Rumford Fireplace (part XVI... final?!)

Designing a house is difficult. Four years ago when I began, I knew virtually nothing about fireplace design. I made accommodations for the fireplace within the timberframe and foundation, but I neglected to include provisions for the fireplace hearth. Ideally, if I could have known everything back then, I would have poured a cantilevered concrete ledge to support the hearth. As it were, I simply framed the house as if the hearth did not exist.

It would not be safe to build a hearth on wood, so I had to perform some reconstructive surgery on the house. I began by cutting out the plywood sub-floor, and the 2x4 and 3x3 that ran beneath that portion of the subfloor. Then I kerfed and chiseled the 6x8 timberframe floor joist that lay beneath that. I did this so that no portion of my concrete hearth structure would be supported by wood within 12 inches of the actual firebox. (Although I consider these measures to be very safe for my own house, this may not meet code in other regions of the country.)

I built concrete forms using aluminum extrusions and hardi-panel cement board. By using fireproof materials, I could leave the forms in place after the concrete cured. Into the forms, I placed reinforcing steel bars and drilled holes into the existing concrete so that the rebar (not the forms) could support the hearth slab on its longest side once the concrete cured. I mixed the quickrete in 5 gallon pales and added two extra shovels of portland cement to each bucket for extra strength. The kids had fun putting graffiti into the concrete... too bad all of that got covered up!

After the concrete setup, my wife and I had a crazy idea... one of those ideas that takes a one day project and turns it into a one week project. "Why not put some kind of pattern into the flagstone hearth?" I sketched a few ideas... Gothic arches in the grout lines, a Rising Sun pattern (the hearth faces east), and an oak leaf motif. We settled on the oak leaf. My wife cut out a piece of cardboard the size of the hearth and I sketched a pattern on it that satisfied both of us. To avoid creating a dust bowl in the house, I took the cardboard pattern outside and cut the stones in the back yard using a gas chop saw and an angle grinder. It took two days to cut the stones, but that included a trip back to the creek to search for more blue silt stones of a uniform color.

I mortared the flagstones down to the concrete hearth using generous amounts of thinset where necessary to level the thinner stones. Then I smeared dark gray grout into the joints, and followed up quickly with a wet sponge and progressively cleaner buckets of rinse water. We were out of "grout easy" so I tried to clean the grout off quickly before it had a chance to stain the flagstones. I used masking tape on the finished ash floor to avoid staining it with grout. As usual, our four year old wanted to help, so she grabbed the shop vac and vacuumed up the bits of dried grout on the floor.

The hearth stones haven't been sealed yet, but that hasn't kept me from starting fires in the fireplace. Here's a picture of the final, if not somewhat dirty, hearth design with a fire in the firebox. Yes that's a timber cut-off in the fireplace -- I have a few of those laying around.

Incidentally, the Rumford fireplace works fabulously. I have never seen an open fireplace that works this well, although I'll readily admit that fireplace inserts give off much more net heat. The fireplace draws wonderfully and lets no smoke into the house, even when the flue is cold and the fire is smoky. The only fault I can find is that it draws so well that it's clearly sucking lots of air out of the house. Although I neglected the hearth in the plans four years ago, I did include a cold-air make-up supply beneath the firebox. When I get that plumbed to the outside, the fireplace will be able to fuel the fire with outside air instead of warm inside air and I think that will contribute to the net heat output of the Rumford. For now, it burns a beautiful flame and radiates warmth!

The preceding post in this Rumford fireplace series can be found here.