granite hearths for open fires

[Matt_boro]

do they really have to be cut into sections when being used with an open fire to avoid cracking??

seems to defeat the object of using a nice slab of granite!

conflicting opinions everywhere...

one website says...

"The hearth can be used with small or medium sized solid fuel fires as the heat given out would be insufficient to crack the granite."

where as another says...

"Granite Hearths are unsuitable for Solid Fuels as they are prone to cracking - only use with Gas Fires."

 

[Richard C]

The area of the hearth immedietly in front of an open fire or stove will reach very high temperatures & will need to expand whereas the granite towards the outer edges of the hearth will not heat up at the same rate or to the same extent & so will not expand as much. It’s not so much high temperatures that cause problems more that the temperature isn't uniform accross the hearth. The large temperature difference that can occur between the bit immediately in front of the fire & the outer edges of the hearth will cause differential expansion rates. This will create extremely high stresses within the granite &, in the case of a single piece of granite, enough to crack it. What you may not be aware of is that on a solid fuel hearth, the granite is cast into a substantial concrete base (usually 50mm) whereas a decorative hearth is usually just a 20mm granite slab with some granite up stands stuck around the edge which makes it less expensive to make. I have a granite hearth fitted around a multi-fuel cassette stove; the centre section is in 4 pieces, 3 small pieces immediately in front of the stove with a larger section behind; similarly the fire surround is constructed in 4 pieces. This allows differential expansion to ocur without the risk of cracking & gives a sort of geometric pattern which, IMO, does not look out of place.

The alternative is to risk it & buy a one piece decorative hearth which may well end up in pieces which are not so attractive to look at!

Additionally, don't use ordinary Gypsum plaster on the wall around a solid fuel fire; it won't stand the heat, will crack, blow & end up on the floor.

 

[Matt_boro]

many thanks for the detailed reply. i will get one that is cut into pieces

Matt

 

[PrenticeBoyofDerry]

found this might help.

Unslabbed Granite hearths are not suitable for use with solid fuel as the intense heat generated may cause cracking

Slabbing
Slabbing is the method of filling the underneath of the hearth with reinforced concrete. This provides a stable base for the tiles or stone and the steel reinforcements help spread the heat more evenly so that the slate or granite is not subject to heat stresses which might cause it to crack.