BUILDING BRIEFING:
Pitched Roofs
Property managers are often called upon to inspect roofs on properties and specify remedial works. Phillip Shaw ARICS looks at some common problems.
Most pitched roofs are covered with either slate, synthetic slate, clay tiles or concrete tiles. They are usually laid upon a framework of timber rafters across which are fixed timber battens.
The rafters are, in turn, supported by timber purlins. Slates and some older tiles are nailed to the battens whereas most tiles are formed with nibs that hook over the battens.
The age of the house will greatly determine which material has been used and whether other elements have been introduced, the two most important being sarking felt, a layer of felt between the battens and the roof trusses, and ventilation.
Common defects
A number of defects arise from poor design and initial construction.
Common amongst these is the use of inadequately sized timbers to support the weight of the roof covering. This can give rise to bowing or even splitting of timbers (seen within the roof space) and dishing of the roof span (seen externally).
When the dishing is slight and there are no split timbers it may be that the introduction of additional support is all that is required to arrest any further deterioration. When severe, some replacement of the roof covering may be necessary after replacing defective timbers with new, correctly sized, supported and treated timbers. If this is necessary it is an opportunity to introduce sarking felt and provide adequate ventilation.
Roofing felt
Sarking felt is an important element to avoid damp penetration of the roof. Driving rain or snow can creep up the surface of slates or tiles and enter the roof. The felt allows it to run off without coming into contact with the roof timbers. If the felt is laid too tight it tends to tear. When laid too loosely it will sag, allowing water to collect which can cause wet rot to the battens and damage the felt.
Many older roofs have no felt. A common device to avoid damp penetration was to provide torching; a mortar mix that fills gaps between the battens and slates or tiles. The mortar has a tendency to shrink and, with age, perish then falling away. It is common sight when inspecting roof voids to see everything overlaid by dust and mortar particles.
When this has occurred it is likely that there will have been damp penetration and moisture will have adversely affected the battens.
Before the mortar has degraded it will have blocked ventilation which, in turn, may have led to deterioration of the roof timbers.
Slipped slates
A common and easily seen problem with slate roofs is when slates have slipped or are missing. To replace a slipped or missing slate requires the use of a clip, which, especially when large numbers are used, can be unsightly.This is mainly due to nail "sickness". There is, inevitably, some moisture between the roof covering and the battens and this can lead to the rusting of the nails securing the slates. They then fail, causing the slates to slip.
Turnerising
In many instances where this and other slate roof problems have occurred owners have resorted to the short-term solution of turnerising - generally an unsatisfactory method of prolonging the life of a failing roof. This involves replacing or re-fixing slates then applying layers of bitumen, that is re-enforced with fabric, to the whole of the external roof spans. When the next problem occurs, and you can be sure one will turn up, the whole roof covering, including any good slates will have to be replaced.
When turnerising is encountered budget for re-covering and also carefully inspect the roof timbers because there will have been reduced ventilation and possible moisture retention.
Problems
With all types of pitched roof coverings the actual material used can, sometimes, deteriorate. The top, exterior surface beginning to flake and wear away often evidences this.
Natural slate roofs
The quality of natural slate can vary dependent upon its source and how it has been dressed. If the slate has some impurities these can react with elements within rainwater to produce sulphuric acid which will begin to attack the slate causing its top outer surface to begin to flake. Once the deterioration has begun it will become progressively worse through weathering and, in particular, frost action.
As with all problems relating to defective roof covering materials, if the problem is extensive then the better remedy is replacement; but, if isolated, spot replacement may be adequate.
Clay tiles
Unlike slate roofs that are intended to be impervious to rain, clay tiles, whilst designed to cause rainwater to run off, nevertheless allow absorption of moisture but are thick enough to enable this to evaporate before penetrating to the underside.
When there is a combination of heavy, prolonged rainfall followed by freezing conditions some moisture retained within a tile may become frozen, it then expands and can cause the top surface of the tile to crack and begin to flake or perish. Further weathering and frost action will accelerate the deterioration. If the problem is not addressed then there is the likelihood of damp penetration affecting roof timbers, the roof void and then progressively ceilings. The older the clay tiles the more likely the possibility of surface wear because of long term exposure to weathering.
In some localities there can be a tendency for moss or lichen growth to occur on roofs. The growth will retain moisture that can then freeze and begin the deterioration process.
Synthetic slate
External deterioration can occur where man-made materials are used. Many natural slate roofs have been replaced with lightweight man-made slate rather than with clay tiles to preserve the appearance of the building and to avoid having to introduce additional roof timbers to bear the load of the heavier clay. Today the quality of synthetic slate is very durable and reasonably matches original slate; however, up until recently there have been some products that have deteriorated within a short time after installation.
A common problem has been the tendency for some of these slates to curl at the edges. If the curling is pronounced then water will be able to penetrate; if slight it will be merely unattractive. Another aesthetic problem is that many of these types of synthetic slates have become discoloured at their edges.
Concrete tiles
Some concrete tiles suffer a similar problem of discolouration and, when poorly manufactured, can become degraded by weathering and, in particular, frost action resulting in top surface flaking and perishing with all the similar progressive consequences associated with natural materials.
With all of these flaking and perishing problems a visual inspection should be sufficient to determine their presence. Binoculars will help to scan external surfaces and a torch will be useful within roof voids.
Summary
As with flat roofs (see previous article), regular property inspections should include a visual check of roof details. A visual inspection will help determine the presence of failing roof materials and other defects before they cause more major and costly damage.
Further Information:
Books and other references:
Understanding Housing Defects by Duncan Marshall, Derek Worthing & Roger Heath, 2nd edition.
Published by Estates Gazette Books, 2003. ISBN 0 7282 0417 7.
Tel: 01444 445335
The Repair and Maintenance of Houses by Ian A Melville & Ian a Gordon.
Published by Estates Gazette Books.
ISBN 0 900361 43 3
Tel: 01444 445335
Estates Gazette Books website: www.propertybooks.co.uk