Forna Natural Slate is dark grey coloured slate from the Galicia region in Spain. It is produced for Harris Slate & Stone by Pizarras Forna one of Spain's leading producers of natural slate and is available in first and second quality. Forna is produced to a very high standard out of some of the best quality slate rock in Spain. It is supplied in 500 x 250, 400 x 250 and 450 x 220mm sizes with a thickness of 4 to 5mm in the first quality and 5 to 7mm in the standard quality. It is the ideal slate for your pitched roof requirements whether it is new or refurbishment, housing, commercial or industrial.
Forna slates are available unholed or preholed to a 100mm lap. Harris Slate & Stone stock Forna at all four of its distribution centres, Carmarthen, Machynlleth, Newthorpe (Notts) and Leigh (Lancashire).
Performance Characteristics
Forna is regularly tested to BS680 Part 2; 1971 requirements for roofing slates. Samples have passed the following parts:
Water absorption tests (clause 6,2,1,)
Wetting & drying tests (clause 6,2,2,)
Sulphuric acid tests (clause 6,2,3,)
Pizarras Forna also guarantee the material for 30 years.
General Design Considerations.
Celta slates when fixed to BS5534 Part1: 1997 Design will have adequate resistance to wind loads, wind uplift and rain penetration under most conditions. The specifier should check the annual driving rain index in BS5534 to decide if the area in which the slates are being laid is in a moderate or severe exposure area. The site exposure rating and the pitch of roof rafters will determine the size pattern and fixing for the slates.
Sitework
The slates should be stacked on their longer edges with timber bearers, battens or boards between layers. The bottom layer should be on a firm base.
Fixing
The slates should be laid in accordance with the code of practice for slating and tiling BS5534: Part1 1997 and BS8000 Workmanship on building sites Part6: 1990. The slates should be cut on site by hand or machine. When additional holing is required the slates should be drilled from the bed towards the face so that the nail head can sit in the small cavity formed by the area of spalling. Care should be taken to avoid undue spalling. At the time of holing the fixers should be grading the slates into three thicknesses. The thickest to be used at the eaves. Individual slates should be holed so that the thicker end is the tail of slate. Slates should be twice centre nailed as described in the British Standard. Harris Slate & Stone recommend the use of copper nails 40mm in length by 3.35mm diameter to BS1202: Part 2.
Underlay
Protex roofing felt should be used or similar product to BS747: 1994. The underlay should be draped over rafters, or fully supported on boarding. It should allow any moisture to drain and should extend over the tilting piece, fascia board and into the eaves gutter.
Battens
Sizes are recommended in BS5534 for pitched roofs and vertical cladding. Harris recommends the minimum batten size to be 50mm wide x 25mm deep.
Leadwork
Any lead used should be treated with patination oil as it is fixed. If patination oil is not used it could cause unsightly staining to the roof slates.
Ventilation
Adequate ventilation is required for all slate roofs. Ventilation provides the following benefits: Prolonged aesthetic appearance. Avoidance of condensation, resulting in superior insulation performance. Long term protection of roof structure. Harris Slate & Stone have a full range of ventilation products available. See our ventilation data sheet for full details. Accessories A range of GRC ridge tiles are available to suit most roof applications. See separate data sheets for full details.
Accessories
A range of GRC ridge tiles are available to suit most roof applications. See the roofing accessories section for further details.
