Hearth Build -Stuck with some questions

[delp]

I posted about this a few weeks ago on the hearth forum, but now that I've actually begun my questions my concerns are more precise. I'm building back the hearth I removed in front of my fireplace and am pretty stuck. Hoping for suggestions, comments and advice.

I hope the attached photos help describe the situation.

Problem 1: I'm framing with 2X6's and the back board, parallel and closest to the firebox, needs support or a spacer from below before I add the joists. The space I need to fill to create the support is about 1 1/2" -that's what that piece of 2X is doing in the photo.

Q #1: can I pour concrete and make a small "wall" to support that 2X6? Or, should I find a square piece of steel tubing that is 1 1/2" and lay it across that part of the firebox floor to support the bottom of the board? I could also just rip the 2X and shim it under the board, but questions 2 & 3 below would be pertinent to that scenario...

Problem 2: The space behind this same board is the firebox floor. Initially, I thought I'd pour concrete and bring that flush with the structural/r-2 value sandwich I'll build above the joists. However, this space is quite deep, over 12" and I have two doubts/questions about it.

Q #2: should I put down a bunch of gravel and then pour cement?

Q #3: there's going to be a problem pouring cement against the framing board (described above), right? something's going to creep, move, shrink, or whatever else. Suggestions? I have no experience, nor do I have tools to tie and/or attach rebar...just saying...

The space inside the red outlining is where I've been thinking to fill with concrete.

I hope this makes sense and that the pictures help.

Thank you!

 

[delp]

ps...meant to say I'm posting here because these seems to be more generic construction worries that hearth stuff --I have all the clearance and r-value stuff figured out (I think)...

 

[fishingpol]

Take out the 2 x 6. Put in a 2 x 8 and drop it in until it hits the existing concrete at the highest point. Make sure it is level. Take a compass and open it up an inch or so and "scribe" a line across the bottom of the 2 x 8. The compass point should be touching along the concrete, and the pencil part gliding along the board. You are essentially transferring the line from the concrete to the board. Remove the board and cut the line with a jig saw or band saw. Put the board back and it will conform to the concrete. Mark the end of the 2 x 8 where it meets the others and make a straight line and cut that off making the new top of the board.

Look up "scribing" on the net. It is frequently used on countertop backsplashes to conform to uneven walls.

 

[Wade A.]

delp, replacing the 2x6 with a wider board is one option, as fishingpol recommended. Me-my-own-personal-self, I'd just nail a board tthe entire lenght of your existing joist that would overlap it on the hearth side and extend down to cover that gap. Then, using that as your form, pour your concrete on the back side and make sure you push mud up under that joist, effectively bringing your sill up to the joist you've already installed. You could then leave that board in place, or remove it. If the concrete settles or shrinks on curing, you'd have only a small gap that you could drive some shims into. No, I don't see a need to fill with gravel first, not being on top of ground and drainage/frost heaving not a problem. If can save you some labor if you wanted to just minimize the amount of concrete you'll need. You can throw scrap brick, stones in first if you wanted to. Rebar never hurts, or some reinforcing wire.

 

[delp]

Thank you, Fishing and Plough. A friend who's a mechanical engineer came by last night and enlightened me on a couple of things. That conversation and you guy's comments help tremendously.

I'm going to reconsider the DIY solid cantilevered slab idea, since the way my friend explained it seems less daunting than I'd imagined. Basically, he recommended attaching angle iron on the perimeter of the entire area (it's sort of an upside down "T" shape), then resting/attaching corrugated steel decking to those angle irons, and then pouring the concrete. I'd probably support it all from the basement for the pour and during the cure time. I'd have to make a template of the firebox/hearth area and then get someone to cut the steel decking to the exact shape, less some fudge room. If I can find the decking, this seems doable and by far the best, truly non-combustible surface.

Otherwise, what fishingpol suggested is a good idea, and I've done really precise scribing before, so that's a known. However, if I go this half concrete/half wood studs route my new thought is to wrap that 2X6 pine beam in poly, scab some luan to the outside of the beam to hold back the concrete and then pour that whole red outlined area, being extra careful to stuff the space under the beam with concrete. Also, I think that if I hang that beam on joist hangers, the chance of deflection at the center will be less likely.

Okay, off to search for that steel decking. I'll be back with the next step, which I hope will lead me forward toward progress...

Alina