April 13, 2008 – 2:10 pm
Roof structures are often more complex than they appear and even minor defects may indicate major problems. Effective treatment depends on thorough analysis of the structure as a whole, without which it is impossible to identify the extent of a problem and its cause accurately. This article describes some of the main roof types and summarises their structural behaviour.
Roof Types
Roofs come in two types: single and framed. ‘Single’ roofs have a large number of braced pairs of rafters, spaced about half a metre apart. Each pair carries its own loading independently of any other element of structure, down to the walls, and the failure of any one pair may overload its neighbour. These roofs include crown post types, scissored and collared rafters.
The more common roof type in older churches is the ‘framed’ roof which comprises a hierarchy of structural elements with rafters supported by purlins off trusses. Each element relies on the one above it in the hierarchy for support, but the design, strength, and spacing of each element is determined by the one below it. Thus a common rafter of a particular size, traditionally determined by rule of thumb or availability of timber, can span a particular distance. This, in turn, determines the spacing of the purlins, whilst the size and strength of the purlins determines the spacing of the trusses. In practice however the truss spacing was often dictated by other architectural features, such as the positions of windows.
Church roofs differ from domestic roofs in that they rarely support a ceiling, being intended rather to give a feeling of lofty space up to high collars or the ridge. There was therefore a disincentive to provide tie beams at eaves level, although these are structurally the most efficient way of preventing the spread of a roof, as described below.
Framed Roofs
The structural development of the framed roof starts with heavy timber beams spanning from wall to wall to produce a flat roof. A low pitch could be achieved by standing one or more vertical posts on the beam to support two inclined principal rafters which meet at the apex. In this arrangement the posts are in compression and the beam is loaded in bending.
From this simple roof structure developed the triangulated truss which is similar in form but totally different structurally. It was realised that the rafters could support each other and did not need to be propped up on posts. The truss relies on the ‘tie beam’ to prevent outward movement of the rafter feet. As this tie acts in tension, it can be very much smaller than the simple beam referred to above. Where a triangulated truss has vertical posts, as in ‘king post’ or ‘queen post’ trusses, the posts act in tension, preventing the tie beam from sagging.
The structural role of the elements in a triangulated truss was only recognised in the late 18th Century when bolts were introduced to hold the tie to the post. Correct identification of a beam or a truss is very important if any defects are to be analysed or corrected.
From triangulated trusses the requirement for greater headroom led to the development of ‘collar trusses’. These are triangulated trusses with the tie moved upwards and referred to as a collar. When the collar has been moved up too far it becomes ineffective, as explained below, and arch bracing can be introduced to provide stiffness to the frame. Arch bracing usually extends down below eaves level in the form of wall posts, supported at their lower ends on corbels. Hammer beam roofs are a further development of arch braced roofs, giving the possibility of a wider span as well as scope for architectural embellishment.
Lateral Support
The basic problem with pitched roofs is that when inclined rafters are loaded vertically, by the weight of the roofing material and snow, a horizontal reaction is required at the eaves to prevent the rafters moving outwards. Consider an analogy of a ladder on icy ground leaning up against a wall. Unless it is held in position at the bottom it will move outwards, and the top downwards. Moreover one can appreciate that the shallower the slope of the ladder the greater the horizontal force needed to hold it in position. So it is with roofs: shallow pitched trusses require greater lateral restraint than steeper ones.
Whereas the horizontal force at eaves level depends solely on the pitch of the roof, the horizontal force in a truss tie or collar also depends on its distance up from the eaves. In fact to generalise on both of these considerations, the tension in a tie or collar, the provision of which is the usual way of dealing with the outward thrust, is inversely proportional to its distance from the ridge to the tie or collar. The loss or deliberate removal of a tie from a truss is unacceptable structurally, and if a tie or collar is moved up to a higher level it must be designed for the greatest tensile stress that it will be required to carry.
Trusses or single roofs that rely on arch bracing rather than ties or collars have very considerable forces locked up in them. A small amount of opening up of their joints can lead to quite large outward forces, and hence movements, at the foot of the principal rafters.
Timber as a material can carry high loads in direct tension: the biggest problem comes at the connections where the forces are transferred through pegs or carpentry detailing. Not only is the cross section of timber inevitably reduced at connections - by forming a tenon for instance - but joints are particularly susceptible to woodworm and beetle attack. Cracks and holes are ideal environments for larvae.
Where the lateral restraint has been weakened or damaged, the roof begins to ’spread’ or move outwards at eaves level. For structural efficiency the spread of a roof truss can best be halted by the provision of a relatively thin steel tie, as near to eaves level as possible acting in pure tension. This may be virtually invisible from the ground in many church buildings, or else it may be decorative or functional, for example, carrying lighting. There are also a variety of different ways in which existing timber ties can be discreetly reinforced with steel or a resin composite. However these methods usually involve cutting channels into the timbers to accommodate the reinforcement, and where historic timbers are concerned major alterations to original timbers may not be acceptable.
