Filling space at the bottom of fireplace hearth

I recently noticed some cold air around the bottom of our fireplace hearth, which is brick and mortar. A quick look showed that the mortar under the bottom brick has worn away, in many places quite deeply. Should I fill with mortar, or could I use some other sort of sealant (foam?) to fill this space? If the former, any tips for someone who has never dealt with bricks and mortar?
Thanks!
-Ben
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I think you are probably a little out of your depth on this project. If this gap is between two items of masonry then you need to use mortar. If this gap is subject to the fire itself you will need to use fireclay based mortar. If this gap is between the hearth and the surrounding floor you may have other options to provide some flexability as the two different materials move at different rates.
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The gap is between the brick hearth and the surrounding wood floor and is not subject to the fire.
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"Ben" wrote

Ben, you say under the bottom brick, IE not between bricks? At the base of the floor?
We'd need to know what the floor is of there to answer. I't certain you could use firesafe mortar but that might actually be the 'problem' if this is on a wooded base (such as an upstairs fireplace may be in an older house).
Can you tell us the underflooring and how 'deep' it is? General dimensions involved? Also of use is if this is how 'tall and deep' the fireplace base is. Mine for example is 12 inches tall and on a cement slab but with flooring wood outwards of it (solid cement under) and a good 18 inches deep before you hit the area you would burn in so this doesnt get hot at the base. I could use almost anything but for safety, would be looking for a clear but firesafe type of caulk if there is such a thing.
Now lets guess a far more different setup, old house, upstairs fireplace. Only 6 inches up and a very thin small fireplace with some sort of asbestos type layer over wood then fireplace built over that. It would not be safe to do that with other than mortar and you'd have to know what you are doing if the gap were deep. In fact, it may not be safe at all to use until dealt with.
What I'd do is see if you can get a free inspection along with paying for a cleaning (tell them upfront of the problem and ask if they can check for safety and write an estimate if needed). They did that for me in my area without a problem. Alternative was pay for the inspection with no cleaning (slightly cheaper). It turned out i didnt need cleaning so they charged me for the inspection which was quite reasonable on their part since it was a package deal. In my case, the damage was significant and we knew that but we were unaware the water pouring in had been enough to make the thing so clean you could almost 'eat off the walls'!
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That's right.

It is on a wooded base.

The hearth comes out about 13" from the fireplace and has a concrete top that is about 14" off the floor. It sticks out about 19" from the side of the fireplace. The front and sides of the hearth are 4 bricks tall. The mortar under the bottom brick is what's worn way. I can't see anything other than the wood floorboards in the space. I assume that the floorboards sit on top of floor joists, since this is the upper floor of a 70s raised ranch.

I would like to get a cleaning, so I'll look into an inspection as well.
Thanks!
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"Ben" wrote

Good and i see from the later parts, well away from any spot that gets hot.

In this case, check with the guys when they inspect and clean but i believe there are simple caulks which will be better this time than re-mortaring it. Certainly easier.
If i get to my local hardware store soon, I will look about but this one isnt going to be hard to find as long as you tell them the right information. What you want is heat resistant but you do not need 'fireproof' as your dimsions show you are well from any flame danger and caulking to a wooded base.

Hehehe I think it's funny the only other responder didnt think to ask if it was upstairs. Not bad, he just didnt think of it.
What you likely have is slow settling, normal to a house of that age. It's created mild gaps so you do need to fill with something that will help vapor and water barrier. If looks matter much depending on the dimensions, backfill with a heat and water resistant cault then you can thinly cover the outermost with some mortar.
Lived in a house like that with a double fireplace (one very small on the floor above, same chimney and some odd older design so one didnt smoke out the other). The upper one had a problem a bit like what you say and was caulked then a thin layer of gritty sandy stuff was in the outermost caulk so except at close inspection, it looked like mortar.

No problem! I dislike 'home cleaning' unless it's an occasional extra between one or a frankin type stove pipe where you just dismount it then take it outside and run a brush through it.
I insist on annual professional cleanings here. Worth their weight in gold. If you have a downstairs fireplace below it, they package deal the price. I call my single fireplace cleaning 70$ *well spent* as we use the thing almost daily in winter.
Oh on inspections, you do not need the fancy camera ones that cost 150$ or more unless theres something about your particular configuration and a normal one shows more indepth is needed. See if you have 'ww.chimneycleaners.com' in your area. They did well by me. (no affiliation other than happy customer)
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