installed in the dry and most rely to some degree or another on clean walls or over site, reasonable temperatures and dry surfaces.

failed bentonite basement waterproofing
failed bentonite basement waterproofing

“It is only by poor design and cost-cutting procurement that the risk of a leak is built into basement construction. All basements can be and should be dry”.

The facts.

1 External waterproofing on its own does not comply with British Standards BS8102.

2 Most water ingress is above the water table. Water ingress is just as likely to occur from surface water draining through the soil.

3 External waterproofing cannot be installed defect free. A precedence set in law.

4 Liability for failure will always be with the designer, architect, engineer. Manufacturers do not accept design liability.

5 Manufacturers do not accept design liability, Manufacturers guarantees do not cover workmanship and materials and are rarely honoured. Also; Manufacturers always rely on the argument that all failures are workmanship issues.

6 NHBC insurance have dealt with or are dealing with 20 million in water ingress claims since 2005. NHBC claims are running at £13,000 per day and this is just the NHBC. Claims for water ingress in 2012 were 4.8 million.

two storey basement being erected under a 112 year old church
A two storey basement being erected under a 112 year old church

Basement Build & Design tips

If you are building a basement for accommodation or recreation you will need to achieve a Grade 3 habitable basement to comply with BS8102 - the British standard for earth retaining structures.

The standard says you should involve a waterproofing expert from the start of the design process and seek their advice throughout. The expert should accept design liability, Not a manufacturer.

Mortgage companies will ask for guarantees when you sell the property. The only meaningful guarantees are Products Insurance backed guarantees and GPI insurance to give some cover in the event that the installers cease trading.

2 Forms of Waterproofing
In order to comply with British Standard’s, you should have 2 forms of waterproofing. if the risk of a leak is high, such as in a high water table or if building from block work or ICF rather than poured concrete.

Or if the consequences of a leak are unacceptable, which they will be in a habitable basement.

Look at your proposed basement as a box in the ground. Our recommended waterproofing (supported by BS8102) is that you waterproof the outside with either a membrane under the box and up the sides to stop water entering or build it with well compacted reinforced concrete with professionally installed hydrophilic joints and external joint protection.

In piled construction the internal concrete wall should be waterproofed with Crystalline slurries after construction as this will repair any minor defects unlike additives added before construction.

Basement Build & Design - Check list

1. Two forms of waterproofing for habitable basements.

2. Concrete should be waterproofed after construction not with additives prior to construction.

3. Post applied Crystalline slurries. This is because concrete additives often fail due to workmanship and site condition defects.

4. Waterproof the whole box down from DPC down to below the concrete slab.

The external waterproofing is referred to as Type A waterproofing.
Type A is an external sheet membrane or coating system which forms a barrier, on a habitable basement Type A should never be installed on its own.

You cannot expect Type A to be installed completely defect free - this is a precedence set in law

You cannot test the effectiveness of the system until the structure is buried in the ground. In the event of a leak, it is too late to repair and it is not possible to maintain.


Large New Build Basement
A gigantic new build basement Waterproofing system designed, supplied and installed by MacLennan

Type B Waterproof Concrete

Type B is an integrally waterproof structure. This means a structure built of well-designed concrete that is in itself waterproof. It is often said that this requires additives to the concrete but this is not true as concrete is waterproof. It is the joins and minor faults in workmanship which allow water ingress.

Type B is notorious for leaking. Waterproof additive companies are always repairing failures with our injection systems and cavity membranes. All of the waterproofed additive companies have to employ injection specialists to put right their frequent failures.

Additives do not stop leaks through cracks, honeycombing and join. The additional reinforcement and joins required by most additive companies add unnecessary cost to the construction.

Concrete waterproofing Should be carried out after the concrete is installed.
To repair and waterproof the faults. If you have a leak after the construction phase the additive companies will not return until fixtures and fittings and even internal walls are removed at the clients’ expense. They will not accept consequential damages.

Type A and waterproof additives should not be used together as type A protection will prevent autogenous healing if type A is installed soon after construction.

Type C waterproofing

Type C waterproofing is often in the form of a plastic membrane with a cavity drain, all built into the structure behind walls.

It is essential that drainage is reliable or that the system is pumped. The pump design is critical.

It is the most effective form of waterproofing for common-sense reasons, If the waterproofing leaks for any reason you have to be able to resolve the problem with the minimum of disruption to the occupier, You cannot do this with A or B systems.

All of this has to be considered during design and liability does fall on the designer if it is not.

A concrete structure without waterproof additives but with well-compacted concrete and no honeycombing and well prepared (scabbled) joins with water bars will provide a Type B structure.

A concrete structure with additives and reinforcement to BS8007 but with less than adequate workmanship in any small area will leak. Honeycombing occurs due to a lack of accessibility to poker due to the amount of steel. Water bars are difficult to detail and install and surface prep of joins is difficult due to steel.

No company can properly guarantee an external system on its own. Guarantees for external systems are not worth anything to the client ( why? because they can’t be remedied when they fail). Contractors and suppliers will always easily find reasons for not honoring guarantees.

Product insurance and insurance backed guarantees are not available for external Type A systems.

A type C system can be guaranteed by qualified waterproofing companies for 20 years as a stand-alone system on a well designed reinforced concrete structure.


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