Re-slating and new slate roofs

Natural slate is tested to a European standard, BS EN 12326, and the results appear as short codes on the supplier's paperwork. They are the quickest way to compare two slates that look identical on the pallet. NBS, the national building specification body, warns that specifying slate simply to BS EN 12326 is not enough, because the standard covers several different levels of quality.

LABC Warranty, which insures new homes, requires natural slate to be rated T1 and S1, and asks for the supplier's consignment document confirming those results. That is a sensible minimum on any house, and the document is worth asking to see before the slate goes up, not after.

Most natural slate laid in Britain now comes from abroad. The British Geological Survey's 2024 factsheet on building and roofing stone says imports of roofing slate substantially exceed domestic production, and that British slate comes mainly from North Wales and the Lake District, with a small amount from Cornwall and Devon. The quarry matters more than the country, which is why the test codes for the actual consignment count for more than the name on the pallet.

Fibre cement slates are made to look like slate and are tested to a different standard, BS EN 492. Being manufactured, they are uniform, so they lay evenly.

They behave differently on the roof, though. LABC Warranty's technical manual points out that fibre cement slates fit closely enough to be relatively airtight, which can trap condensation beneath them, so the space between the battens needs ventilating, using counter battens, in line with BS 5250. They are fixed with copper nails, with a copper disc rivet holding each tail down against wind chatter.

On an older house, the deciding factor is often simply matching what is there: colour, size and thickness, so that a part re-slated roof still reads as one roof.

BS 5534, the code of practice for slating and tiling, expects every slate to be mechanically fixed, and the fixing to be worked out for the roof's own site. The wind load on each slate depends on where the building stands, its height, the ground around it and what lies under the slates. LABC Warranty calls the calculation complex and advises getting a site-specific fixing specification from the slate manufacturer.

Mortar on its own no longer counts as fixing a ridge. LABC Warranty's manual says mortar does not give enough bond to resist wind uplift and that ridge tiles only bedded on mortar are not acceptable. Ridges and hips are fixed mechanically, either with a dry fix system made to BS 8612 or with mechanical fixings through the mortar bed.

Verges come under the same code of practice, and a dry verge system should be one made to BS 8612, as LABC Warranty's manual sets out.

On a strip and re-slate, sound old slates are taken off and kept. LABC Warranty notes that slates should be graded on site for an even finish; with reclaimed slate that means sorting them by thickness so each course lies flat, and new slate needed to make up the numbers can go on one slope, often a rear one, rather than being scattered through the old.

The battens and underlay are renewed. Many old slate roofs were laid with no underlay at all, so a re-slate usually adds one, and that changes how the roof space breathes. Ventilation and condensation are governed by BS 5250, and the type of underlay matters: under LABC Warranty's guidance, a low resistance, breathable underlay can allow less loft ventilation than a traditional one, if it is installed to its third-party certification.

Re-slating a roof renews its covering, which Approved Document L treats as renovating it. Where more than half of the roof is done, the government's own worked examples set a target U-value of 0.16 W/m²K, and for a house with a cold loft they reach it at ceiling level: about 250mm of mineral wool or cellulose between and across the joists, or a top-up to that if 50 to 100mm is already there in good condition.

Where there are rooms in the roof, the insulation goes at rafter level instead, between and below the rafters or between and above them. Where that would thicken the roof, particularly on a terrace, the guidance says a lower target may be necessary.

Older houses get some latitude. Approved Document L says listed houses and those in conservation areas need not comply fully where doing so would unacceptably alter their character, and that traditional buildings should only be insulated in ways that will not cause long-term damage to their fabric. Separately, the Planning Portal says that a new covering 15 per cent or more heavier than the old one needs building control approval, which is worth checking whenever slate is being swapped for something else.

None of these is unusual to ask for, and a roofer working to the standards will have them to hand.