I am looking for ideas, opinions and any suggestions on how to handle my 3/4 HW installation around my stone and mortar fireplace hearth. Please see the photos below.

I've gotten lots of good info on fireplace solutions on this site and other sources. As is often said though, every job is bit different and of course the devil is usually found in the details. I've looked closely at the Crain 812 and Crain 820 and was planning on undercutting the hearth about 1" deep and simply running the HW under it to about half that depth to provide some expansion space.

What I found however, because of overhanging stones and other protrusions, the Crain tools may not be able to reach in and cut the bottom of the hearth to a 1" depth. (In some spots, they may not reach in far enough to undercut at all.) Another problem with a 3/4" undercut that can be seen in the photos is that the bottom of the hearth drops back sharply in some areas right where the top of the 3/4" cut would be. I'm pretty sure this would result in blowing out chunks of mortar and result in the HW having to run even farther in under the hearth.

It appears the original hearth was built on a 1/4" sheet of masonite which was pulled out after completion which resulted in a 1/4" space under the entire hearth (except for about 2 - 3 inches at the rear corners (see photos below.) This made me think it might be better to slide in a border of 1/4" slate, tile, or even 1/4" oak plywood to picture frame the hearth and run my HW up to that border providing expansion at the juncture with a reducer molding.

Another idea I've had is to continue out from the existing 5/4 x 6 scribed trim boards on either side of the fireplace with 3/4" thick - 1 x 6 oak with a matching dark stain onto the floor and around the hearth. The 1x6 could be scribed to the hearth and caulk any spaces or even router down to 1/4" in the scribed portion to slide under the hearth. This would provide a clean continous trimmed look around the entire fireplace and hearth without adding another contrasting feature, color or material between the fireplace and HW floor as would be the case with a border of slate or tile. The HW could then run up to this 1 x 6 wood trim and provide exapnasion at this juncture under a T moulding.

I apologize for the length of this post but thought it necessary to fully explain my situation. I would appreciate any ideas, advice or other suggestions on how best to handle this from an esthetic perspective and of course varying degrees of difficulty.

My Crain 820 will undercut that with a diamond blade and a good vacuum for dust control.

AFG! Thanks again for your replies. I have only had my hands on a Crain 812 at the local rental place and think the 812's upright motor configuration will prevent reaching in at the blade level in some places.

I was hoping the 820 might reach in farther under the protruding stones with its low profile right angle configuration. I appreciate the testimonial on the 820!

How about the area shown in the front of the hearth in the third picture?
Won't the blade completely remove the "mortar lip", moving the undercut back to where the mortar starts to rise? This rise occurs 1.5 - 2" back from the "mortar lip" in some spots. Do you see that as a problem?

Nope, don't see it as a problem.

Cut it and take your time chiseling it out. You may want to make two cuts. One on your mark and one lower to make getting that backer board/ plywood out.

Thanks again AFG! I do think undercutting and running the HW under the hearth will give the best look.

Backer Board? You may have misunderstood my description. I don't think there is any backerboard in there at all. I think the hearth is all solid rock and mortar but I could be mistaken.

It looks to me like the mason built the hearth on a sheet of 1/4" plywood or masonite but then removed it when he was finished. The existing 1/4" space under the hearth goes all the way back and all the way across. At least as far as I can reach with a 1/4" thick paint stirring stick.

If I am correct, don't you think one cut will be enough?

The Crain 820 will cut that and the depth guard is removable. Probably just need the one cut. Be very careful on the chiseling out part. Putting tape over the rock or saw guard will keep the rock face from scarring as you cut.

Thank you Jerry! Can you tell me how the Crain 805 masonry blades perform? Or is the 822 diamond blade the only way to go for this sort of cutting?

How about the Roberts 10-55 Longneck undercut saw? Have you ever seen or worked with one of them on hearths?

The abrasive masonry blades do OK on cultured stone and are barely adequate on brick. Real stone requires the 822 blade. I don't know anything about the Roberts saw.

Those masonry blades are a waste on stone or even brick. If your just cutting mortar they work OK.

Thanks Jerry! Thanks AFG!
I appreciate your replies and advice.
Can you tell me how effective is the vacuum dust control on the 820? Would you recommend not laying any wood in the room until after the hearth is cut?
ZT

I agree. However, they work ok on soft stone like marble and limestone. I just undercut a hearth faced with marble tile with one of those masonry blades. It was slow but the tile was only about 1/2" thick. It took maybe 15 minutes tops.

Well guys! I just made my first few cuts in the last of the jambs with my new Crain 820. Wish I had bitten this bullet before cutting the majority of the jambs and undercut most of the drywall with a hand jamb saw. It would have saved me a lot of time. The 820 put another big dent in my budget but I don't think there's a better way to do what I'm trying to do with the fireplace. I'll switch to the diamond blade and start on the hearth this evening.

I appreciate everyone's advice and the nudges in the right direction.
Thanks to all who have helped me with this project. I will post some pix when I'm done!

ZT