Technical - Concrete Block
Habitable / Grade 3
What are my options?
Concrete block and Stepoc construction are very porous.
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 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.
If you are building a basement for accommodation or recreation, you are required to achieve a Grade 3 Habitable basement to comply with BS8102, 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 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.
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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.
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.
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.
Concrete Block - Technical Download Area
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What else do you need to think about when waterproofing a basement?