Slatework generally lasts somewhere between about 75 and 150 years before it becomes inevitable that it should be replaced. It may or may not be possible to reuse a proportion of the slates depending on their condition. Two things generally fail: the nails or the slates.
Image : Scotch Slatework that is some years old but still performing its function
Galvanised steel nails tend to rust. The quality of galvanisation in the past has varied. Some very old galvanisation was extremely good. Some more modern galvanisation was extremely poor. Copper nails are more generally used for good quality work now.
Image : Galvanised Nails
Slates often get soft around their heads. Slatework lasts better where it dries out completely after rain. Steeper roofs promote this. Roofs that get the sun generally last better. Moss and lichen can make slatework age prematurely.
Shallowly pitched roofs require bigger slates. Slatework is often laid to diminishing courses as it goes up the roof with the larger slates at the bottom. When slates are reused they are usually “dressed” removing their heads and making them shorter. The larger slates are then in short supply.
Image : Large Welsh slates on a low pitched early 19th century roof
Poor quality slates can be damaged by freezing/thaw cycles.
Slates come off for a variety of reasons. Sometimes they are affected by vortexes of wind that make them come off in the same place again and again. Slates come off because they have not been well fixed in the first place, even on quite new roofs. In Scottish slating practice slates that are fitted after the roof irons (brackets that provide temporary support for the slaters) are removed are often less well fixed than the others. Slates should lie flat on a roof and not be subject to wind uplift, but they should also have sufficient head and side lap to prevent wind blown rain from getting up and under them.
When slates are refixed into a roof by way of isolated repairs it is often best if the fixing is allowed to be visible. Mechanical straps, tingles, hooks etc are generally better than adhesive which is often used because it is invisible. They also give a clear indication of how much repair work has been required.
The weakest point in a slated roof may be the junctions which are often formed with lead, zinc or copper flashings or with mortar fillets or with bitumen felt. The failure and replacement of such elements may trigger the need for disruption to the slatework, in turn making it desirable to renail larger areas of slatework.
The oncost of scaffolding is often a consideration in deciding how far to take roofing repairs. It acts as a distorting factor pushing owners into doing more at any one time than they might do if the roof area concerned could be accessed at lower cost.
The underlay below slates has an important influence on the performance of a roof. Synthetic vapour permeable membranes are most commonly used now. Bitumen felt (which is not vapour permeable) was used universally for many decades up until a decade or two ago and is still sometimes used. Hair felt was often used before bitumen felt. Sometimes slates were bedded in lime. Often no membrane was used in the past. Some slaters claimed that it should not be necessary to have an underslating membrane if the slates were properly fixed and they considered it undesirable to restrict airflow. Hair felt was mostly used from the late nineteenth to the early to mid twentieth century. If water gets past the slates and runs down the membrane it is important to consider where it will end up. When bitumen felt starts to break down the run-off may occur behind the eaves, into the wall head.
Image : Scottish Slatework nailed over bitumen felt with a vent to overcome the fact it is otherwise impermeable to vapour.
Slates are double or single nailed. Courses of slates in a single nailed roof are double nailed at intervals. The tradition of single nailing small Scottish slates aids subsequent maintenance. Double nailing fixes the slate more securely.
The selection of new slates requires great care. Merchants have often stocked poor quality slates driven by price. Such slates are a decidedly false economy.
Image : Roof nearing completion before roof irons and batons have been removed
Image : Crisp, freshly renailed Scottish Slatework