How Much Weight Can the Floor of a Masonry Built Firebox Hold?

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Aug 14, 2013
Upstate New York
Hi All,

I have been shopping to replace the wood grate in our traditional, wood burning, masonry fireplace. Some of the better, more expensive ones sold at a local fireplace store seem to weigh a lot more (~25 lbs) than the ones sold at Lowes (~10 lbs). It got me wondering how much weight the floor of a firebox can hold? We all know Santa Claus can come down the chimney and land in the firebox with a bound. Is there any risk of putting too much weight in the firebox from using a heavy duty grate with some big logs in there?

Our masonry fireplace has a raised hearth with fire brick lined along the sides and floor. There is an ash dump in the middle of it. I am also curious as to what holds the fire bricks up around the ash dump? Here's a picture of our fireplace after it was built about 10 years ago:
How Much Weight Can the Floor of a Masonry Built Firebox Hold?

Thank you.
A lot. Couple hundred pounds (a thousand??) per square inch before the bricks fractures.

I don’t think it’s anything you should worry about.
Look underneath the fireplace in the basement or outside. You most likely will see it is masonry right down to the ground. The weight of the grate is not going to affect the structure. The fireplace could accommodate a 400-pound insert without issue.