Building a hearth (and it's kind of a weird one)

[MaybeErnie]

I got lots of good tips for framing up a raised hearth from various old posts here. I had to get a little creative to adapt it to my particular situation because it's an old house and I didn't want to damage the oak flooring. Additionally, I wanted to frame it so that if we eventually get a smaller more efficient stove, I could downsize the hearth to suit the reduced clearances of the new stove. That is why, if you look at the image, there is a "hearth within a hearth" (outlined by the red marked boards), so that I can rip out the outer region back to the inner hearth. I don't know that that will ever happen, but I wanted to plan ahead to make it a possibility without completely starting over. The hearth will be covered with (in order): 3/4" plywood, 1/2" Micore 300, 1/2" Durock, and ceramic tiles.

To my question -- As you can see in the image, there are a lot of shims beneath the framing. That is because 1) the old floor has a lot of high/low spots and 2) no matter how carefully one selects boards, 2x4 lumber often has bows in it. I planed the upper surface and got it within reasonable spec for flatness, but there were still minor gaps on the bottom. Hence the shims. I can not detect any deflection, even before adding the shims, which are only inserted loosely (not hammered in), just enough to be snug.

Are the shims a good idea or should I just leave it slightly floating and allow it to settle over time? Would anyone recommend using spray foam to fill in the remaining gaps below? Although I know foam is sometimes used in construction (e.g., below sill plates), I worry that the expanding foam might lift the whole thing up a bit.

Oh yeah, I put down a double layer of heavy construction black plastic below the whole thing to protect the floor. Good idea?

Thanks for any and all tips. All observations are appreciated!

 

[Whitenuckler]

Hi - Were the 2x4's kilned dried? I'd get the plywood on ASAP and get rid of the plastic. If you plan on somehow taking it apart I'm not sure if you can use adhesive before you screw the plywood down? You will want to try and level it with as few shims as you can. The unit needs to be done bending if it's still drying out before shimming. The faster you get the plywood on the better as that will help it dry true. Use wood shims to level it, then cut the shims with a saw to the exact length and glue to the framing.
The shims inside you don't need to cut unless it's because of plan B. You don't need all those shims normally but because of plan B you need to have some on the small pad too.
I found a picture of a base I made for a gas fireplace. Mine was not as complex.

 

[MaybeErnie]

Thanks for the input! I got the lumber from a reputable lumber yard and they claim that it is kiln-dried (I called and asked).

The plastic was recommended by someone (can't remember if it was here or in one of the Reddit forums) to guard against scuffs and also to protect the floor from moisture that might build up in the hearth structure. I wasn't clear why moisture would ever be an issue in the hearth, but whoever suggested it seemed certain that it would inevitably get in there. Maybe they're assuming it would come from wet, snow-covered wood and water leakage down through teeny cracks in the masonry. Not sure, but you don't think that's a concern?

The plywood was actually screwed down previously, but I pulled it up to add a couple of additional boards. The plan for converting to a small hearth was to release the whole structure from the wall (using a demo saw to sever the hearth-to-wall screws), lift up the front end of the hearth to get some working room, and then cut around the inner outline with whatever demolition saw worked best. I plan to have a tile break that matches the inner hearth outline to avoid any extra tile work. So I think I'm OK using construction adhesive for the plywood. Again, the downsizing might never happen but this hearth is pretty dang big (about 60" each side).

I'll go back and reassess the shim layout as you suggested and then start putting on the upper layers. I was kind of surprised at the lower gaps but it is a 50-year old house with lots of settling and flooring issues that only become apparent when I need it to be flat (LOL).

 

[Whitenuckler]

Thanks for the input! I got the lumber from a reputable lumber yard and they claim that it is kiln-dried (I called and asked).

The plastic was recommended by someone (can't remember if it was here or in one of the Reddit forums) to guard against scuffs and also to protect the floor from moisture that might build up in the hearth structure. I wasn't clear why moisture would ever be an issue in the hearth, but whoever suggested it seemed certain that it would inevitably get in there. Maybe they're assuming it would come from wet, snow-covered wood and water leakage down through teeny cracks in the masonry. Not sure, but you don't think that's a concern?

The plywood was actually screwed down previously, but I pulled it up to add a couple of additional boards. The plan for converting to a small hearth was to release the whole structure from the wall (using a demo saw to sever the hearth-to-wall screws), lift up the front end of the hearth to get some working room, and then cut around the inner outline with whatever demolition saw worked best. I plan to have a tile break that matches the inner hearth outline to avoid any extra tile work. So I think I'm OK using construction adhesive for the plywood. Again, the downsizing might never happen but this hearth is pretty dang big (about 60" each side).

I'll go back and reassess the shim layout as you suggested and then start putting on the upper layers. I was kind of surprised at the lower gaps but it is a 50-year old house with lots of settling and flooring issues that only become apparent when I need it to be flat (LOL).

Go ahead and use the plastic if you know why it's part of the design. Try not to make it airtight so it can breath. Leave some gaps in places that can't be seen. If moisture ever gets down in there you want it to get out. It's looking good.

