1. Article Body
Historic roofs usually fail through their fixings and their details rather than their coverings. Slate and tile outlive the nails holding them and the metal around them, which is why so much historic roof work is repair rather than replacement.
Built in gutter failure
A defining problem of period houses and the most expensive one.
A built in gutter sits within the roof structure at the eave rather than hanging from it. When its lining fails, water discharges directly into the structure: the cornice, the wall head, the joist ends.
Damage is concealed and progresses for years. By the time it shows inside, the repair involves carpentry rather than roofing.
Any inspection of a period house should establish whether built in gutters are present and what condition the lining is in.
Nail fatigue on slate
Slate does not usually fail. The nails holding it do.
Iron or steel nails corrode over decades and slates begin to slip, first individually and then in numbers. The slate itself may have a century of life remaining.
A roof shedding slates is often a candidate for reslating using sound salvaged material rather than for replacement.
Original flashing at end of life
Lead, copper and terne flashings last a long time and not forever. They crack at folds, split at fixings and thin at exposed points.
Flashing failure at a chimney, a valley or a dormer is the most common leak source on a historic roof and one of the most repairable.
Unsympathetic past repairs
Cement mortar where lime was used. Mastic and bitumen smeared over flashing. Modern sealant in place of a proper detail. Mismatched replacement tiles.
These cause active harm. Hard cement against soft historic material cracks it, and bitumen over metal flashing traps water and makes future repair harder.
Removing bad past repairs is frequently part of the job and should be priced as such.
Deck and sheathing
Original sheathing may be spaced boards rather than continuous deck, which is normal and correct for some coverings.
Replacing spaced boards with continuous sheathing changes how the assembly breathes and may be conditioned by a review body. It is not automatically an improvement.
Rot is concentrated where built in gutters, valleys and chimney flashings have leaked.
Ventilation and moisture
Historic roofs were built to breathe. Adding modern insulation and vapor barriers without understanding the original assembly traps moisture in timber that has been dry for a century.
Condensation problems in historic attics are frequently created by well intentioned modern upgrades.
Fire code conflict on wood
Where the historic covering is wood shake, current fire codes in many jurisdictions restrict its reinstatement. That creates a direct conflict between preservation and safety requirements, resolved through the review process rather than on site.
Vegetation and biological growth
Moss and lichen hold moisture against slate, tile and timber. Removal must be gentle, because pressure washing destroys historic material quickly.
