What could possibly go wrong…

Cellar perils

I seem to have been plagued by ‘virulent’ cellars of late. Two particularly extreme cases spring to mind. Both were typical 18th century underground brick cellars that projected underneath gardens. In both cases, modern building works had been carried out immediately above the cellars, and their original vents to the outside air had been removed. A large expanse of concrete that included a damp proof membrane had been laid over one cellar around 40 years ago, while a new extension had been built over the other cellar around 15 years ago. And the results…

Where the concrete covered cellar was, rainwater from the extensive concrete was draining over the edges and saturating the ground around the cellar. None of the water spreading under the concrete could escape, so was percolating into the cellar – which had effectively become a drainage sump for the concrete! Inside the cellar, water was continually seeping through the walls and ceiling to form stalactites, the floor was underwater, and the humidity was 100%. This had provided ideal conditions for severe wood rotting fungal attacks to take hold in the timber floors of the house above, centred on one wall in common with the cellar.

As for the other cellar, the extension above it ran just past the end wall of the cellar, with a patio then being laid beyond. This created a very similar situation to the concrete covered cellar, but this time rainwater from the patio was draining down beside the extension and into the blocked chute for the old cellar vent. Again, the humidity inside the cellar was 100%, and as the water couldn’t evaporate, it had spread through much of the cellar brickwork. In this case the wood rotting fungal attacks were still modest as the causal changes were relatively recent, but extensive and severe dampness was spreading through the house above from the one small wall section in common with the cellar.

One potential consequence of keeping fish

Every so often, I come across a home suffering from mould or condensation caused by the humidity generated by a large aquarium filled with tropical fish. When asked to investigate widespread mould growth and a damaged woodblock floor in a bungalow though, the humidity source turned out to be in a whole new league of its own. When I walked into the worst affected room, I was greeted by... a large raised garden feature complete with a central fountain and 3 tiered fish ponds linked by waterfalls! Evaporation from the ponds, waterfalls and fountain, was overloading the room with water vapour. The resulting high humidity was then providing perfect conditions for mould to grow on the walls and ceiling. The woodblock floor was absorbing the excess water vapour, causing it to swell and lift in the centre of the room. Fine spray from the fountain itself had also been drifting onto the adjacent woodblock floor, and had caused some of the blocks to start rotting. As is often the case, patterns in the mould growth also showed up defects in the cavity wall insulation which are normally only visible with a thermal imaging camera. In case you are wondering, the solution was not to take out this impressive garden feature, but to fit appropriate vapour extraction and adjust the fountain slightly.

Ignore the warning signs at your peril

One of the worst problems I have encountered concerned a thatched cottage in Norfolk, built around 1650. The walls of the cottage were timber framed with wattle and daub infill panels, all covered externally with cement render. The plumbing to a ground floor shower over a bath had developed a leak which had gone undiscovered for several years, despite numerous visible signs of problems – severe rot appearing in all of the exposed timbers where they emerged from behind the tiling, the tiled panels themselves dropping out of position, plaster blowing next to the tiles, and decaying plaster and decorations in the adjacent room on the wall backing the bathroom. The leak was only finally stopped when water starting running out from underneath the bath onto the raised tiled floor! Even then, no attempt was made to dry out the structure, or even to repair the holes in the tiling where the panels around the shower had dropped. By the time I was called in, the wattle and daub infill panels behind the tiles had collapsed into the space under the bath, much of the timber frame had totally rotted away, and the rot had spread behind the cement finishes to severely damage about a quarter of the building. The first floor and roof in that part of the building seemed to be supported entirely by the thick cement render on the outside of the house – and even that had developed some stress cracks that indicated a sudden failure was on the cards.

Experimental 'cavity' walls are earlier than you think

A Norfolk rectory had the earliest forerunners to the cavity wall I have so far come across. The house was built around 1852 and had two corner wings. Both wings had two hollow outside walls. (True cavity walls in the modern sense are quite rare prior to about 1950, before this date they are generally just hollow walls.) Superficially, all outside walls to the house resembled solid walls. A wall in one of the wings had a strange damp patch at the base inside the house – despite its location, it didn't resemble ground moisture and there were no other apparent moisture sources. The investigation revealed the wall was hollow, but being so early an experiment, the inner and outer sections were held together by hardwood blocks! Many of these blocks had become loose and could be slid straight out so I had to devise a way of re-fixing them. As for the damp, it turned out to be blocked guttering that was allowing water to enter the top of the hollow wall and run down inside, before collecting at the bottom and soaking through into the house.

Plastic bags or the latest in building technology?