 

[MaybeErnie]

One more question --

When layering plywood/ Micore 300/ Durock, some old threads said there was no need to use any adhesive on either side of the Micore because it could be locked into place by using long screws through the Durock into the plywood. Does that advice still stand? It makes sense to me.

The only uncertainty I have is that the longest Durock/Cement Board screws I found are 1-5/8" and normally I'd want at least 2" when traveling through 1" of material (1/2" Durock + 1/2" Micore). I guess 5/8" of bite will be enough if I use 6" or 8" grid spacing -- that's a lot of screws.

 

[Whitenuckler]

One more question --

When layering plywood/ Micore 300/ Durock, some old threads said there was no need to use any adhesive on either side of the Micore because it could be locked into place by using long screws through the Durock into the plywood. Does that advice still stand? It makes sense to me.

The only uncertainty I have is that the longest Durock/Cement Board screws I found are 1-5/8" and normally I'd want at least 2" when traveling through 1" of material (1/2" Durock + 1/2" Micore). I guess 5/8" of bite will be enough if I use 6" or 8" grid spacing -- that's a lot of screws.

Do you need this Micore 300? I would put some screws into the Micore just to hold it in place then the longer screws through the cement board into the frame. No adhesive. Yes, normally the cement board is screwed into plywood. Maybe use longer Tapcon Screws.
I was not able to see what you are putting on it? A wood stove? What kind? Any modern one with an ash pan on the bottom should be fairly cool on the bottom? All the clearances should be to something combustible and cement board and tile are not.

 

[MaybeErnie]

Yeah, I think I need the Micore in the mix. Our current stove is not UL listed so I'm adhering to the clearance specs for unlisted stoves. After some reading here, I learned that the Micore has a higher R-value than concrete board and is also really light.

It's weird stuff - made with silica and I don't know what else. It's fragile until its secured in place and it doesn't like drilling or holes. I think it'll work fine as the middle layer in the sandwich providing I don't destroy it in the process. I just need to pick up the longer screws.

 

[Whitenuckler]

Yeah, I think I need the Micore in the mix. Our current stove is not UL listed so I'm adhering to the clearance specs for unlisted stoves. After some reading here, I learned that the Micore has a higher R-value than concrete board and is also really light.

It's weird stuff - made with silica and I don't know what else. It's fragile until its secured in place and it doesn't like drilling or holes. I think it'll work fine as the middle layer in the sandwich providing I don't destroy it in the process. I just need to pick up the longer screws.

Are you sure you are able to get that stove approved? Here in Canada we would need the stove to have an approval stamp. Who gives you the inspection on the stove and chimney for your insurance?

 

[MaybeErnie]

Yes, that is a good question.

It's an old Lopi that's been modified a bit. It was originally in a different location in the house and it has a lot of sentimental value to us and the kids. I had one installer come out and he wasn't totally sure if it would pass inspection but was overall encouraging about the installation plans (hearth, wall protection, chimney...). Once it's done, I'll contact our insurers to see how they want to evaluate it. Unlisted stoves can be a grey area. If it's a thumbs down, then I'll just have to snag a different stove. That's the main reason for the "modular" hearth approach.

Another issue is, of course, cost -- once I priced out all of the other components it was clear that this might have to happen in stages. Years ago, I installed 2 wood stoves in a different house and it's obvious that since then the tech, the cost, and the scrutiny have all ramped up. That's all good but it does make modern woodstoving a different ball game.

 

[Whitenuckler]

Yes, that is a good question.

It's an old Lopi that's been modified a bit. It was originally in a different location in the house and it has a lot of sentimental value to us and the kids. I had one installer come out and he wasn't totally sure if it would pass inspection but was overall encouraging about the installation plans (hearth, wall protection, chimney...). Once it's done, I'll contact our insurers to see how they want to evaluate it. Unlisted stoves can be a grey area. If it's a thumbs down, then I'll just have to snag a different stove. That's the main reason for the "modular" hearth approach.

Another issue is, of course, cost -- once I priced out all of the other components it was clear that this might have to happen in stages. Years ago, I installed 2 wood stoves in a different house and it's obvious that since then the tech, the cost, and the scrutiny have all ramped up. That's all good but it does make modern woodstoving a different ball game.

Yes I understand. I did one wood stove and vent pipe brand new from start to finish in a cottage. It is a big process and expense. My understanding is that the insurance company will not approve anything. That will need to be done by whatever process the underwriter says they need to follow. Here in PEI my company specifies is be inspected by a WETT certified inspector and a certificate issued. The certificate has all the measured clearances documented and the model numbers of the pipe and stove, all of these need to be approved by some other organization. Then the insurance company adds the rider and you pay more each year. Here they even had a condition that I can't burn more than 3 cords a year. Mine being a pellet stove I ignored that. I was in close contact with my agent during the whole process from saying I'd like to get one, to asking for what I need to get approval, to getting it done. You should be able to get the approval plan ahead of time. No since waiting as it's going to happen regardless.