"Ben" wrote

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?

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'!