Tegral & Fibre-Cement Slates: Why They Need Different Treatment

A very large number of Irish houses built from the 1970s onwards are roofed in fibre cement slate, and a lot of owners do not know that is what they have. It is often assumed to be natural slate, or lumped in with concrete tile. It is neither, and the difference matters as soon as anyone proposes cleaning it.

What fibre cement slate is

It is a manufactured slate: cement reinforced with fibres, pressed into a flat slate format. It gave the look of natural slate at a fraction of the weight and cost, which is exactly why it became so common here.

The Irish name attached to it is Tegral, which manufactured in Athy for decades. If you go looking for Tegral today you will find that tegral.com now redirects to Cedral, the brand the range sits under. Current roof slate ranges include Thrutone, Rivendale and TECTA Texture. Worth knowing before you conclude the product no longer exists.

How it ages, and why growth takes hold

A fibre cement slate leaves the factory with a finished surface. Over decades of Irish weather that surface weathers, and as it does it becomes progressively more open and more absorbent than it started.

That is the key point for cleaning. A more absorbent surface holds moisture for longer after rain, and anything that holds moisture in this climate grows algae, moss and lichen. So an older fibre cement roof is not just dirtier than a newer one, it is more hospitable, and it will green up faster after any clean than a newer roof in the same position.

On this material the surface is not a coating over the slate. It is the slate.

Why aggressive cleaning is a worse idea here than on concrete tile

A sound concrete interlocking tile is dense and robust, and on the right one mechanical removal is reasonable. Fibre cement is a different proposition, particularly once it has aged.

Attack an already-weathered fibre cement surface with pressure and two things happen. You remove material from a slate that is thinner than it was when installed, and you leave the surface more open than you found it, which means faster regrowth and faster ageing from that point on. The roof looks dramatically better for a season and is in worse condition than before.

Lichen is where this goes wrong most often. Lichen bonds tightly and does not lift easily, so it invites exactly the escalation in force that this material cannot afford.

The delamination question

Delamination is when the layers of the slate begin to separate, and it shows as flaking, lifting or crumbling at the edges and corners, sometimes with slates becoming soft. It is a condition of the slate itself, arrived at through age and long exposure.

What matters practically is that a delaminating slate is not a cleaning candidate. There is no technique that reverses it, and working on a slate in that state risks breaking it. If a significant number of slates on a roof are visibly breaking down at the edges, the conversation is about the roof, not about cleaning it.

What safe treatment looks like

The approach on these roofs is deliberately conservative. Assess the slates properly, including their condition rather than just their appearance from the ground. Remove growth by the gentlest means that will actually work on that roof, not the fastest. Then treat the full plane at low pressure so the remaining growth is killed rather than merely lifted, which is what stops it returning quickly, as covered in why biocide matters.

The general principle of matching method to material is the same one set out in does roof cleaning damage tiles. Fibre cement is simply the material where getting it wrong is least forgiving.

One practical note: if your roof is relatively recent and may still be covered, check the terms before anyone works on it. Cedral publishes its roofing guarantee information, and it is worth five minutes to read rather than assume.

When the answer is a roofer, not a cleaner

We would rather tell you this than take the job. If slates are delaminating across the roof, if there are numbers of cracked or slipped slates, or if the fixings are failing and slates are moving, cleaning is not the right spend. It will not address any of that, and it puts work over a roof that needs a different trade.

A clean makes a sound roof look after itself for longer. It does not buy time on a roof that is genuinely at the end of its life.

Common questions

Related services

• Roof cleaning
• Gutter cleaning
• Softwashing for render and walls