FIG 6.7 In Victorian London almost every room had a fi replace with the fl ues gathered together in shared chimney stacks over the party walls. Lots of scope here for downward damp penetration. Redundant fl ues should be vented at the base with hooded pots on top to keep them dry. Often only a limited view will be available from ground level even with the use of binoculars but many of these roofs have external trapdoors from the roofspace, especially the centre-valley or “ butterfl y ” London roofs, so the surveyor can climb out onto the roof to view the construction at close quarters.
Gutters
Generally, all good roof design should incorporate the provision of adequate guttering and rainwater pipes to carry all roof water away to a storm drainage system. It is impor-tant that stormwater be taken away from the perimeter of the building to prevent damp penetration through walls and to avoid subsoil erosion from continual discharge of roof water into soil adjoining the main walls.
Certain traditional roofs encountered in period construction will not have gutters or downpipes, particularly in the cases of thatch and traditional stone slab construction where a deep eaves overhang is provided instead. It is vital with this type of construc-tion that the area around the building should be paved, with a suitable drainage fall away from the main walls and with a good clearance between exterior and interior lev-els. A rendered plinth treated periodically with a bituminous waterproofi ng paint would also be advised. An examination of a number of thatched cottages over the years will reveal that dampness at the base of the main walls, especially on the weather-sides, will invariably be a problem unless the run-off of water from the roof slopes is adequately catered for. A combination of thatched roof, solid rendered walls of stone and lime mortar, and high exterior ground levels is often encountered; not surprisingly in such circumstances, invariably there will be a damp problem at the base of walls internally with a history of plaster repairs, or false walls applied over damp plaster.
A client proposing to buy a cottage with a thatched roof may have seen it through rose-coloured spectacles, especially if he fi rst viewed in summertime. Such a client may have thought it quaint that visitors step down into the building instead of up, and will view uncritically the moss-covered and loosening rendering around the base of the building.
The vendor’s panelling to dado height around the ground fl oor rooms could be consid-ered part of the charm. It may not be until the following winter that the client begins to detect the eff ects of damp penetration at the base of the walls internally and that the carpet underlay is becoming mouldy around the skirtings. Perhaps then the signifi -cance of the high exterior levels and the nature of the construction will be appreciated and the client may feel inclined to look out the surveyor’s report to see what was said on these subjects.
Often, small projections devoid of gutters or rainwater pipes are built and this will be commonly encountered in small bay roofs and porches of all ages. No matter how small the projection, guttering and downpipes are always advisable unless the roof over-hang is very deep. Where these are omitted it will often be found that exposed sills and woodwork beneath have deteriorated from the eff ects of dripping rainwater. Staining down walls is also commonly found where guttering is omitted and this can cause con-siderable discolouration on north sides where moss growth is then encouraged.
Rainwater will occasionally be found taken to water butts instead of to stormwater gul-lies and this may be unsatisfactory, especially if the roof area is large, since overfl owing from the butt will be undesirable. If water butts are needed it is suggested that gullies also be provided to take surplus water from the butts.
Modern PVCu rainwater systems have been found durable and maintenance-free if correctly installed with adequate brackets and sponge seals. Older rainwater systems
103 Gutters
FIG 6.8 Modern gang-nailed trussed rafter roofs require diagonal bracing and gable-end strapping to prevent racking.
FIG 6.9 Hampshire thatch with eyebrow window details.
will be encountered which may be less durable or suff er other drawbacks. Cast iron can be long-lasting if properly maintained but maintenance is often expensive and conse-quently poor and the hidden surfaces such as gutter interiors are often left unpainted.
Asbestos-cement rainwater systems appear to have a useful lifespan of about 30 years and then become porous. Pre-cast concrete ‘ Finlock ’ gutters, commonly used in the nineteen-fi fties, need periodic internal bituminising to avoid downward penetration of water into the walls beneath, especially at joints. Where an existing rainwater system needs repair and repainting it is often advisable to consider complete renewal in PVCu since the cost is often comparable and the fi nal result should require no further main-tenance in the foreseeable future.