If your roof is over 100 years old and is of its original materials the repairs and maintenance needed will be constant. Although many of the original slates are still serviceable, the rusting nails and old slate battens will make the roof covering vulnerable to the elements. 
Most roofs of this age would have been re-covered in the past, unfortunately many with cement fibre slates ( asbestos ). These man made slates do not weather well, becoming brittle, porous and attracting moss and lichen. 
Some roofs have been recovered with concrete tiles which also become porous and attract moss. Concrete tiles are aesthetically wrong on period property. 
New roof covering 
The choice of the new roof covering will effect the character of the property and the longevity of the roof. It is advisable to use the material that originally covered the roof. In the majority of cases this would have been natural slate. Good quality natural local slate can have a useable life of 300 years. Therefore if 150 year old reclaimed slate is used, it still has 150 years use remaining – far longer that new man made materials. 
The two main alternatives for re-roofing materials would be: 
Use new natural slate. If the existing roof has natural slate, they can be sold to an architectural salvage company and the profit used to purchase the new slate. 
Carefully remove and clean the existing natural slate and re-use. Normally, 20% of the existing slate will be defective or will be damaged during removal. These can be replaced with exact matching slates from architectural salvage firms.  
Below is a typical procedure for re-roofing a period property. 
Scaffolding will be erected to all elevations at eaves level. 
Removal of Existing Covering 
Carefully remove all original ridge tiles and slates for future use. 
Remove all existing slate battens, old felt, etc. 
Existing Roof Structure 
Check the existing roof framework for defective areas and rot. Replace any defective or rotten timbers. Clean all dirt from rafters, purlins, etc. 
If required, replacement or repairs to fascia boards, soffit boards and guttering is undertaken at this stage. A felt support tray is installed over the fascia boards. 
Felt and Battens 
Lay a breathable membrane roofing felt over the rafters and fix preservative treated premium timber slate battens. Battens are fixed using 75mm galvanized clout nails. 
Install code 4 or code 5 lead flashing and valleys. All new leadwork is treated with patination oil. 
Fixing Slates 
Hang slates to all roof elevations. These can be new slates, reclaimed slates, or the existing slates that have been cleaned prior to re-fixing. All slates are fixed using two copper nails and a stainless steel slate hook. 
Ridge tiles, Hip tiles and Finials 
The original ridge tiles, hip tiles and finials are thoroughly cleaned and placed back onto the roof. Any tiles or finials that are found to be defective or missing can be replaced with an exact match from an architectural salvage or reclamation yard. 
Repairs to roofs 
The most common defects on period roofs are easily repairable.  
Replace slipped or missing slates with matching slates. 
Replace loose or damaged ridge and hip tiles with matching tiles. 
Replace broken finials with matching originals. 
Re-new defective or worn leadwork. 
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