Welsh Slate Roofs In Eccles And Monton

Key takeaways

• Good Welsh slate is dense, thin and splits cleanly, and it can outlast the building it was laid on.
• The nails, the battens and any torching underneath all have far shorter lives than the slate.
• A slate that rings when tapped is sound. One that thuds has absorbed water and is delaminating.
• Reusing existing slate on new battens keeps the character and costs far less than buying new.
• Replacing Welsh slate with concrete tile adds significant weight the original structure was not designed for.

Why this material lasted so long

Slate is a metamorphic rock that splits along a natural plane, which is what allows it to be produced thin, flat and light. Welsh slate in particular is dense and has very low water absorption, which is the property that matters most: a material that does not take up water does not suffer when that water freezes. It is also chemically stable, so it does not degrade in the way that manufactured materials do. The practical result is a covering that can serve well beyond a century with no maintenance at all, which is exactly what has happened on a lot of the older housing in this part of Salford.

The three things that fail before the slate does

First the nails, which on original work were iron and have been slowly corroding in a damp roof space ever since. Second the battens, sawn softwood that has spent the same period in the same conditions and eventually softens where water has reached it. Third the torching, the mortar and hair mix applied to the underside of the slates on many older roofs to close the gaps, which crumbles and drops into the loft over decades. When people say a slate roof has come to the end of its life, what they almost always mean is that one or more of these three has, and that the slate is simply no longer being held.

The tap test, and what a slate sounds like

A sound slate held loosely and tapped gives a clear ring, almost a musical note. A slate that has begun to absorb water and delaminate gives a dull thud instead, because the internal layers have started to separate. Roofers use this constantly when sorting slates during a strip, and it is the single most useful way of deciding what can go back on the roof. Poorer quality slate from other sources, which was sometimes used on later or cheaper work, delaminates far sooner than Welsh slate, and it shows as flaking, swollen edges and a chalky surface long before it fails outright.

Sizes, and why they matter when matching

Slate was produced in a range of named sizes, and a roof was laid with a specific size to a specific gauge, meaning the spacing of the battens and the amount each course overlaps the one below. That geometry is not adjustable after the fact. It means a replacement slate has to match not only in colour and thickness but in size, or it will not sit down correctly and the overlap that keeps water out will be wrong. This is why sourcing genuinely matching reclaimed slate matters, and why a repair using whatever was in the van tends to announce itself both visually and in the next spell of driven rain.

Stripping and relaying, which is what these roofs usually want

Because the slate outlasts everything under it, the appropriate work on a roof of this age is usually to take the slates off, sort them, and relay them on new battens with new stainless or copper nails over a modern underlay. The character of the building is preserved, the material cost is largely already paid for, and the resulting roof has a service life measured against the slate rather than against a corroding nail. Expect a proportion of the slates to be unusable, and expect that proportion to be established by sorting on site rather than by an estimate given from the ground. A roofer who tells you the figure before anything has come off is guessing.

The weight problem with switching to tile

The temptation on an ageing slate roof is to recover in concrete interlocking tile, which is cheaper per square metre and quicker to lay. The difficulty is weight. Concrete tile is considerably heavier than natural slate, and the roof structure of these houses was sized for slate. Adding that load to original rafters and purlins is how roofs develop the slow deflection that shows up years later as a dipping ridge line. It also changes the appearance of the building considerably, which matters on a street where the housing was built as a set. If tile is being proposed, the weight question deserves a proper answer rather than a shrug.

What to check on your own

Take a zoomed picture from the pavement and count what has moved, paying more attention to whether the losses are spread across the whole roof or grouped in one place. Look along the courses for a stepped or wavy shadow line, which shows slates that have moved slightly without falling. Look at the ridge and the verge for missing mortar. In the loft, look for fallen torching on the floor, which is a clear sign the roof is shedding its original detailing, and look at the battens where they are visible for softness or splitting. Look too for previous repairs in a different colour or size of slate, since they tell you the history of the roof.

Planning the work rather than reacting to it

Roofs of this age rarely fail suddenly. They give years of warning through gradually increasing slippage, which means there is time to plan, get more than one opinion, and choose the season. That is worth using. Work of this kind wants settled weather and a properly set up scaffold, and it is considerably better done as a planned job in spring or summer than as an emergency in February. It is also worth establishing early whether the property sits within any local designation that affects what can go back on the roof, because that answer changes the specification and it is much better known before quotes are sought than after.

What to ask before anyone starts

Three questions separate a considered quote from a guess. First, what proportion of the existing slates does the roofer expect to reuse, and on what basis: an answer given before anything has come off the roof is an estimate rather than a finding. Second, what will make up the shortfall, and will it match in size, thickness and colour, since a mismatched slate is visible from the street for the rest of the roof's life. Third, what fixings and what underlay, since those are the components that determine whether this work lasts thirty years or a hundred. It is also worth asking to look in the loft together during the quote, because the underside tells you about the battens, any remaining torching and the state of the timber, and none of that can be assessed from a ladder outside.

Frequently Asked Questions

How long does a Welsh slate roof last?

The slate itself can serve well beyond a century, and often outlasts the building's other components. What limits the roof is the nails, the battens and any torching beneath, all of which have far shorter lives. Most slate roofs are replaced or relaid because the fixings have failed rather than the slate.

How can I tell if a slate is still good?

Held loosely and tapped, a sound slate rings clearly. One that has begun to absorb water and separate into layers gives a dull thud. Roofers sort slates this way during a strip, and it is how the reusable proportion on a relay is actually decided.

Can my old slates be put back on?

Usually a large proportion of them. The normal work is stripping, sorting, and relaying on new battens with stainless or copper nails over modern underlay. It preserves the appearance of the building and costs considerably less than buying new slate, since the expensive material is already there.

Is it cheaper to recover in concrete tile?

Per square metre, yes, which is why it gets proposed. The complication is weight: concrete tile is much heavier than natural slate and the roof structure of these houses was sized for slate. That extra load is a common cause of the slow deflection that shows up later as a dipping ridge.

Why does my repair patch look so obvious?

Because slate was laid in a specific size to a specific gauge, and a replacement has to match in size and thickness as well as colour or it will not sit down correctly. A mismatched slate reads as a bright patch and often compromises the overlap, which is why sourcing matching reclaimed slate matters.

There is grey debris on my loft floor. What is it?

Most likely torching, the mortar and hair mix applied to the underside of the slates on many older roofs to close the gaps. It crumbles and falls over the decades. It is not dangerous in itself, but it indicates the roof is shedding its original detailing and has no secondary barrier.