Slate hearth cracked - advice please?

[rickbert]

Hi folks - first time poster. Hoping you can help me.

We recently had a 4kw wood burning stove installed. The stove sits on top of a single piece of slate that measures 1100 wide x 800 deep x 20mm thick.

Underneath the hearth stone there is a section of floorboards cut out, so a recess which a little smaller than the dimensions of the slate. The hearth spans this void and sits on top. The installer put some cement down in the void under the stone.

A few weeks after installation, the slate cracked right across from the front to the right hand side, when we put a basket of logs down on the front right corner of the hearth.

The installer visited again to work out what caused this. His advice was that the floor is not completely flat and because we used slate, which is a 'natural stone' and has imperfections, it has cracked under the weight. Looking at it now, I can see there is a gap of about 6 or 7mm under the front left corner, and the hearth is 'floating' unsupported, whereas the back left corner is flat to the ground. I imagine if I put anything heavy on that front left corner it would probably cause another crack.

So I would like a second opinion on what has caused the hearth to crack. You'll see in the photos how the installer put some cement into the void. Do you think that's sufficient to level and support the stove and hearth stone? Should the installer have used anything else to support the edges and ensure the stone was level and supported all the way around? In your opinion, what is the best way to rectify the situation?

Photos:


Thanks in advance for any pointers!

 

[Rearscreen]

Yup unsupported. So unfortunate, and yes stone has "grain". I have Marble and the same thing had happened from the previous owner. Unsupported and it cracked. Last week I bought slow curing epoxy and fixed it. I bet if I mixed up a little marble dust I could make the fault line vanish.

 

[nola mike]

Hi folks - first time poster. Hoping you can help me.

We recently had a 4kw wood burning stove installed. The stove sits on top of a single piece of slate that measures 1100 wide x 800 deep x 20mm thick.

Underneath the hearth stone there is a section of floorboards cut out, so a recess which a little smaller than the dimensions of the slate. The hearth spans this void and sits on top. The installer put some cement down in the void under the stone.

A few weeks after installation, the slate cracked right across from the front to the right hand side, when we put a basket of logs down on the front right corner of the hearth.

The installer visited again to work out what caused this. His advice was that the floor is not completely flat and because we used slate, which is a 'natural stone' and has imperfections, it has cracked under the weight. Looking at it now, I can see there is a gap of about 6 or 7mm under the front left corner, and the hearth is 'floating' unsupported, whereas the back left corner is flat to the ground. I imagine if I put anything heavy on that front left corner it would probably cause another crack.

So I would like a second opinion on what has caused the hearth to crack. You'll see in the photos how the installer put some cement into the void. Do you think that's sufficient to level and support the stove and hearth stone? Should the installer have used anything else to support the edges and ensure the stone was level and supported all the way around? In your opinion, what is the best way to rectify the situation?

Photos:
View attachment 173743 View attachment 173744 View attachment 173745 View attachment 173746

Thanks in advance for any pointers!

Don't know what the process is normally for installing a piece of slate like that for a hearth, but it certainly looks like an incorrect substrate underneath. That wouldn't fly for a tile floor. Any gaps or flex to the subfloor will cause a large piece of slate to be vulnerable to cracking. I'd be tempted to get a second opinion as to the correct way to lay that (or wait for some forum members to chime in).

 

[firefighterjake]

Should have had more support under the slate . . .

My memory may be going . . . but wasn't there another person here from the UK that had cracked slate from an installation . . . not to this degree though?

 

[mitchell721]

So are they offering to replace it? In my opinion depending on hearth requirements it should have been self leveling compound used. Would have found its own level and provided a stable flat surface. Even if that one corner was to be "fixed" who know how supported it is in other locations. If for hearth requirements you were to use concrete or motar or something similar it seems like it should have been troweled flat atleast. That just looks like the old "throw a few globs in there, SHOULD be fine" mindset. Just my 2 cents

 

[FionaD]

Should have had more support under the slate . . .

My memory may be going . . . but wasn't there another person here from the UK that had cracked slate from an installation . . . not to this degree though?

Your memory serves you right -

https://www.hearth.com/talk/threads...-after-new-installation-of-woodburner.150457/

Oddly enough I was just wondering today if that earlier issue had been resolved with the installers.

Hi, Rickbert, sorry to hear about your problem. It does seem to be a common way to lay hearths over here. Mine was done the same way, although, 'touch (kiln-dried) wood', with no serious problems... Probably only because mine is 40mm granite..... though I do hear the cement underneath the stone crackle if I stand on the edge of the hearth

I have no idea why it's done this way in the UK.. I can only think because installers think in terms of laying over existing stone hearths from earlier fireplaces, rather than on floorboard... Pre-existing hearth stones would be way smaller than wood stove ones anyway, so it none of it makes sense.

