Tanking may be one of the more unglamorous sides to waterproofing but it can sometimes be the most critical. Most building owners, developers, consultants and architects agree that of all the problems that can beset a building, inadequate waterproofing above and below ground creates the greatest havoc. The entry of moisture does not just ruin décor and equipment, it can set up a whole chain reaction. Soil and atmospheric chemicals can become activated and attack the very structure of the building, without being immediately detectable. Thus over a period of time, irrepairable damage is done.
The amount of water pressure exerted on a basement structure will depend on two related factors
> the maximum water table level
> the depth of the basement below the water table
In practice it is best to assume that the head of water is developed from ground level to the full depth of the excavation to allow for the seasonable fluctuation in the water table.
Water pressure increases with depth and is determined by the formula
Pressure in kN/m2 = 9.8 x Depth in metres
This pressure is exerted equally in all directions horizontally and vertically irrespective of the angle of the surface on which the pressure bears.
It is essential that the ground water table is kept below the level of the base concrete throughout the entire tanking operation and for this purpose site de-watering or pumping must be provided on a continuos and uninterrupted basis until the loading slab and concrete walls have hardened or in the case of blockwork walls, the mortar, has set.
TANKING is a method of protecting habitable areas below ground against the penetration of ground or subsoil water.
The purists in the construction industry refer to tanking as a complete encasement of the building foundations below ground with a continuous membrane beneath the floor slab of the basement, the lift pit and up all external walls to (and sometimes above) ground level.
In practice there are many types of Damp Proofing , Waterproofing or Roofing systems which are commonly referred to as TANKING.
External Tanking (where ground water pressure forces the membrane against the structure (i.e. basements))
Internal Tanking or Tank lining (where the water is held behind a structure ( i.e. planters, roof gardens))
Vertical Tanking (where perhaps only a retaining wall is waterproofed)
Horizontal Tanking ( where the membrane is applied like a roofing system, but is buried beneath a protective layer (i.e. under tiles on building podium levels, under road asphalt or concrete screeds on Carparks))
When the membrane is applied to the external walls of the basement structure, the “base / site slab” of un-reinforced site concrete should extend for at least 300 mm beyond the outer face of the basement wall. The horizontal membrane is laid to the base slab, the membrane applied to cover the full base slab. A sand cement screed or protection boards are then applied over the membrane depending on site conditions, ensuring the membrane is not damaged during assembly of the reinforcing steel rods of the loading / finished slab.
This is when the most common cause of failure of “Tanking” occurs, the Steel fixer assures the Builder “ Our guy's are careful, they won't damage the tanking membrane”, the Builder accepts this, cuts out laying the protective screed to save money, the membrane is damaged during steel assembly and the whole chain reaction begins.
Assuming the protective screed is applied, this should be laid on a building paper over the 300 mm “pickup” outside the line of the external wall to avoid contamination of the membrane during construction of the wall.
It should be the work of the Tanking contractor, or at least done under the Tanking contractors supervision to carefully remove the protective screed, ensuring it is free of all cement slurry, mortar droppings or other contamination in order that complete fusion between the horizontal and vertical membrane is achieved.
The vertical membrane should be applied as specified, with a minimum of a double thickness of membrane at the joint between horizontal and vertical faces when using self adhesive or Torchon membranes and if using mastic asphalt, a 2 coat fillet should be applied between the angular junctions between the vertical tanking and the horizontal “pickup”.
Attention to any pipe penetrations through the tanking membrane is essential, collars around through pipes must be treated with care and sealed completely to the main membrane.
Protective boards such as Hardiebacker, polystyrene insulation, soft board, filter board/membranes etc. should be applied against the tanking membrane , preferably by the tanking contractor, immediately the tanking is completed.
No back filling should be carried out until all the Tanking and protection loading is completed.
INTERNAL TANKING OR TANK LINING
When the tanking membrane is to be applied to the internal surfaces of the planter or containment area, the concrete base and the structural walls of blockwork or concrete should be completed and allowed to cure for a minimum of 28 days prior to the membrane being applied.
To ensure a sound waterproofing, continuity in the membrane application is essential to avoid any contamination by mortar droppings, slurry, foot traffic etc. by other trades, which will compromise jointing between sheet materials or bonding of the membrane to the primed vertical walls.
Attention to any pipe penetrations through the tanking membrane is essential, collars around through pipes or electrical conduits must be treated with care and sealed completely to the main membrane. Jointing of conduits within the tanking area must be 100% bonded together.
Protective boards such as Hardiebacker, polystyrene insulation, soft board or propriety brand name protection boards etc. should be applied against the tanking membrane , preferably by the tanking contractor, immediately the tanking is completed.
FILLING WITH DRAINAGE MATERIAL AND SOIL.
No filling should be carried out until all the Tanking and protection loading is completed.
Whether a vertical wall is part of a complete tanking system or an independent retaining wall, attention to surface preparation is essential.
Concrete walls should be fully cured, with 28 days curing time allowed, removal of surface laitence should be by wire brushing, grinding or water blasting treatment.
Blockwork joints should be lightly raked and well brushed out or preferably flushed off.
The entire wall surface should be primed with a solvent or waterbased bitumen primer (depending on membrane type) prior to the application of the membrane.
Terminations at the top of vertical tanking should be turned into a chase and pointed up with sand and cement mortar, compatible sealant or a proprietary metal flashing fitted over.
