Extending a hearth

I s'pose cementing some rebar or something into the existing edge wouldn't be a bad idea if you're just going to pour a concrete addition onto what's there. I've no idea how high this hearth is above the existing floor, but the other idea that occured to me was to set masonry pieces in mortar...standard bricks or non-standard bricks...which would serve the purpose of extending the hearth, while also "trimming it out". A pic of the existing location would certainly help. Rick

There is an article in the HearthWiki about making hearth extensions, but that was mostly focused on doing extensions over combustible flooring in ways that would meet code requirements. Doing an extension on a non-combustible floor will be much easier since R-values and such aren't really a concern, as long as you don't use combustible material in the extension...

I think your idea of putting some lags or rebar into the old hearth is a good one, I'd also make sure to rough up the cement you will be building the extension on as well, and possibly put a couple of lags in that as well - you need to bond to both the old hearth AND the original floor...

Gooserider

I like your idea of making a form and pouring the concrete into it, but personally, I don't think you're saving much time or energy by tying the new pour into the existing hearth. The right way would be to drill holes, blow-out the holes (to remove the dust), add a chemical anchor, and insert bolts / threaded rod / rebar, so that the new pour is tied to the existing one.

Personally, I think it'd be just as easy to break out the existing one, and make one monolithic pour inside your form. The bagged concrete's you buy at the boxes are super strong. If you use something like quickcrete, you probably don't even need any reinforcement. Concrete is cheap and relatively easy to work with.

Will you tile over it? If not, you need to make sure you get all the bubbles out along the form. Plenty of gentle tapping of the form with a mallet or hammer should help - be sure to spend a little extra time in the corners too. If you decide to tile over it, no worries, you'll just have extra surface area for your thinset to adhere to.