The construction of pitched roofs can vary with the age and type of the house and the regulations that covered the construction when the dwelling was built.
The construction of older roofs will generally involve a lot of very substantial timbers with big chunky joints. Roof trusses of this type were used in varying forms until timber rationing after the Second World War.
After the 1940s there were a variety of systems for building roofs, some of which involved the use of structural steel. The need to conserve timber required that the trusses used less timber but were equal in strength. Various systems were employed until the pre-formed truss was invented.
In most new build properties the main roof structure is of pre-formed trusses (gang nail trusses). This form of construction is popular because the trusses can all be manufactured using mass production techniques and are therefore more cost effective than the more labour-intensive older-style trusses. Additional benefits are that they are very strong and do not impose the same loadings on the structure below.
In older roofs the roof covering was constructed on to battens with mortar applied to the underside between the slates and the batten to stop rain and snow being blown into the roof space. The additional benefit of this mortar is that it limits the movement of the slates/tiles and therefore prolongs the life of the roof. This system allows the roof to ventilate in the normal cracks between the slates.
Later as construction developed the roofs became under-felted. This was initially a fabric covered with a bitumen compound but later evolved into the modern reinforced plastic sheeting. This is almost entirely used in new build and in resurfacing of existing roofs. The sheeting replaces the mortar back-pointing and also acts as a second line of defence against damp penetration.
When insulation is installed it can be stuffed or blown into the eaves of the roof. The ventilation of modern roofs is generally through the eaves and therefore the insulation can severely restrict ventilation to the roof space.
If there is inadequate ventilation an environment is created that can enable timber decay to occur. Ventilation should be maintained at all times to roof spaces.
The roof surfacing material will have a direct influence on the size and extent of the structure required to support it. Any change in the roofing material will have an effect on the supporting structure.
The materials used in the roof coverings are varied. By far the most common are slates, clay tiles and concrete tiles. Other roofing materials include asbestos tiles, bitumen, thatch, profiled metal sheeting and more modern composite slates.
The relative weight of the material and the required gradient will dictate the style and form of the structure above.
The materials used to provide the covering will also age and weather in differing ways. The method of securing the slates can also affect how the roof ages.
Wind loadings on roofs can cause significant stresses that over time can cause the deterioration of the structure. In modern pre-formed truss roofs it is a Building Regulation requirement that diagonal wind bracing be installed. Older roofs are at risk of wind damage to structures where the additional bracing was not installed. Older-style roofs are not immune from wind damage; however, the more substantial jointing makes the roof more able to withstand high wind loadings.
If caught early the repairs can be relatively inexpensive. However, we would strongly recommend that a building surveyor be engaged to provide a specification. Alterations to roofs should only be carried out with specialised knowledge.
Roofs will deteriorate because of tiles lifting in strong winds. If missing slates or tiles are not replaced through general maintenance the wind through the roof space could hasten the deterioration of the roof and cause further damage. Typically a roof will have one slope with the wind loading the surface and the other elevation there will experience negative pressure (a lifting force). The result is that slates and tiles will work loose or more catastrophically the whole roof could be lifted off.
It is essential to keep the roof surfaces well maintained. Check the roof surfaces after gales and have good household insurance.
Roof spread typically occurs if roof trusses are inadequate or if the roof has been resurfaced with a heavier tile than the roof was originally designed for.
The defect is identified by sagging of the roof surfaces and sometimes cracking and distortion of the main walls at eaves height.
The repairs can vary in cost and extent. In extreme cases the whole roof and supporting structure will require replacement. In most cases, the roof can be repaired by providing additional bracing within the roof space. Calculating the loading of a roof and recommending additional bracing is a specialised area and should be to a specification calculated by a building surveyor or structural engineer.
This defect is caused by the deterioration in the securing nails in a roof. Nails in older roofs are often only mild steel and are prone to rusting and breaking. This result is that the roof will require more frequent maintenance. As the problem with the nails worsens, the cost of repairing the roof will escalate to the extent that the roof may become beyond economic repair. At this stage the only repair is renewal of the roof covering.
Once a roof has become seriously affected by nail sickness the only appropriate method of repair is to strip and resurface the roof. When a roof requires resurfacing, care should be taken to choose a covering with a similar weight to the old roof surface. (See roof spread, above.)
A nail-sick roof will have many loose and slipping slates; many more may have been rewired into position. Our advice is to seek the advice of a qualified roofing contractor.
If your roof is in need of complete resurfacing, how about considering the installation of a roof that can supply electricity to your home?
On the whole roof trusses are of timber and are of a softwood material. Timber can split as it dries out and seasons. In older roofs the roof timbers will have these splits and there will not be any undue effect on the structure. Problems will occur where the splitting goes through the whole of the timber and affects the structural integrity of the beam.
Where this occurs you will often note a sagging of the roof similar to roof spread but the damage will be more localised. The repair is often carried out by replacing the damaged timber or sandwiching the timber with new timbers. The new timbers will be bolted and bonded in place. This is not a DIY repair and should be carried out by either a roofing contractor or a joiner working to the specification of a building surveyor.
Rot and Woodworm
Rot and woodworm will over time affect the load bearing capacity of the roof timbers in both tension and compression as the decay reduces the effective size of the structure.
Leaking roofs and defective flashings to the roofs can cause rot and woodworm. A well maintained roof is much less at risk. Outbreaks of rot and woodworm in roof spaces will need specialist treatment to treat and prevent further outbreaks. (See timber and damp)
The tiles and slates on any roof will deteriorate with age. How quickly this happens will depend on the location, the likelihood of frost, whether the surface is north or south facing and whether there is any vegetation (moss or ivy).
Some materials such as asbestos tiles are particularly prone to weathering. These were often used in the 1940s and 1950s and in most cases are now weathering and becoming porous. Where asbestos is the material used, the costs of removing the material can be significant and the job should only be carried out by an approved contractor with experience in this area.
Normal slates and tiles will also deteriorate by splitting, cracking or exfoliating. In most cases repairs can be carried out through routine maintenance, though in some cases a bad batch of tiles may deteriorate more rapidly and require that the roof surface be renewed. Slates or tiles of a similar weight should always be used when resurfacing roofs.
Flat felt roofs have a limited life expectancy and ongoing and accelerated maintenance should be anticipated. How many times have you seen that on a survey or valuation report?
Flat roofs are on the whole made of bitumen. This material is almost fluid when heated up by the sun in hot parts of the summer and will be rock hard in the winter. The heating and cooling of the bitumen will through time cause blisters and pleating to occur in the material. The constant movement will also stress the joints. It is inevitable that the roof will deteriorate more rapidly than other materials. The average life of a flat roof is 13 years.
If moisture does enter the cavity below the decking, then older felt roofs may not have enough ventilation to remove the moisture, and deterioration of the supporting structure can occur. If there is no vapour barrier between the accommodation and the roof cavity, condensation can occur, which can have a similar affect possibly promoting rot.
Felted roofs should not be left alone. If there are a lot of blisters, water remains on the roof for some time, the felt is old in appearance or the joints are cracking then it is nearing the time where your cheque book may have to be made ready. Fortunately in most cases the resurfacing of a felt roof is not prohibitively expensive.
Other flat roof surfaces can be of lead and copper. On the whole these surfaces are far less likely to experience the problems of felt roofs. The materials do however expand and contract and if poorly constructed and detailed then tears and splits can occur in joints, flashings or edging, which can allow leakage. The temptation to fix a defective roof by painting it with a thick coat of bitumen tar should be avoided; this is only a temporary repair at best. By far the preferable repair is renewal with a similar material; if funds are tight a bitumen felt roof could be installed.