Tuesday, February 10, 2009

What the Victorians Didn't Do for Us - Part 4

The Roof.

The pitched roof spans between the two brick party walls, and in this case is slate. The slates are overlapped so that at any point on the roof surface you get three slates thick. This is designed to stop leakage through the gaps (not entirely, but the law of averages means that during rain most of the water flows down the roof slope rather than through the gaps). The slates are nailed to timber battens, which in turn are supported on the roof rafters.

Unlike a modern roof, there are no trusses, and the structural frame is braced by the purlins and cross braces at centre span. In addition there is no roofing felt under the battens and over the rafters. In a modern house the felt stops drafts, and ensures that any water that gets through the slates has a path down the slope to the eaves. The Victorians didn't have an equivalent to this, and their buildings were built this way to ensure maximum ventilation to both the roofspace (to prevent the growth of wood rot) and also to allow air in to fuel the coal and wood-burning fireplaces.

You can see on the right that the rear chimney breast has been removed, and the remaining stack is now supported on a concrete lintol bearing on a steel L-section bracket. This is in accordance with the latest regulations, but it would have helped if the previous builder had cut the lintol back and had enough confidence in their work to remove the additional timber props under it! Perhaps refixing the purlin support would have been a good idea as well?

The battens and slates continue over the party walls and form a continuous slate roof between the adjacent properties. There are no firestops at this point, and the pointing on the party walls is intermittent, so there isn't much of a smoke barrier either, in the event of a fire next door. Where the roof abuts the chimney the rafters are trimmed, and the slates are cut and made weatherproof by using lead flashing. This is thin sheets of lead, tucked into the slate courses and bedded into the vertical surfaces of the brick chimney.

The chimney itself normally has a slate course at the top acting as a dpc to stop damp moving down the brickwork from the top. Similarly lead sheet is used as a valley gutter where the rear outrigger of the house hits the main roof. At this point it sits on what is known as a valley rafter (here this is just a couple of boards, rather than a full beam).

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