Published 25th August 2012
I’d hate to count how many times I’ve written about dampness over the last ten years or so. Rising damp from the ground, falling damp through the roof, damp penetrating horizontally through a wall and even the dreaded condensation damp caused by the unseen source which is most typically you the occupier!
The other day however we were asked to comment on dampness that had started to occur near to ground floor level, on a chimney breast in the middle of a house. The house was a fairly typical 1950’s semi with two reception rooms, both of which had chimney breasts blocked off for many years before the clients bought the house. The clients first noticed the damp patch a couple of weeks ago.
The chimney stack that served the fireplace was a combined one in the centre of the ridge with four chimney pots, two for each of the semi’s. All the chimney pots were still in place and three of them had cowls over but not the one serving this fireplace. Also, the pointing around the pots was breaking away. The pointing to the brickwork of the stack wasn’t good but I’ve seen worse, a lot worse in fact.
To understand what is going on here you have to understand a bit of construction theory. When the house was built, the fireplace at ground floor level would have a flue not just going upwards but leaning over towards the centre of the house. It would do this leaning over in two bites, one at ground floor level within the wide chimney breast and another at attic level. At first floor level it would go straight up. During the 1950’s they started to line flues with round clay pipes rather than the traditional method of a square brick box lined out in mortar applied in a curve to get rid of the square corners. The round clay pipes reduced the soot build up and let the smoke flow faster and more efficiently up the chimney Accessories.
The chimney was intended to be in regular use therefore the possibility of rain soaking into the stack at high level was never a problem with hot smoke flowing up the flue drying out any damp brickwork. Even in summer warm air ventilating out of the living room went up the chimney to the outside. But that isn’t the case now.
When this fireplace was removed and the opening bricked up, the owners failed to incorporate an air vent in the new wall. Presumably they thought leaving the chimney open at the top would be sufficient to keep the flue dry. Unfortunately it isn’t.
The problem here is a combination of factors, all exacerbated by the lack of the air brick at the bottom of the old flue.
The dampness hadn’t been known to happen before, probably because the house hadn’t been subjected to such a prolonged period of regular rainfall as it did from April to July this year. On top of that, just when the brickwork to the chimney was well and truly saturated along came ‘storm Thursday’ and shortly after, the heavy rains of early August. These heavy downpours simply went straight through the brickwork and into the flue. The clay pipes of the flue, being nice and shiny, let the water trickle down the inside right to ground floor level where it dripped onto the old concrete hearth. From here it soaked into the side walls of the chimney breast and bingo, apparent rising damp.
Possibly if the chimney pot had a cowl then less water would have entered the flue. Also if the pointing on top of the stack and the pointing in the mortar joints had been sound then less water would have soaked in over the four month period. But for me the critical factor was the missing air brick at the bottom. Had warm dry air been constantly flowing from the downstairs room up the stack it would have evaporated some of the rainwater penetration and I don’t think the damp would have been seen.
Ah well, they’ve got an air brick now.