Where a retaining wall is separate from the building or adjoining the main building, the vertical ends of the membrane should be treated as for top terminations, or carried around the thickness of the wall.
The average life of a well designed, correctly applied, exposed waterproof membrane covering is about 25 Years, with the addition of heavy protection, this figure is increased to 30 years and with the use of extruded polystyrene foam insulation placed under gravel (IRMA) a life of 45-50 years is not uncommon.
The type of application will depend on the structure of the deck and the usage of the finished surface.
There are 3 main methods of applying the membrane,
Independent (loose laid)
Independent applications are laid on top of the substrate, sometimes with another separate layer laid dry so the covering will not adhere.
Quick to apply.
Less sensitive to cracking due to substrate movement.
Vapor is diffused under the membrane
Less resistant to foot traffic and impact during construction.
Only suitable for slopes less than 5%.
Requires careful application of heavy protection.
Difficult to find leaks.
Adhered (fully bonded)
The covering membrane is completely adhered to the substrate. The substrate needs to be fully supported and stable and the membrane must be resistant and elastic to cope with substrate movement.
Better resistant to foot traffic and impact during construction.
Applicable on any inclination.
Resists wind uplift well.
Does not require heavy protection. (I.e. Tiles may be stuck directly to the membrane without screed)
Easier to trace leaks and minimizes spread of water if leak occurs.
More stable in extreme hot and cold situations.
Application takes longer (more costly).
Greater sensitivity to substrate movement (cracking).
Blisters form more easily in poorly adhered areas.
When laying fully bonded, it is common practice to apply bridging strips, approx. 100- 200 mm width, laid over joint edges of prefabricated panels or plywood sheets and adhered to one side of the join before the main covering is applied. Bridging strips help to counteract the effects of differential movement between the panels by distributing the force of the expansion and contraction over the area of the strip rather than in a narrow 1-2 mm line in the covering itself. Bridging strips may be made of thinner versions of the membrane being used if available to avoid over-thicknessing at this point.
Semi Independent (spot stuck)
Used when the substrate is flexible, or may be damp, and therefore likely to create vapor blisters under a fully adhered membrane. It is a compromise between the two previous systems and fixing can be as follows;
Adhesion through perforated base sheets.
Spot sticking in strips or areas by use of hot or cold adhesives or partial flame bonding or of the lower side of the covering.
Mechanical fixing on timber, underlap fixing or in the case of 2 ply systems stagger fixing over the whole of the base sheet.
Does not require heavy protections.
Vapor can be diffused through venting.
Only suitable for slopes less than 20%.
Leaks are difficult to trace.
PROTECTION OF HORIZONTAL TANKING.
A min. 20 mm bed of plaster/concrete laid on an isolating layer of polythene sheet or bitumenized building paper over the membrane as the bedding, with tiles set into the wet screed and pointed up with grouting.
Prefabricated paving slabs ie. 400x400x50 laid on proprietary plastic supports resting directly on the membrane, without a filler between the slabs, or on polystyrene insulation boards laid direct to the membrane, (IRMA, Inverted Roof Membrane Assembly).
Self locking brick type Pavers may be laid on a 10-20 mm thick sand bed, which in turn is applied directly onto a protective polythene sheet loose laid with 200 mm overlaps over the waterproofing membrane.
A layer of filter material such as non woven polyester or proprietary filter sheet should be loose laid over the membrane, with unbonded laps and then washed gravel should be spread over the whole surface. The size of gravel depends on the membrane type and the system of attachment to the substrate, see suppliers for details.
Generally a layer of 16-32 mm round washed gravel would be laid 40-50mm thick.
The need for Rooftop parking is becoming wide spread, due to the high cost of real estate in our cities and the need for more off street parking, due to higher density housing/apartments.
The principle of roof top carpark tanking, is to sandwich the waterproofing between the main floor structure and the wearing surface.
The aggregate in the asphalt should be rounded. It should be applied directly onto the waterproofing in the case of bitumen membranes laid by the Torchon or pour and roll methods at a temperature of 1400C .
The thickness will depend on expected traffic loading, but a thickness of no less than 40mm is accepted practice.
The membrane will be subjected to thermal shock when the hot binder is applied and to great puncture stress when the tarmac is being rolled. For this reason membranes used for rooftop parking normally have heavier duty reinforcement than in other tanking situations. This is very important when choosing a membrane, a variety of Torchon membranes available may be 4mm thick, but the polyester reinforcement weight can vary between 100gms/m2 to 350gm/m2.This is the critical difference between a membrane working or failing, when applied beneath roading asphalt.
As the road asphalt (tarmac) needs to be compacted by roller and on site loading weights of heavy rollers will have to be considered at the design stage if specifying road asphalt.
The membrane is applied to the horizontal substrate by the chosen method, with the membrane fully bonded to the substrate on access ramps. To protect the waterproofing layer, a non-woven polyester fabric 500gm/m2 and a polythene sheet 0.15 to 0.20 mm, should be applied prior to the application of the concrete screed. On larger areas the screed should be saw cut into squares and allowance made for expansion joints.
INSPECTION / CONCLUSION.
In all of the above Tanking situations it is critical that the applicator inspect the finished membrane prior to the laying of protection, no matter which type of membrane or protection is used. Concrete screeds, tarmac, board insulation, tiles and pavings are not waterproof, there are no second chances in some situations, attention to detail and quality control on site are the only sure ways of ending up with a waterproof building.
Thanks to Ruberoid UK for the following detail drawings.