When thatch reaches the end of its useful life it will need replacing in its entirety.
Replacement may be undertaken with a diff erent covering such as slates or tiles or with new thatch if thatchers are available. In many cases re-thatching will be realistic and not more expensive than tiling, bearing in mind that tiling will certainly involve new battens and sarking felt, renewal of or addition to rafters and the provision of gutters, rainwater pipes, fascias, soffi ts and diff erent detailing around stacks. If the property is in a conserva-tion area, or is a listed building needing special consent from the local planning authority, it may be that a change in the type of roof covering will not be permitted.
The fi re risk associated with thatch will obviously be greater in older timber-framed buildings where the ceilings and walls may be of lath and plaster or similar construc-tion which is itself a potential fi re hazard in some circumstances. The need to maintain the electrical equipment in a property with a thatched roof in good order, and to test the wiring regularly, is self-evident. This is extremely important since when the thatch is renewed or repaired, the old thatch is often left lying in the roof void. This dry, brittle, dusty material is highly combustible, particularly in the case of straw thatch.
One fi nal warning concerning thatched roofs is the prevalence of insect infestation – in particular furniture beetle and the death watch beetle where oak timbers are involved.
There is hardly a thatched roof in the West Country which does not contain some form of insect infestation, usually Anobium punctatum .
Roof Coverings in General
Lead is a traditional covering for both fl at and pitched roofs with the life dependent upon the lead thickness adopted. Heavy cast lead was used in period houses, churches and other buildings and may last hundreds of years. Modern leadwork is generally the thinner milled lead which is more likely to suff er from ‘ creep ’ on slopes, fl ashings and other details, and from splits in exposed locations or where subject to movement.
Roof Coverings in General
Generally a lifespan of 80 to 100 years is typical for modern leadwork. Modern lead is clearly preferable to mineral felt for fl at areas and detailing, and the author prefers to specify leadwork where possible if the client’s pocket is deep enough, especially in locations with a southerly aspect where mineral felt tends to rapid deterioration.
Large areas of leadwork without rolls or drips at suitable intervals may fail due to ther-mal movements which cause rippling and splitting. In its original state the lead is a bright silver colour but on contact with the atmosphere this changes to the familiar matt grey due to a coating of lead carbonate which forms on the surface. This surface carbonate protects the lead from further deterioration. The only chemical deterioration likely to occur is from strong acid attack and acid can occur in rainwater if this is dis-charging on to the lead from a roof area where there is moss growth, or material sub-ject to moss such as ageing shingles. A point discharge such as regular dripping from a mossy roof will cause pitting to the leadwork due to such an attack.
Zinc was often used as an alternative to lead for fl at roofs and detailing, especially in chimney stack fl ashings and soakers, and small fl at bay roofs in traditional dwelling con-struction. In pollution-free localities the life of such zincwork is generally in the range of 20 to 40 years and sometimes even longer. Good detailing to zincwork is essential for durability, and the rolls and drips are necessary to accommodate thermal movements.
When chemical pollutants are present in the atmosphere, the life of zinc is shorter and failure in the form of pitting and splits is possible after as little as 15 years. As with lead-work, a particular problem can arise with a constant drip of rainwater at a specifi c point over a period causing the acid in the rainwater to wear away the zinc and create a point for leakage. Zinc, like lead, also develops a carbonate on the surface which aff ords some measure of protection but this is thinner than the lead carbonate and does not provide as eff ective protection, especially where the atmosphere is polluted by sulphur dioxide.
Asphalt is often used for better quality fl at roofs and is more durable than built-up felt-ing provided that the surface deckfelt-ing is sound and free from excessive thermal move-ment. A lifespan of 20 to 40 years is generally found to be possible, depending upon the quality of the original job and the degree of exposure to sunlight. Deterioration is indicated by the appearance of surface splits and cracking, and by surface bubbles which form from the expansion of vapour trapped in the decking. Minor surface craz-ing may not of itself indicate general deterioration since the surface asphalt is trow-elled into place and the trowelling is often overworked, as it is on stucco renderings which craze for similar reasons. This overtrowelling brings to the surface a skin of bitu-men which may tend to craze due to slight diff erences in rates of expansion and con-traction on the surface compared with the deeper material.
A normal thickness of asphalt is about 20 mm and anything less than this may be regarded as sub-standard and of a potentially shorter lifespan. Asphalt is not a suit-able material for point loads, and will indent and spread under the weight of water tanks and other imposed weights. Very often considerable damage can be done to an asphalt roof by indentations created by chair legs, fl ower tubs, duckboards and the like. Most problems arise with the asphalt detailing. If the material is laid at angles to slopes and vertical upstands without suitable tilting fi llets, creep will occur, especially in a south-facing location.
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Copper is a traditional roof covering used for small areas of fl at roofs for detailing, and for pitched construction in domes and spires where the distinctive green patina is required for aesthetic reasons. Copper sheet used for this purpose is thin but surpris-ingly durable, although not quite so durable as lead. In time the metal ages from the eff ects of thermal movements, and splits and ripples to the surface which will appear indicate that renewal is required. Rainwater with a high acid content attacks copper in the same way as with zinc and to a greater degree than in the case of lead. The thin metal – especially the rolls – is prone to damage from any foot traffi c or ladders which may be used for maintenance.
Detailing
As a general rule all fl at roofs should have a natural drainage fall and should not hold water. The reason for this is that ponding on the surface, especially with asphalt and built-up felt, will result in marked diff erences in temperature between the wet-cold areas and the dry-warm areas, especially in warm sunlight following overnight rain. This causes relative thermal movements between cold and warm parts which result in rapid deterioration of the surfaces. Generally, in new construction a fall of at least 1-in-40
Detailing
FIG 6.5 Asphalt is used for fl at roof covering, basement tanking and as a waterproof surface coating to steps if there are vaults or other accommodation beneath.
Vapour blowing and creep can be a problem. Creep to the asphalt on these south-facing steps results from poor preparation and application of the asphalt exacerbated by trapped moisture and exposure to the full heat of the sun.
is desirable to allow for possible bedding down of the construction in later life and to accommodate any minor defl ection in timber joists. In existing fl at roofs where any bedding down and defl ection has ceased, a shallower fall may be acceptable but this should not be so shallow that the surface holds water.
Experience tends to indicate that most problems with roof construction arise at the detailing rather than the main areas of covering. Rain penetration of fl ashings and soakers of stacks and fi re walls or to upstands and fl ashings at the perimeter of fl at roofs is common. On close examination many roofs will exhibit a history of repairs to such details, the repairs often being of a highly unsatisfactory nature. Unfortunately the average building owner or lessee will rarely carry out any close inspections him/
herself and in the event of a leak will rely upon the judgment of a roofi ng contractor.
The eventual bill will be paid without knowing whether or not the work done was nec-essary, suitable or reasonably priced. Often it is only when the building is subjected to the scrutiny of a surveyor that the nature of the past repairs comes to light.