Questions to ask a roofer before slate work begins

Start with the specification, not the price

Two quotes for a slate roof can differ by thousands of pounds and still look identical on a single line of paper. The difference usually sits in what has been left out. Before you compare figures, ask each roofer to provide a written specification: the type and size of slate, the fixings, the underlay, the battens, the flashing material and the extent of any timber repairs. A quote that simply says "re-slate front elevation" tells you almost nothing about the finished roof.

If a roofer is reluctant to put the detail in writing, treat that as useful information. A thorough contractor will happily explain what they plan to do, because they expect to be judged on it. Once you have two or three specifications side by side, the cheapest quote often reveals itself as the one that has quietly omitted the underlay or the ridge work.

Insurance, accreditations and who will be on site

Ask for a current certificate of public liability insurance and check the date and the level of cover. For domestic roofing, £2 million is a common minimum and £5 million is better; anything less is a warning sign. If the firm employs anyone, they also need employer's liability cover, which is a legal requirement in the UK. Ask whether the certificate covers subcontractors, because many roofers bring in labour-only teams for larger jobs.

• Which trade association or competent-person scheme do they belong to?
• Will the people on your roof be direct employees or subcontractors?
• Who is the named person supervising the work day to day?
• Can they provide two or three local addresses where similar slate work has been done?

A quick look at a finished roof five years on will tell you more about workmanship than any brochure.

Slate fixing methods and materials

Slate is unforgiving: a poorly fixed roof can look perfect for a season and then start to shed slates in the first gale. Ask how the slates will be fixed. Traditional Welsh and Scottish slate is normally double-nailed in copper or stainless steel, but larger or heavier slates sometimes need hook fixing or additional rivets. Nail type matters, as does nail length: it should be long enough to bite into the batten without splitting it.

Then ask about the layers beneath. What underlay is proposed, and is it a breathable membrane or a traditional bituminous felt? How will the battens be graded and spaced, and does the gauge match the slate size? For older properties with diminishing courses, ask whether the slates will be graded by size and laid to match the original. On listed buildings or in a conservation area, confirm who is dealing with consent and whether reclaimed or new slate has been agreed. These questions separate a roofer who understands slate from one who treats it as a commodity.

Scaffolding, access and permissions

Scaffolding can account for a substantial slice of the bill, so establish early whether it is included and who arranges it. Ask how long the scaffold will stand, who pays if the job overruns, and whether protection boards, netting and brick guards are included. If the scaffold has to sit on a pavement or road, the contractor will need a licence from the local highway authority, and that takes time to obtain.

Access also affects cost. A terraced house with a shared alley is a different proposition from a detached property with a driveway. Ask how gutters and downpipes will be protected, how the scaffold will be tied into the wall without damaging brickwork, and whether gutters will be cleared before the roof is stripped. Chimney access, valleys and dormers are often priced separately, so make sure they are either in the quote or clearly excluded.

Waste removal, skips and salvage

Old slate is heavy, and disposal is not free. Confirm whether the price includes a skip, where it will stand, and who is responsible for any permit needed to place it on the highway. Ask about the tonnage allowance, because a large roof can easily exceed a standard skip and trigger an extra charge.

Salvage is worth discussing too. Reclaimed slate in good condition has real value, and many roofers will offset its worth against the cost of the work. If you want to keep spare slates for future repairs, say so before work starts; a stash of matching slate in the garage is worth its weight when a single tile blows off in ten years' time.

Payment, guarantees and weather

Finally, agree the money and the paperwork. A small deposit is reasonable, but large upfront payments are not. Ask for a staged schedule, a written start date and an expected completion date, and confirm whether the price is fixed or subject to remeasurement if timbers prove rotten. Get the guarantee in writing: how long does the workmanship warranty run, what does it cover, and is it transferable if you sell the house?

Roofing is weather-dependent, so build in a little patience. A roofer who refuses to strip a roof before a settled forecast is being sensible, not slow. Ask how they will keep the roof watertight overnight and what happens if rain arrives mid-job. The answers you get to these questions will tell you as much as the quote itself.