Cracked slate hearth after new installation of woodburner...

[jeffesonm]

too bad that is a nice looking piece of slate

 

[Pebbles112]

IMHO, rwhite and other previous posts have your answer on all counts. That crack clearly originates from the front leg of the stove, indicating problems with the install. Likely due to not leveling the floor and floating/screeding the mortar bed correctly. Or perhaps the mortar wasn't sufficiently cured and the weight of the stove caused the slab to deflect right in the middle of its length.

Just for clarity and confirmation did you observe the installer and can you describe the process used in completing the install?

For example, did your installer pull back all the carpet to one side of the room, then use some type of curbing to lay out the mortar bed with a level across the curbs first, screed the mortar bed off, then lay the slab, level it, lift it and back butter with just a portland cement mix and tamp back to level? Or did they simply cut the carpet, pour a bucket of mortar out and smear it around and then slap the slate on it? The former would likely have prevented the cracking and the later will increase the chances of a poor bed which will allow deflection and cracking. Any dips, hollows or voids under a slab that size and thickness will lead to problems.

That slab install should have sat for a minimum of one day and two would be much better before placing the stove.

Thank you for all your advice.
I can confirm the installer cut away a square in the carpet and put a circle of the mortar mix onto the concerete floor and then laid the slab on top of that. The stove was placed within two hours of the hearth being put down.

 

[begreen]

Sounds like the mortar never had a chance to set up.

Wondering how they installed the stove without them standing on the hearth?

 

[Pebbles112]

We have now found more cracks. To make it easier we have drawn them out as this is attached. It is a simple plan view of the hearth from above with the stove legs marked.
We have now contacted HETAS, who is the UK body for stove installers.

 

[begreen]

Yes, I'm not mason but I would have waited a couple days for this to set up and would have take the mortar out to within an inch of the edge for full support.

 

[rwhite]

The 48 hours you mentioned should have been the mortar set time and not the stove start time. If they set the stove within 2 hours I can guarantee it happened just as I stated. Was the installer a mason or stove installer (or jack of all trades and master of none?). Surely any mason worth his weight would have known. Even with a properly floated floor you don't put that kind of weight on mortar in 2 hours. Here is the thing I don't understand, this was a small job and I would think that if I were the contractor I would eat the cost rather than risk my reputation.

 

[jtaves85mcss]

^+1 rwhite. I would suspect either this was the installer's first attempt at this or he has a long list of unsatisfied customers.

 

[Pebbles112]

The 48 hours you mentioned should have been the mortar set time and not the stove start time. If they set the stove within 2 hours I can guarantee it happened just as I stated. Was the installer a mason or stove installer (or jack of all trades and master of none?). Surely any mason worth his weight would have known. Even with a properly floated floor you don't put that kind of weight on mortar in 2 hours. Here is the thing I don't understand, this was a small job and I would think that if I were the contractor I would eat the cost rather than risk my reputation.

It is his own business and he is qualified to install stoves and installs the hearths as well.
He has reported back to us he has done 1500 installations over the years.
He is adamant that the fault is ours as we stepped on the edge of the slate and caused the crack! He also states that the slate was in perfect condition when left. Well it would be...the stove wasn't lit then.
I agree, his reputation is at stake here.
We have a battle on our hands now. Thank you for backing us up with such good technical info and advice.

 

[begreen]

He may have done 1500 or 15 installations, that's irrelevant. What's more important is how many floating slate installs he's done.

 

[pvfjr]

It is his own business and he is qualified to install stoves and installs the hearths as well.
He has reported back to us he has done 1500 installations over the years.
He is adamant that the fault is ours as we stepped on the edge of the slate and caused the crack! He also states that the slate was in perfect condition when left. Well it would be...the stove wasn't lit then.
I agree, his reputation is at stake here.
We have a battle on our hands now. Thank you for backing us up with such good technical info and advice.

I can't believe he's sticking to his guns on this. You should be able to have a weight-watcher's tap-dancing party on top of a well-supported piece of stone without having it crack! Think about how thin slate tiles are, 3/8" to 7/16"? They're made to walk on, but don't crack when you mortar the substrate properly.

Slate has a compressive strength of 95 MPa, which is very, very strong. It only has a tensile strength of 3.5 MPa, however. When sitting on a floor, the only way to apply tensile stress within the slate is to apply a bending moment. The only way this can happen is if a downward force is applied to an unsupported portion, and is most likely to happen if that unsupported region is at the edge. Basically, the edge of that thing was probably cantilevered, and you stepped on the far end. So yes, you provided the necessary load for failure, but the true cause was the installation.

 

[jtaves85mcss]

Pebbles112, I hope your hearth is warm tonight, and you're family is at ease. Get this one resolved in the new year.

 

[RandyBoBandy]

I have been working with stone for half my life. (17 years). My cabin has a natural stone hearth that my VC Defiant has sat on for 7 years. No cracks no heaving, only great fire watching. By the way I laid my own hearth. Depending on the type of stone used in your install and how well of a mortar bed the installer put down is what is at question. I can do jumpin jacks on my hearth with out worrying about any cracking. The fact that your installer is blaming you for stepping on it is a load of S%*T. Not to mention installing a 400 Pound stove two hours after he laid the hearth....sketch city....your installer is a clown.

 

[Shari]

When he said you should not have stepped on the hearth, did you ask him how you were to add wood to the stove without stepping on the hearth. Assuming you have at least 18" slate extension in the front of your stove, I know I can't reach 18" with a piece of wood in my hand to put the wood into the back side of my stove.