There is a special hearth stone grout you can buy over here, that comes in various colours for people who want to keep the colour of hearth stones and grout consistent. We have it on our hearth, Between the four large granite pieces... it's exactly the colour of the granite and may work well to patch up your slate. It has a degree of elasticity too, so it 'gives' a little.

The bad news is, I have no idea of the brand name, I'm so sorry! But apparently it's made by several companies who produce hearth stones - so id like to think that one of them would be able to help you... I'm sorry that's infuriatingly vague, but maybe it'll lead to something. I hope so.

 

[fran35]

Unfortunate. Slate can be very weak if not set perfectly. However, was this whole hearth installed by the same contractor? If so, my thoughts are that they should bear some burden in the remedy. It looks to me like it was a erroneous install. As mentioned, the floor should have been cut away to encompass the entire slate piece and then self leveling compound utilized.

 

[RandyBoBandy]

Make the installer redo it. The correct way. As stated above the entire stone should have been supported by the mortar bed. Some sort of extra framing and support should have been done directly under the hearth area. It also appears the installer did not do a good job cleaning and or buttering the back side of the stone due to such a large piece breaking off.

 

[electrathon]

Very much the fault of the installer. Stone must be supported, not left to float out in the air.

 

[rickbert]

Thank you all for your responses. Seems the consensus is that the installer is to blame (yes, they did the entire install job). They have offered to replace the stone and re-install but encouraged us to consider granite as it is stronger. I'm dubious about that, and we prefer the matt slate look. Slate should easily be able to support the stove, if installed correctly, right?

 

[RandyBoBandy]

what is directly under the stove? basement, crawl space, etc?

 

[rickbert]

what is directly under the stove? basement, crawl space, etc?

Nothing, it is ground floor. Solid wood floor boards, then suspended chipboard floor. Modern built house (2006)

 

[firefighterjake]

Thank you all for your responses. Seems the consensus is that the installer is to blame (yes, they did the entire install job). They have offered to replace the stone and re-install but encouraged us to consider granite as it is stronger. I'm dubious about that, and we prefer the matt slate look. Slate should easily be able to support the stove, if installed correctly, right?

The slate really shouldn't have cracked if it was supported properly underneath . . . that said . . . perhaps unpolished or semi polished gray or black granite if you are looking for a matte finish.

 

[drz1050]

Strength really has nothing to do with a hearth surface. You can have glass, and as long as it's properly supported underneath, it won't crack. For there to be a crack, there needs to be some give. There should be no give at all in a proper install.

 

[mark cline]

I would have use a 1" deep bed of beach sand . Set the slate on it , then vibrated to settle the sand under the slate. The edge would have to be finished somehow to keep the sand in place and allow for a clean edge.

 

[FionaD]

Glad the installer is playing fair Rickbert.

As Firefighterjake has said, you can get different finishes to granite. I would not have wanted my granite hearth to be at all shiny ... so ours is that most matte of the three options we were given - what they called 'leather finish', very matte with some noticeable texture to the surface when one touches it. Of course it's gets even more matte looking once there's a permanent fine film of ash dust on it!

Glad you reached out to Pebbles in her old thread about the same issue.. hope she replies, I'd like to know how things went with her too.

 

[RandyBoBandy]

Nothing, it is ground floor. Solid wood floor boards, then suspended chipboard floor. Modern built house (2006)

The reason I ask is flex in the subfloor. If there was a way to bridge and possibly double up the floor joists under the stove this would help strengthen things up. Unless everything is on a slab when you say "nothing" is directly under the stove area.

 

[Simonkenton]

What size are the floor joists? How far do they span? How far apart are they. What is the subfloor on top of the joists.
My subfloor is overbuilt. 2x10 joists, spanning just 10 feet, and 16 inches OC. [On Center, the centers of the joists are 16 inches apart.]
When I built this subfloor, I doubled the joists that were to be beneath the wood stove. Three of the joists are doubled. Cost me an extra 36 bucks!

On top of that I have a sheet of 3/4 OSB, and on top of that 1/2 inch of plywood. Total subfloor is one and a quarter inches.
I suspect that your subfloor decking, the plywood, is not thick enough, thus dooming any masonry install you might do upon it.
If I were laying that pretty piece of slate I would lay a bed of mortar one inch thick, on top of the plywood, and put the slate on top of that. That way, there would be direct contact between the entire piece of slate and the mortar, no 6 mm gaps.

You got an install from someone who did not know what they are doing.
Go down and measure your joists and give us the specs and we can go from there.

 

[Pebbles112]

Hello Rickbert. Sorry to hear you have got a similar problem as us. I am pleased your installer came out to assess his work quickly and has agreed to redo it.
We had to fight our corner and things got pretty heated at one point. We think our problem is due to the mortar mix not curing properly and the stove being placed on the slate hearth too quickly.
We will be watching closely when he comes back to redo the work.
Pebbles.