Sometimes buttresses are used to oppose the outward thrust of roofs. Often these are little more than pilasters, which give minimal structural benefit. Buttresses that are features of a church are often of doubtful use because they must be quite large to be effective. Often they settle under their own weight leaving a gap between the buttress and the wall, which means that further movement in the wall must take place before the buttress can be effective.
The reverse scenario can occur when a wall starts to lean outwards due to a foundation problem, requiring the roof to hold it back. Any ties in the roof then become over- stressed. In this case, if the trusses are robust enough, an area of the inner leaf of the wall around the truss bearing can appear to pull away from the main part of the wall, whereas in reality the truss is trying, unsuccessfully, to restrain the wall from moving out. Clearly, correct diagnosis of the problem is essential. It would be inappropriate to strengthen a spreading truss if the problem in fact rests with the foundations of the external wall!
Damp
Ideally timber should not be in contact with masonry: all external walls are more or less damp, including mortar in particular, leading to rot in the timber. Wherever possible a ventilated space should be left around timbers where they are embedded in walls, such as beam ends, and wall posts should also be kept very slightly away from walls, not plastered in.
In some cases contact with the masonry cannot be reduced significantly and timbers such as wall plates which have one or more surfaces hidden from view are of greatest structural concern. The wall plate is a piece of timber running along the top of the wall to provide a bearing for the feet of the rafters. It is particularly vulnerable where located beneath a parapet gutter as the gutter may leak or overflow. Minor leaking from the gutter may be hard to detect until an outbreak of dry rot is discovered. The wall plate presents a classic problem as it usually cannot be seen and it is often unventilated. Therefore ideal conditions for dry and wet rot decay can lie unseen for years, and when decay is discovered, the problem is often far more extensive than could be imagined.
Wall plate and rafter feet decay is often started by leakage from gutters, but in most cases the cause is overflowing of the gutter due to blocked outlets or the accumulation of leaves. It may seem an obvious point to make, but how many churches have beautifully polished plate and waxed pews while an external inspection reveals blocked gutters and gulleys? Unfortunately most churches have at least one gutter which is difficult to get at ‘for maintenance. Perhaps, prompted by the CDM Regulation (duties of designers), grant aided repairs should include some provision for easier access for maintenance, such as a roof access door from a tower, which is so rarely seen.
Most churches do not have ceilings in the domestic sense, except the grander Georgian ones and those with ‘wagon’ ceilings which follow the line of the arched bracing. Where there is a ceiling, an accessible roof space enables the roof structure to be inspected at close quarters with relative ease. However, unless an inspection is made, the ceiling can conceal leaks and intercept that tell-tale drip on a member of the congregation. Leaks through slating or other roof coverings are of course disastrous for timber roofs. Small leaks can be hard to locate if they occur above a principal rafter because the water can travel down the rafter before dripping onto the floor or into the wall some distance from the actual leak. In its passage along the timber, the water initiates decay unseen.
Repairs
It is essential that timber joints are made correctly and this requires a good joiner. Poor workmanship is often the cause of problems later on. Wherever possible timber of the same type should be used rather than steelwork but in many situations steel is the only practical answer. Traditional timber joints are generally most vulnerable in bending, so where a piece of structural timber, such as a purlin or rafter, requires strengthening or repair near its mid point, some sort of steel plating may be inevitable. Connections at the ends of timbers, however, can often be remade without metalwork.
There are two considerations in relation to roof repairs that are often overlooked. One is the appropriateness of past repairs and the other is the extent of repairs that are actually required.
Past repairs that cause accelerated decay in roofs include poorly detailed lead work, resin repairs to timber, inappropriate materials (such as untreated softwood and impermeable roofing felt) and changes to the heating and ventilating regime.
To determine what repairs are strictly necessary it is essential that the real defects are correctly identified after a detailed inspection and analysis. It is not good enough to start from an assumption that complete re-roofing is essential. Very expensive re-roofing schemes have been carried out in the past in the belief that a roof was leaking, whereas the problem was in fact condensation, which was not addressed.
Roofs are often complex structures, but even the simpler structures are often misunderstood. If problems are suspected it is always prudent to seek professional advice from an engineer or architect experienced in dealing with old roofs who can also assess the historical significance of individual timbers. Although some outlay in fees may be required, a correct diagnosis at an early stage can save a large repair bill later
Other timber related subjects:
http://www.woodfx.co.uk/timberdecay.html
http://www.woodfx.co.uk/laminate.html
http://www.woodfx.co.uk/woodfinish.html
Article written and submitted by UK DIY Forum member: joiner_uk
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