Basements waterproofing

Figure 1: A traditional London town house.

Houses that are rendered will offer great opportunity for rising dampness if they do not possess a reliable horizontal damp-proof course. A veritable lobster pot for dampness: once in rarely out.

What any surveyor would consider ‘damp stains’ were apparent in the lower ground floor at waist height. Surveyors will invariably find themselves heading downstairs to a lower ground floor or basement to survey a damp problem.

Most brickwork begins to be significantly damp when its moisture content is 1–3%. So at A and B the wall is definitely very damp, at 5.2–6.9%.

At height C the brickwork only contains a marginal amount of moisture (0.5%) – the amount of moisture the brickwork would pick up from typical British dampish air, i.e. not from damp penetration, rising damp, a plumbing leak, etc. So at C we could refer to the wall as ‘air dry’. This is in effect the control reading for the wall.

But why stains at waist height – please look closely at figures 2 and 3. All is revealed after a little plaster removal.

Figure 2: At ‘A’ the decision was taken to cut out a square of plaster not far from the front room grand – in an attempt to find out why plaster is failing to contain dampness at this height above floor level.

Figure 3: The wall section – typical detail at top line of waterproofing. The author’s understanding of how damp patches have resulted at position A.

At A, where we cut a square of plaster, figure 3 shows how dampness within the wall is finding a way out. The damp damage coincides with the junction of two different waterproofing systems.

Vertical analysis of damp readings and symptoms at test position

Test	Height	Plaster type	Carbide MC	Search mode	Pin probe	Salts
C	1850mm	Existing cement render	0.5%	154	16 R/R
B	1500mm	Existing cement render	5.2%	171	17 R/R
A	950mm	Existing cement render	6.9%	1000	60 R/R	Nitrate 75mg Chloride positive
Plaster join	900mm			1000	100 R/R	Salt damp
	150-850mm	Synthaprufe + Carlite		200-210	11-20 R/R
	Skirting				10.2%

Diagnosis

Before cutting out a plaster square, you might have expected you were witnessing rising dampness – that has just managed to climb a little above a waterproof render system that was applied from floor to 900mm height. However, it can be dangerous to limit rising damp in our minds to the common 1m rise!

As carbide test results of drilled samples showed, dampness seems to be rising within this dividing wall (between the two main lower ground  floor rooms) much higher than the oft-quoted 900mm. As you can see from carbide meter tests, the walling was very damp to a height of 1500mm.

The truth was that moisture seemed to be finding a way out at the junction of two quite successful waterproofing coatings. Electronic moisture meter readings were taken vertically up the walling from floor to ceiling on the line of the plaster cut-outs. The readings were always low, except at the junction line of the two plaster types, where high readings were recorded.

Looking at the plaster cut-out at A, the bottom half of the exposed substrate was black. This is synthaprufed brickwork. In the 1980s, the existing cementitious rendering was hacked off up to a height of 900mm. The walls had originally been cement rendered to full height.

The walls from floor to 900mm height were then ‘waterproofed’ with three coats of Synthaprufe. A gypsum-based undercoat was then applied onto the Synthaprufe, finished with a gypsum skim coat. (Many surveyors may take a deep intake of breath here – gypsum plasters are always considered unsuitable for below-ground walls. However, applying gypsum plasters to three coats of Synthaprufe has proved a successful waterproofing method at many properties over the years, as long as gypsum plasters make no contact whatsoever with any damp floor or (more importantly, whether chemically injected or not) the wall surface, to create a bridge.)

According to Synthaprufe technical literature, the method of waterproofing at the subject property would not be considered suitable for below-ground situations. At this property the lower ground floor was around 500mm below ground (probably not deep enough to really feel like a basement), so below-ground basement waterproofing requirements would apply.

Three coats of Synthaprufe plus gypsum plaster = ‘damp-proofing’ rather than ‘waterproofing’.

The problem lay in the presumption that dampness would not defeat a 900mm wall barrier.

Key point: inner walls that are waterproofed on both sides really feed dampness upwards if there is no effective horizontal dpc present.

Here we also needed to consider the fact that the external ground was around 500mm above the internal floor level. The least you would need to do would be to add 500mm to the 1m (standard assumption) – so we could expect dampness to rise a minimum of say 1500mm up the basement wall. Safer still to give the dampness a 2m high jump challenge.

The damp inside the wall actually petered out at around 1700mm. Dampness reaching to ceiling height or thereabouts would be a concern, as this would be where ground floor timbers would be located.

Remedy options

Extending the Synthaprufe coating would prove practically impossible, as the Synthaprufe peels away as plaster is hacked back.

You would need to hack off the existing waterproofing as well as the render coat above, and apply a new waterproof system, probably to full height in all the rooms exhibiting damp patches. This would be expensive, but a guaranteeable option.

Whether the route is a multi-coat render system or a cavity membrane air gap system, the cost of producing a reliably dry decorative finish for this flat would not be cheap.

The water threat requires a belt and braces approach. Before you begin to question the wisdom of hacking out three squares of plaster, sometimes invasive investigation of buildings can cause very little disturbance or aggravation to occupiers. In this case, the occupier had agreed to some invasive investigation – as long as the cut-outs were behind one of the large wall paintings!

Alternative basement waterproofing scenarios

Figure 4: The ideal basement: no internal dividing walls, simple plan shape, no changes in levels.	Figure 5: Waterproofing discontinuous at dividing wall intersections to earth retaining walls. Vertical chemical injection at X would be doomed to  fail, as chemical injection  is designed only to withstand moisture movement by capillarity. Water below ground can be under much greater hydrostatic pressure.

Figure 6: Basement  with dividing walls, waterproofing discontinuous. Lateral penetration is likely at the junctions of earth retaining/dividing walls.

Figure 7: Difficulties of waterproofing junctions of dividing walls and earth retaining walls solved – the dividing wall is physically separated from the earth retaining wall. This may be by a partial height removal of wall material, or by connection of dividing wall to earth retaining wall by a restraint joint after waterproofing – where the divider is tied to the outer wall by sleeved connectors  set into  sealed pocket housings in the outer wall.

Figure 8: Waterproofing continuous around dividing walls. There is a good chance of success, providing door frame fixings do not compromise the waterproofing. Any subsequent internal alteration would compromise waterproofing integrity.	Figure 9: Internal partitions are removed, and structural support provided over if necessary. This opens up the basement space and would offer a good chance of success for waterproofing with less junction and intersection detailing required.