A 17th century brick farmhouse in Suffolk had a particularly severe damp problem affecting two outside walls of a ground floor room. On one of the walls, the moisture pattern and location suggested water was leaking from a concrete covered drain taking roof water to a drainage ditch. On the other wall, the moisture was building up between internal cement plaster, and cracked external cement render, about 2ft up the wall. When the concrete paving over the drain was broken up, it was discovered the top of the drainpipe was completely missing. However, to stop the concrete filling the pipe when it was laid, the builder had stuffed plastic bags into the pipe! On the other wall, when the cracked cement render was removed, the mortar in one of the horizontal brick joints running across the wall had been removed and... stuffed with plastic bags then rendered over, presumably to create a proprietary damp proof course! All of the work concerned appeared to date from the 1970s and was presumably carried out by the same builder. This innovative use of 1970s plastic bag technology does not appear to have caught on elsewhere, but how many more houses in that part of Suffolk are still enjoying the benefits of this former builder's innovations?

Dodgy alterations are nothing new

In the late 18th century, a large late 17th century detached house in a Norfolk market town was reorientated. This was done by building a large full width extension across the back of the old house, to provide a new double-height entrance hall and side wings. The old back wall was retained, and became a spine wall to provide the main support to all of the floors, and for the old and new roof structures. So far so good... However, the top of the spine wall/old back wall had visibly sunk where it supported the later roof, it had developed diagonal cracks in the bedrooms immediately below that, which had broken through new plaster, and the bedroom floors had pronounced dips against the wall. In the ground floor rooms, the wall was hidden behind panelling and kitchen units, and the floors were level replacements. However, there was a cellar accessed from the older part of the house, which had been extended when the 18th century extension was constructed. This had required the removal of part of the old back wall/new spine wall. The section of wall that was removed was directly below the problems higher up! So how was the wall supported? The only support was provided by some standard 18th century oak floor joists that had been run across the underside of the wall, from one side to the other. The floor joists were badly bowed, and had been allowed to become very damp so were riddled with a severe Death Watch Beetle infestation, which had almost completely destroyed large parts of them – in fact, only about a 1” depth of sound timber was left at the ends of some of the joists.

The joys of damp proof courses…

Damp proof courses can be useful in certain circumstances, but are easily made useless, and can be far from benign if not thought through or understood correctly…

A rectory in Suffolk was built in 1876, this was the first year that damp proof courses were compulsory, so all ground floor walls incorporated a slate damp proof course. This was likely to be the first time the builder had ever had to include a damp proof course but it was properly constructed, continuous, and placed a suitable distance above the external ground level. However, as was normal at the time, the ground floor of the extensive service wing was lower than the principal accommodation, resulting in the damp proof course being well above floor level throughout this area. This had concentrated moisture against the underside of the slates in the service wing, and the plaster had then allowed it to bypass the damp proof course. Fed by the moisture that had collected under the damp proof course, this doubled the height the ground moisture could rise to.

A large early Victorian house in an East Anglian city was extended in the second half of the 19th century. The extension had a slate damp proof course which was well above the maximum height that ground moisture could reach. The ground floor joists rested on top of a horizontal timber beam that was above the damp proof course. There was a fatal flaw in this design though... the beam under the joists sat directly on top of the slate damp proof course. Although ground moisture failed to even reach the damp proof course (the brickwork immediately below was completely dry and salt free) and the brickwork around the joists above the beam was also completely dry, the underside of the beam sitting on top of the slates was wet and large areas had rotted away. Every time a brick wall gets wet in the rain, the moisture spreads out through the brickwork before evaporating again. As with any damp proof course, the moisture spreading down through the brickwork couldn't get past the damp proof course, so instead, ran across the top of it and deeper into the wall. In this case, the damp proof course carried the moisture straight under the beam which then absorbed it.

A rectory in Cambridgeshire was built in 1868 with an experimental damp proof course, consisting mostly of bitumen, but with slate sections where door openings and steps were present. Being experimental, while the damp proof course itself was just above ground level, the ground floor of the house was raised almost 3ft above the ground, so even if the damp proof course failed to work, no ground moisture would normally be expected to reach the interior. Again there was a flaw in the design though – only the principal rooms had suspended timber ground floors. The reception hall, corridors, and service wing all had solid floors consisting of tiles on a thin bed of chalk lime mortar, which had been laid over soil added to make the floor levels match the rest of the house. The damp proof course in these areas was therefore under 2 feet of internal soil. Ironically, if the house had just been built as normal, with the ground floors being 9” above the external ground levels and the solid floors being laid over a layer of thick sand and/or a thick bed of chalk lime mortar, they would not have been troubled by damp.

About David M. Kinsey

I am a semi-retired Norfolk based independent Surveyor, specialising in damp and the problems of period houses. I used to cover pretty much all of England, Wales and Southern Scotland, but these days, I only undertake surveys in Norfolk, Suffolk, Cambridgeshire, Lincolnshire, and north-east Essex.

My building surveying career dates back to the late 1970s. I have also been a Chartered Information Systems Engineer since the mid 1990s.

In common with other registered professionals, I have to keep my knowledge up to date. This is achieved by attending professional seminars and courses, as well as keeping up to date with published research from various bodies. I am also the author of the paper titled ‘Meeting the challenges of structural damp diagnosis in older buildings’ published in the Journal of Building Survey, Appraisal & Valuation (volume 8, number 3) by Henry Stewart Publications.

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