Cracked slate under log burner

Walshy252 · 20 January 2019 at 3:35PM

In October 2017 I had a new log burner installed by a local company. They ripped out the old gas fireplace and fitted new hearth, burner and lining kit. Very pleased with the result at the time.

Just under a year later I've noticed a crack in the slate hearth directly below the burner which goes right through the piece of slate that is now in two pieces. The piece of slate at the front is not damaged at all, and this fact that the crack is below the burner means it's impossible for something to have dropped on it to cause the damage. I reported it to the company as soon as I saw it, and they said they would call to discuss. Three emails later, and with the lifting between the two pieces of slate getting worse, no call and no closer to understanding where to go.

Any thoughts on this would be appreciated. The company offer no specific timeframe of a guarantee though do guarantee "100% customer satisfaction". I can only assume the slate was laid on an unlevel surface? Has anyone else experienced similar or could offer some advice on how to escalate?

Photo of damage below

https://drive.google.com/file/d/1ncRqQreO4iQ1X0zXAuVHFiCWUpaAg0IC/view?usp=drivesdk

telemarks · 20 January 2019 at 3:39PM

Is it possible, that the installation did not allow for expansion of the slate when it gets hot?

Walshy252 · 20 January 2019 at 3:42PM

Possibly

How would I be able to prove that as I suspect the company are being purposefully evasive. Also worth noting that we haven't hadn't had the fire on for at least 6 months before noticing the crack , so would have to have been natural expansion if that exists

FreeBear · 20 January 2019 at 4:01PM

That is not a minor crack !

It is a complete failure of the slate due to (most likely) poor preperation & bedding down of the slate. If it had been bedded down correctly, there is no way it would have lifted like it has in the middle. I would hazard a guess that it will sound hollow if you gently tap the slate in various places with your knuckle.

Keep_pedalling · 20 January 2019 at 4:15PM

There is a large amount of lift and slate is fairly impervious to thermal expansion, so something weird is going on here

owen_money · 20 January 2019 at 5:47PM

I agree with FreeBear, it seems like an installation issue, getting an even bed of mortar, without air pockets is key. When I laid my slate hearth I used a special fixative specifically for slate on to concrete, it had a bit more flex I think. Not had any issues.

Walshy252 · 20 January 2019 at 5:49PM

The odd thing is that the lift has increased over time. When we first noticed it the crack was back to front with only a slight lift. It has since developed to be the size in the photo.

Assuming that there is either a fault with the slate or with the installation, what would cause the continued lifting?

owen_money · 20 January 2019 at 6:40PM

The continually applied weight of the log burner? As you know they are very heavy. From the photo it looks like the leg/foot of the burner is right on the edge of the slate, almost on the join. I'm no expert, but I wouldnt have thought this was ideal, as you need to spread the load over a wide area. Where it is now seems to concentrate the pressure on the edge of the slate, and if the mortar isnt a constant bed it might crack it?

As I said I'm no expert, just offering an idea on the problem, hopefully there will be an expert out there?

keith969 · 20 January 2019 at 7:04PM

It appears that you have two pieces of slate, the one under the burner - which is cracked and lifting at the crack - and one in front. Is that correct?

That would be quite common for a typical installation, the bit at the back goes in the hearth enclosure and the bit at the front is wider and extends beyond the hearth.

If the back piece had insufficient expansion jointing to the left/right edges of the hearth, it could indeed crack that way. So it would appear to be an installation issue and you should ask for the stove to be removed, the slate replaced with suitable expansion joints.

Having said that I'd never have slate for hearths (have had in the past), it scratches far too easily. Granite looks better imho and has a lower thermal coefficient of expansion.

Walshy252 · 20 January 2019 at 7:57PM

Keith yes that's right and the front piece is fine and undamaged.

That's great advice thank you and gives me a bit more confidence going in to a conversation with the company

Davesnave · 20 January 2019 at 8:24PM

I shall now confuse the issue. Slate can just split along the bedding plane. This is called delamination.

I know this because the rear part of a matching pair of hearth stones delaminated in the car whilst bringing them home from the local slate company. I had three rear hearths by then!

So, this problem you have could be a result of:

insufficient expansion space
uneven pressure from the weighty stove
delamination

or any combination of these.

However, having looked again at the picture, this doesn't look like delamination, which would cover the whole piece. It seems that the company organised the cut in the worst possible place for this stove and failed to support one side correctly with bedding mortar....but only an investigation via lifting the slab might prove this.

Give them a chance to do it, but be present. Also, be aware that a replacement rear slab may not match exactly in colour.

Cracked slate under log burner

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