The short answer
Yes, a little. Slates are fixed with nails through two holes and hang from battens, so they are more fragile to disturb than a tile. Old slate roofs also often carry torching, a lime mortar pushed into the gaps from underneath, and foam can bond to that. Taking the foam off slate takes patience, and some slates may crack and need replacing.
Slate is thin, brittle and nailed at the head. Foam that has bonded to it comes off slowly, and a few slates may not survive.
Why slate is more delicate to work around
A slate roof is built from thin, flat slates arranged in staggered rows, called courses. Every slate is held by nails driven through holes near its top into the battens, the horizontal timber strips beneath. Nothing else keeps it there, except the weight of the slates above.
What does that mean for foam removal?
Slate is hard but brittle, and its edges chip easily. A tile has body and takes a knock. A slate can snap along its length if it is flexed, levered or struck. Foam bonded to the back of the slates has to be freed without bending them, which means cutting round the nails and working slowly, so each slate stays where it was fixed.
The nails matter too. On older roofs the nails can be rusted, and their grip on the batten may depend on the foam holding the whole thing together. Once the foam is gone, a slate that looked secure can slide. We check the fixings as we clear each area, and slates that have lost their hold are re-fixed before we move on to the next.
Torching and foam bonded to the slates
Many older slate roofs carry torching: a layer of lime mortar pushed into the gaps between the slates and battens from the underside. It stops wind driving rain and dust in, and it also holds the slates steady against uplift.
Can foam be removed without losing the torching?
Not always. Foam sprayed over torching bonds to the mortar and to the slate behind it. Cutting the foam back often brings the torching with it, because the mortar is the weaker layer. What comes away is lime that was already old and dry.
Where torching is lost, there are two routes. Re-torch the slope with fresh lime mortar, which keeps the traditional detail. Or leave it bare and rely on the felt and the slates' own overlap, which suits some roofs and not others. A Victorian terrace in a conservation area deserves the first, and the right choice for your roof comes out of the survey. A listed house may need listed building consent for changes to its character, and Somerset Council's planning team can confirm the route.
Saving the slates you have
Natural slate can last a very long time, which is why the quarry-fresh look of a new slate is not worth chasing. CORC, the Confederation of Roofing Contractors, says that slate, if well maintained, can last 100+ years. That is a reason to take care. A foamed slate roof that is cleared patiently can have most of its slates back in service, and old slate is worth more than new slate that does not match.
Which slates will not survive?
Slates that are cracked, delaminated (splitting into layers) or snapped at the nail hole will not go back. Others come away cleanly and can be washed, checked for hairline cracks and refitted. Where a few need replacing, a matching slate can be found from reclaimed stock or from the new slates that suit the roof. Our answer on slate roofing describes how repair and replacement are judged, and replacing a single slate shows how one is fixed in place.
We lift only what has to be lifted, keep sound slates together by slope, and show you photos of the ones that fail, with the reason each one failed. That way you see what was lost, and why it was lost.
What the survey checks on a slate roof
On the inside, we look at whether the foam is bonded to the slates, the felt or both, and whether any torching remains. We check the nails and battens for rust and rot, then walk the outside to check the slope, ridge, valleys and lead flashings.
Do slate roofs need scaffold for this?
Often only if outside repairs follow. The bulk of the removal happens from the loft, as the page on scaffolding for foam removal explains. Slate roofs with a steep pitch or high eaves are more likely to need a platform for repairs, and the survey will tell you which applies.
Tell us what the roof is covered with, where you can see it, and send a photo of the underside from the loft hatch if you can. Slate can be hard to identify from a distance, and an old roof may have been patched with something else over the years. Once we have surveyed, one written price follows, and the spray foam removal page sets out the job from the first visit.
More on foam and slate
Will my slates be damaged by foam removal?
A few may be, especially older, thinner ones that have bonded hard to the foam, since slate does not flex. We work slowly to keep damage low, and we photograph every slate that fails. Sound slates are cleaned and refitted, each one back on its own slope. Where replacements are needed, we match size, thickness and shade as closely as we can.
Is foam on a slate roof a problem for a sale?
It can be, because surveyors tend to flag it. Ask your lender, or the buyer's lender, what evidence it wants. The RICS consumer guide advises checking with your mortgage provider. A dated record of the removal and the roof's condition afterwards helps either way.
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