© Central Conservation Ltd 2014. All rights reserved

London (Head Office) 020 8886 4144  Home Counties 01753 437407  Lines are open Monday - Friday  8am - 6pm Consultants & Contractors
Call me back

"Preserving the past for our future"



Agrément Approved.  Certificate No. 86/1591/1592/1627

Tanking involves a specialised application of cementitious render to a previously prepared substrate.

Tanking operations are carried out to structures subject to hydrostatic water pressure as well as liquid retaining structures.  The application will turn a once redundant and uninhabitable area into functional accommodation.

Surface Preparation

All surfaces to be mechanically roughened, wire brushed and thoroughly washed down.  Immediately prior to application the substrate must be soaked with clean water and any standing water removed.  All fixtures must be removed.  Cracks, porous patches and generally defective areas should be cut out and made good as the work proceeds.  Infiltration of water must be stopped with Sika-4a, alternatively the water table must be lowered.

Dilution & Mixing

One part Sika-1 with ten parts clean water, (with wet sand 1:8, with dry sand 1:12 ).  Stir often and use within 12 hours.


Renderings and screeds must be kept moistened for a minimum 7 day period after application to stop rapid drying out.  They should also be kept insulated from adjacent sources of heat to ensure the temperature does not exceed 90ºF.  Protect fresh mortars from frost.


Only washed, well graded, sharp sand may be used.  The basic sand required is 3mm down, (4.5mm down for floor coat).  All dry mixed sand and cement should be sieved prior to use.

1.  Moisture & Damp Resistant Rendering (Above Ground Level)

Surface Preparation = as above.


Apply the first coat of a 1:1 (cement: sand) mortar with Sika-1 not less than 6mm thick, taking care to cover the whole surface.  Mix to a sloppy consistency and cast on vigorously.  (See plaster specifications).  Apply second coat of 1:2.5 (cement: sand) mortar with Sika-1 not less than 6mm thick as soon as the previous coat has stiffened sufficiently (typically 4-5 hours).  Finish with a wood float.

2.  Rendering to Resist Water Pressure

Surface Preparation = as above.


First Day -

Second Day -

Lap Joints -

3.  Waterproof Floor Topping

Surface Preparation = as above.

Grout Coat

Mortar 1:1 with Sika-1, of a sloppy consistency, vigorously applied with a brush or broom.  To be laid in strips and not to be walked on.

Bonding Coat

Mortar 1:1 with Sika-1, to a plastic consistency, spread with a trowel, not less than 10mm thick.  To be laid in strips and not to be walked on.

Main Floor Coat

Mortar 1:2.5 to 1:3 with Sika-1, preferably using 4.76mm down sand, laid in a semi-dry state, while the bonding coat is still wet, to a thickness of not less than 28mm.  The surface to be tamped vigorously until moisture rises to the surface.

Lap Joints

To ensure water tightness careful attention must be paid to all joints.  The edge of each strip of bonding coat must be covered by the next strip.

Wall / Floor Angle

The first and second coats of the wall rendering should be carried down and out onto the floor 200mm and 100mm respectively.   Before laying the floor topping these exposed strips should be prepared and treated as per the main floor area.  The cover in the main floor coat should be formed with a cove trowel or bottle to help strengthen the joint between the wall rendering and the floor topping.  Where solid floor is not being tanked, form “V” detail in solid floor and extend tanking into.

Waterproof Membrane Systems

Membrane systems are a well tried method of securing underground areas from water and damp.  Any size or shape of room can be protected quickly without the problems of disruption, wet works or long drying out periods.  They are particularly suitable for areas that are subject to high vibration or where wet applications are not desirable.


The fixing surface should be firm and clean.  Any damaged, loose or crumbling plaster, all timber, fixings, adhesive residues and other material should be removed.  Any depressions should be filled in with render.


The membrane should be cut to fit using a sharp knife or shears.  The manufacturers recommended plugs should be used to fix the membrane securely to the structure.  The membrane should be pulled tight and square while fixing, this will avoid sagging or bulging.  Fixing centres to be as per manufacturers specifications.  All jointed areas to be tapped over and sealed using the manufacturer’s appropriate product.


Various finishes can be employed to membrane systems.  The system can be completed by finishing with a coat of plaster.  Alternatively battens can be attached to the membrane and then dry lined and finished with a skim coat of plaster.

Any water entering through the structure can be drained behind the membrane to whatsoever draining system is applicable.  A sump/pump may need to be installed if natural drainage is not an option.  The air gap created behind the membrane allows the wall to breathe and even to some extent to dry out.

Additional Notes For Client’s Guidance


Tanking systems do not cure condensation and may highlight effects that were not evident before the lateral moisture was remedied.  Consideration may have to be given to the installation of a dehumidifier or extractor unit in such cases.


Delay any decoration for at least 4-6 weeks following works and allow for good natural ventilation to enhance drying.  After this time period only use a non vinyl based emulsion.  Do not apply heat or accelerated drying methods. Final decoration should be delayed for at least twelve months and even then the use of gloss paint, wallpaper or other impermeable coatings is not recommended.

Note: The initial decorations should be regarded as temporary, the final decorations not taking place for at least 12 months after the completion of the works.  

Skirting Boards

Timber or plastic skirtings can be fixed to the tanked walls either by the provision of manufacturer’s special fixings, or with a suitable adhesive.

Drying Out Period

It is vitally important that the new system be allowed to dry out slowly.  Too rapid drying out will result in cracking, and eventual breakdown of the system.  Heating or forced drying should be avoided.

We reserve the right to alter the provisions of this specification in such respects as may be found necessary during the course of the works.

Internal Basement Tanking / Structural Waterproofing

BackBack to top