Below every roof covering is a roof structure. Most historic roofs have a timber structure. A timber roof may be any one of a variety of types – it may have purlins, rafters, trusses or it may be a cruck roof type, found in vernacular buildings. It is important to understand the design and function of any historic roof structure which will work in tandem with the roof covering. Trusses alone come in a huge variety of patterns - hammer-beam and king-post or queen-post are amongst the most common types. More unusual types include Belfast trusses. Consideration will have been given during the design and construction of the roof to the angle and porosity of the roof structure and covering to control ventilation, run off, expansion and contraction as well as acting as structural support and a cover to the building below. An engineer experienced in conservation will be familiar with these common types of historic roof, and any assessment of soundness should be carried out by a suitably experienced person.
One of the first priorities in maintaining an historic roof is ensuring that the roof structure is sound. Where there is significant sign of movement in the roof structure, advice from an engineer experienced in the repair of historic structures is required. When designing remedial structural repairs, a minimum intervention approach is required for both technical and aesthetic reasons. Historic timber to roof structures should be replaced with appropriately treated material after detailed inspection for rot, insect attack and structural weakness, and spliced to sound timber. Wholesale replacement of roof timbers is rarely necessary – new timber can be cheek-bolted to old. An entire structural element such as joist or truss should be replaced only where a significant proportion of the original has been lost.
Repair the damaged timbers using new preservative timbers run to the original profile and treat rot or insect attack locally as required.
Where possible separate timbers from damp brickwork with a damp proof course and allow free ventilation where practicable. Check the provision of ventilation to roof voids. If extra ventilation is required, locate it discreetly and create openings using traditional materials-such as lead.
Slate or Tiled roofs
- Regular visual inspection of slated or tiles roofs is important, and in particular after storms or high winds.
- Pin up any slipped slates or tiles, and replace any that are missing and reset ridges if necessary.
- Replace or repair damaged and missing ridge tiles using terracotta ridge tiles to match any existing, and re-point.
- Any replaced slating or tiling should be fixed with non-ferrous nails: copper, aluminium alloy or silicon-bronze.
- The original clipped eaves and verge details should be retained without the addition of timber trimmings, fascias, etc.
- Original materials and methods of fixing should be respected and fibre cement slates and concrete tiles should not be used in repairs.
Lead roofs, valleys and gutters
Lead sheets should be inspected for holes or splits that can lead to water penetration. The internal sign of water penetration is often far removed from the defect, as water will often move horizontally through the structure before finding a vertical path. Small repairs may be made using lead patches or whole sheets may need to be replaced. Either way this is a job for a specialist contractor.
Slipped lead flashings should be refixed, and damaged ones replaced, as soon as possible.
Thatched buildings are an integral part of our vernacular heritage. Since the 1950s an estimated 15,000 thatched buildings have been lost and today only 144 examples with an uncovered thatch roof are known to remain.
As an increasingly scarce feature of our landscape, the Department is committed to encouraging the retention of traditional thatched buildings through priority grant-aid.
Listed buildings with thatch under a tin roof are included in this category.
- See also Maintenance Checklist: Thatch
Bituminous Sheet Flat Roofs
Occasionally, a bituminous sheet flat roof may have been added to an historic property. Flat roofs pose a number of problems different from those encountered in pitched roofs. As they are not as proficient at shedding water, any defect can have serious implications for the building. They also may not receive the attention they require as inspection is sometimes difficult.
Flat roofs basically consist of joists, sarking and a waterproof covering built to a slight fall to drain off rain water. ‑e most common covering material is bituminous or roofing felt . ‑e felt covering is built up in layers each bonded with a bituminous substance such as tar. ‑e upper surface is sometimes covered in a layer of stone chips set in tar to make it more robust, weatherproof and better able to deal with solar gain (this is the amount of heat which the roof structure absorbs from the sun).
Check for early signs of problems
It is important to regularly inspect the external surface of a felt covered flat roof to check for early indications of problems and defects. If spotted, these should be rectified as soon as possible. Where practical to do so flat roofs should be inspected from inside as well as from ground level and if possible from the surface of the roof itself. Some points to bear in mind when inspecting flat roofs are: If damp patches appear on the ceiling below a flat roof it means that there is a leak in the covering. It should be sourced and repaired as early as possible to avoid further damage. It is important to note that the leak may not directly correspond to where the damp patch appears and some searching for the source of the problem may be required.
- Soft patches underfoot are indicative of deterioration of the substrate usually caused by water penetration.
- Gutters and other water outlets should be inspected for blockages particularly in autumn.
- Pools of standing water are indicative of poor drainage.
- The surface should be checked for signs of deterioration such as cracking, blistering or crazing.
- Walking directly on the roof can make the problem worse.
Where a flat roof meets an adjoining masonry wall, a parapet or a slopping roof, metal detailing known as flashing is used to ensure water is kept out of the junction. Flashing was installed in such a way as to allow for a little movement in the structure of the roof whilst still remaining water tight. Where flashing are sealed with tar or cement these often do not last as they become brittle and fracture.