Wednesday, February 4, 2009

What the Victorians Didn't Do for Us - Part2

I'll start at the bottom with the below floor stuff

Firstly, in this part of London the soil is clay and subject to a fair amount of movement and subsidence. If you look carefully at the photo of the front facade, you can see that the bay on the front is in a different brick to the rest of the walls and most likely has been rebuilt in the past.

The Victorians approached structure from the sole point of keeping things standing. Damp-proofing was rudimentary, and the brick structure was flexible enough to withstand most ground conditions. At that point in time they didn't have a knowledge of plastics so dpms (damp proof membranes or polythene to most people) did not exist. The brick external walls are solid and 9 inches thick. These are made of bricks that are quite soft by today's standards and would have been bedded in lime mortar (importantly this is roughly the same softness as the bricks- ie. it's slightly crumbly - these bricks still hurt if dropped on your head!). At the bottom the bricks step out to rest on the concrete strip foundations and this helps distribute the load since the footings were not reinforced.

They were aware that damp rose in the porous brickwork and that this could damage timber, so they built layers of slate 6 inches above the outside ground level into the brick walls. The slates were lapped and stop the water rising from the ground below. In addition to this, the void under the timber floor is ventilated to further reduce the moisture level in the bricks and timber. So all the external walls are punctured by air bricks below the level of the slate dpc (damp proof course - these days this is usually bitumen felt rather than slate). The timber floors are suspended above the ground on brick stub walls (built with holes in the coursing to allow air through). This means that the ground floor is supported independent of the potentially damp outer walls and ventilated from below. To stop moles and provide some stability to the party walls there is a ground slab laid between the concrete footings.

The important thing to realise is that if you compromise the ventilation the damp-proofing doesn't work and the chances are that your timber will start to rot. People tend to block the airbicks accidentally, or else deliberately to reduce the cold air drafts internally. The traditional airbricks are also great for letting in mice which is another excuse to block them. Repointing the brickwork with modern cement mortars can also cause the soft bricks to spall from frost damage and crack with movement.

Another issue with this type of suspended floor at ground level is: how do you insulate it? The stub walls are deliberately sited to reduce the depth of the joists needed to span under the floorboards. On this house this means that they are only 4 inches deep (even with the best insulants this will not meet the current Building Regs standard).

The next installment will talk a little more about the walls....

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