Waterproofing rarely fails because of the material. In most leaking work the product is right and the sequence or a detail is wrong. Below are the real reasons waterproofing stops performing early in its life, how each one is recognised and how each is prevented.
1. Trusting waterproofing without drainage
Waterproofing and drainage are not alternatives, they complete each other. Drainage carries water away and lowers the pressure on the waterproofing. Without drainage around the foundation the waterproofing works under constant hydrostatic pressure and tires long before its design life.
How it shows: the leak increases a day or two after rain and stops completely in the dry season, meaning water collecting around the building is not being removed.
Prevention: a perforated pipe wrapped in filter geotextile around the foundation, washed gravel over it and a drainage board on the wall face. The pipe must run to a collection point on a fall.
2. Leaving waterproofing unprotected
After foundation waterproofing is installed, sharp stones and plant movement puncture the membrane during backfill. The damage usually goes unnoticed because once the fill is in place nobody can inspect it.
How it shows: a leak appearing at a specific level and as an isolated patch points to mechanical damage.
Prevention: a drainage and protection board over the waterproofing, and backfill placed in compacted layers using screened material.
3. Forming the fall above the waterproofing
On terraces and roofs the fall must be formed below the waterproofing layer. When the fall is created in the screed above it, areas of standing water form at waterproofing level. As long as water sits there it will find the smallest weakness.
How it shows: slow, continuous dripping that carries on for days after rain.
Prevention: the screed to falls is laid before the waterproofing, with a continuous fall to the outlet and no reverse falls or level areas.
4. Trying to resolve details with the main material
Corners, pipe penetrations, joints and outlet connections need separate products and separate solutions. Covering them with the sheet or liquid used across the open surface is the most common error of all.
How it shows: if the leak is near a corner, a pipe or a joint, it is almost certainly a detail failure.
Prevention: fillets at internal corners, corner tape at corners, sleeves at penetrations and joint tape at joints, all selected from the same manufacturer as the rest of the system.
5. Insufficient thickness
With liquid systems the applicator cannot judge thickness by eye. The manufacturer states a consumption per square metre, and if the site falls below it the waterproofing does not perform as designed. This error is invisible when the work is finished and appears two or three years later.
How it shows: widespread, diffuse damp with no single source that can be found.
Prevention: record the total material used and divide it by the area applied to compare against the data sheet. Apply several thin coats rather than one thick one.
6. Poor laps
Laps not fully welded in bituminous membranes, or welding temperature set wrongly in synthetic membranes, is the classic cause of leaks. The sheet itself is sound, the joint is not.
How it shows: a leak advancing along a linear path is following a lap line.
Prevention: a continuous, thin bead of bitumen should emerge at the edge of every lap in bituminous membranes. In synthetic membranes the weld is checked mechanically with a probe afterwards.
7. Bringing incompatible materials into contact
PVC membranes touching bitumen, certain liquid systems touching particular adhesives, or primer and main products from different manufacturers being mixed, all lead to chemical degradation. The result is usually softening, blistering or loss of adhesion.
How it shows: tackiness, blisters or discolouration on the membrane surface.
Prevention: place a geotextile separating layer between incompatible materials. Select primer, main layer and detail products as one system.
8. Applying to a damp or dirty surface
Primer and waterproofing do not bond to a dusty, oily or saturated surface. Water travels freely beneath waterproofing that has not bonded, and the point where it enters can be metres from where it appears.
How it shows: the source of the leak cannot be found and it appears alternately at different points.
Prevention: clean the surface with compressed air or a brush, remove formwork oil residue and observe the moisture limit in the manufacturer data sheet.
9. Not carrying the waterproofing high enough
When foundation waterproofing is not taken above ground level, surface rain and splashback enter at the point where it stops. The same happens on a terrace if the waterproofing is not run far enough up the wall.
How it shows: damp as a horizontal band along the lower part of the wall, level with the ground.
Prevention: take foundation waterproofing at least thirty centimetres above ground level and terminate the top edge mechanically. On terraces run at least twenty centimetres up the wall.
10. Laying the finish without a flood test
The cost of a leak discovered after the finish is laid is many times the cost of the test. Even so, the test is skipped on most jobs.
Prevention: plug the outlets temporarily, fill the surface with water and leave it at least twenty four hours. Check the ceiling below. Do not start screeding or tiling until the test is passed.
How to narrow down the source of a leak
A few observations before work begins will save unnecessary demolition.
Timing. Does the leak increase immediately after rain or days later? Immediately points to a surface entry point, a delayed increase points to water collecting in the ground.
Seasonality. Damp continuing all year is usually related to the water table. Damp seen only in the wet season points to surface water and drainage.
Pattern. An isolated patch means mechanical damage, a linear stain means a lap line, diffuse damp means inadequate thickness or condensation.
Possibility of condensation. If damp concentrates on cold surfaces, in poorly ventilated spaces and in winter, the problem may be condensation rather than a leak. Waterproofing carried out without making that distinction will not produce a result.
Summary
Four things determine how long waterproofing lasts: the presence of drainage, correctly resolved details, achieving the specified thickness and protecting the waterproofing. Get those four right and differences between materials become secondary. Get them wrong and even the most expensive product stops performing within a few years.
If you are not sure of the source of an existing leak, an on site inspection before any stripping begins reduces both cost and unnecessary damage.