 

[JSoko]

looking at the install photos
thats a pi$$ poor install "expansion gap around the edges" WTF ???
it was only a matter of time till it cracked
I'm willing to bet when you research him he has plenty of complaints

 

[sportbikerider78]

I just had 750 sqft of really nice, thick porcelain tile laid by a great old Ukrainian guy.
You should have seen how meticulous he was with the mortar under the tile. It is all about getting the motar level and even...all the way to the edge. He was criticizing some of my tile work because i left just an 1/8" of mortar off the edge of a few tiles. LOL

 

[RandyBoBandy]

My guess is that there is an air pocket in the middle of the mortar bed. And/or the stone already had a hair line crack in it before homer installed it.

 

[Pebbles112]

Wow thank you for all your advice and help on this! Without you all I think we too would have cracked!!
My husband was present when he was installing and overheard him calling out the ratios of the mortar mix to his assistant. Apparently the 'muck' was not going off properly. We now know the consequence of that.
We are going to ask him for a third time to at least come out and assess his work. If he still refuses, we have to go through Citizens Advice Bureau and write to him with a view to taking him to small claims court.
We will be armed with all your technical info and cannot thank you enough for all you have done for us.
Saving grace... the wood burner is working beautifully. We spend most evenings fascinated by the woods and how differently they burn!
Wished we had bought one years ago.
Happy Holidays to you all, from a very wet and windy village in Kent!

 

[FionaD]

Wow thank you for all your advice and help on this! Without you all I think we too would have cracked!!
My husband was present when he was installing and overheard him calling out the ratios of the mortar mix to his assistant. Apparently the 'muck' was not going off properly. We now know the consequence of that.
We are going to ask him for a third time to at least come out and assess his work. If he still refuses, we have to go through Citizens Advice Bureau and write to him with a view to taking him to small claims court.
We will be armed with all your technical info and cannot thank you enough for all you have done for us.
Saving grace... the wood burner is working beautifully. We spend most evenings fascinated by the woods and how differently they burn!
Wished we had bought one years ago.
Happy Holidays to you all, from a very wet and windy village in Kent!

Your description of how the installer laid the cement and slab is EXACTLY what my very bad installer did. My saving grace was I already had a granite slab in front of the open fire that was there previously - the bit he added was just a hearth extension for the stove and that didn't actually have the stove sitting on it, otherwise I suspect I would have been in just the same position as you. What did happen to me though was the cement between the older slab and the new one in the front cracked and started crumbling badly... Reading your story just reminded me of that..

Just point yer installer in the direction of this thread if he needs a little more convincing.

So glad that your stove is working well and giving you pleasure. Have a great Christmas Pebbles.. And let us know how things work out.

 

[Woody Stover]

Just point yer installer in the direction of this forum

Hopefully he will take it in the spirit in which it's meant when he reads that he is a "clown."

 

[rickbert]

Wow thank you for all your advice and help on this! Without you all I think we too would have cracked!!
My husband was present when he was installing and overheard him calling out the ratios of the mortar mix to his assistant. Apparently the 'muck' was not going off properly. We now know the consequence of that.
We are going to ask him for a third time to at least come out and assess his work. If he still refuses, we have to go through Citizens Advice Bureau and write to him with a view to taking him to small claims court.
We will be armed with all your technical info and cannot thank you enough for all you have done for us.
Saving grace... the wood burner is working beautifully. We spend most evenings fascinated by the woods and how differently they burn!
Wished we had bought one years ago.
Happy Holidays to you all, from a very wet and windy village in Kent!

Pebbles - seems you and I are in the same boat. We're in the UK and recently had our stove installed. The slate hearth cracked in two on Xmas Eve. Full story here - https://www.hearth.com/talk/threads/slate-hearth-cracked-advice-please.152833/

I think our installer is replacing it but the issue of cost for any reinstallation labour hasn't come up yet. Suffice to say, I am fully expecting him to make it good without charge, or we'll also go via the small claims court.

 

[Pebbles112]

Finally after 6 weeks the installer saw fit to come and assess his work. After much discussion he still did not admit to a problem with the installation but agreed to re-do the job with new slate if we agreed to pay half each of slate cost.
He said he would not change his method. We sincerely hope that it works this time.
Thank you to all of the forum - it armed us with information that surprised him and put him on the back foot.
We are just glad it is just coming to a conclusion.
Thank you for all your help and advice.

 

[elmoleaf]

He said he would not change his method. We sincerely hope that it works this time.

Insanity: to follow the same method and expect a different result.
Ensure the slate gets installed with full mortar bed to its edges and that the mortar is given time to cure before being loaded with weight or be doomed to failure.

 

[mitchell721]

Yea sounds like the other issue that was posted on here dealing with slate. there's no reason to put the slate on while the motar is wet this is not a tile it's a flat peice of slate. It's got a stove on it. It's not going anywhere. The motar needs to be screed and allowed to dry then the slate put on. So it can be assured it's a flat sound substrate. If you do it again with this guy you need to be there to watch. I have a feeling he is going to try and just pick the slate up and put the new one down and save the labor of pulling up the old motar. Which will leave you with the same result some point down the rd. When the old one is pulled up you will most likely see the busted motar and air pockets

 

[FionaD]

.... And fight to the death to stop him putting the stove back in place till two days after the cement has been laid !

Glad to hear it's being sorted though Pebbles.. I was wondering about you just recently.

 

[elmoleaf]

Yea sounds like the other issue that was posted on here dealing with slate. there's no reason to put the slate on while the motar is wet this is not a tile it's a flat peice of slate. It's got a stove on it. It's not going anywhere. The motar needs to be screed and allowed to dry then the put the slate on

No, that is wrong. Bed slate in the damp (uncured) mortar to ensure full contact with back of slate. He may need to backbuttter the slate with mortar first or at least dampen backside of the stone. Tap down during install with rubber mallet and check for level.