Mahoney Heater

This spring, we were given the chance to work with a couple, Sharon and Chris Mahoney, on their soapstone masonry heater. After visiting a few project sites, the Mahoney’s chose an oversized double downdraft Finnish style masonry heater with polished soapstone benches.

Soapstone wood burning heater

We ordered all the bench veneer and cap sanded soapstone pieces precisely cut from Les Pierres Steatite in Quebec and we also ordered several pallets of split faced large raw stock from these same folks who are good friends (Scott designed and built his own little soapstone wood burning heater using this lovely high talc and figured Quebec stone). The sanded measured slabs and the many pallets of stone were delivered to our home on a large tractor trailer and we transferred the load to Manchester with three round trips with our own flat bed trailer and a locally rented forklift at the other end.

With a year to complete their house renovation project, we were able to take over a basement level two bay heated garage and transform it into our fabrication and polishing workshop. With three large work tables holding all of our small tools, we also set up a l4″ wet diamond saw in a catch pool against a plastic protected wall.

As winter was approaching, we moved all the sawed slabs immediately indoors to three or four pallets and left the split faced stock outside, tucked under a deck with a tarp so we could work in the rain. With a gas powered hand held l4″ wet diamond saw, Arthur proceeded to methodically work in a few days through all of the very large slabs reducing them to sizes no larger than l3″ by 36″. Albie and Dave Luce carried each piece into the indoor wet saw and proceeded to square each piece into maximum sizes starting with l2″ high stone and then dropping to l0″ stone, 9″ stone, 7″ stone and 5″ stone. By the time we had processed all of the rough split stone we had over 300 square feet of rectilinear stock to work with, in addition to a large quantity of sawed and sanded stock from the quarry.

We had as many as ten pallets spread around the perimeter of the room stacked three or four vertical layers high of each size of stone. Each stone was scribed and chiseled back to a uniform depth on all edges and the three men took every stone outside and power washed them on pallets to remove discoloration and surface debris from each piece. We then moved them all back into the heated garage. Load by load we took all of the scrap home as we generated it.

The work on the soapstone did not in fact get underway until after we demolished the tall stone chimney and set up roof jacks and staging plans and ladders and protection on the roof. The stone chimney and foundation took about a week to remove and dispose of all the broken debris. We saved all the used block and flue tiles. Albie and Arthur, following Albie’s plans, laid out and formed a new expanded reinforced footing and then Albie, Dave and Arthur built the new foundation, capped it with a heavily reinforced concrete slab cantilevered out beyond the foundation to support the heated bench.

Masonry heater albiecoreOver the next few days, we built up the refractory Albiecore with some more elements and the double downdraft heat exchange channels on each side of the heater as well as the beginning of the concrete block inner chimney with two clay tile lined flues. Once the firebrick and refractory element core was up, we laid out the heated bench channels with miter cut and clean out port penetrated clay flue tile liners, set them on mineral wool and bonded them together with water proof refractory mortar, double sealed with saddle gaskets and high temp silicone over each joint. Where the tiles ran over the ash door air entry, we sleeved the tile with a stainless steel matching and gasketed duct, so no long term accident could cause intake air to mix with the outgoing bench exhaust air. Once the core and bench tiles were in, we were ready to set the first of the soapstone pieces.

We wet diamond polished all the bench support stones after drilling holes for corner “u” clips and chamfering the edges with a carbide tipped router and located and cut out holes in appropriate stones for clean out doors and the ash box door. Upstairs, we had carefully laid out and leveled the saturated Skamol calcium silicate 3″ structural insulation sheets and had topped them with a mortar bed and carefully leveled and weighted l/2″ thick concrete board before we began the core.

heated_bench_installationWith the core built and bench tiles in place, a border of perfectly level concrete board remained all the way around the masonry heater for Albie to set the bench support stones. Albie took up all the support pieces and set and mortared and pinned them all with soapstone dust and water glass mortar and “u” clips. Arthur and Albie designed a full scale plywood template for the 3″ thick bench cap stones and Arthur started working all of the very heavy bench stones with hand held diamond saws, routers, sanders and diamond polishers, which were laid out to exactly fit the template. All the bench stones were rounded end to end in a continuous curve and rounded over on the top front edge down to a squared bottom edge. We carried each stone up the cellar stairs on a padded and strapped appliance dolly with Dave and Arthur lifting and pushing below and Albie pulling from above.

Once upstairs, we set little level soapstone shims in the corners under each stone position and then carefully lifted each stone into place on a soapstone mortar bed using a nylon strap to hold up the inner edge of each piece. We mortared the ends of each piece with a bead of high temperature clear silicone and then tweaked and leveled every stone for a perfect fit. On the chimney side and rear side Arthur had fabricated a band course matching the bench to carry the curved edge and curved length motif of the 3″ bench slabs all the way around the heater. We supported these cantilevered pieces with blocking until we could add more weight from the veneer wall above to lock them into a level position.

On two identical rugged plywood topped work tables in the basement we laid out exact perimeter measures of the heater with magic markers. With two tables we were always able to have two successive courses in front of us, which allowed us to avoid stacked joints and to pick complementary stones. End corners of each course were polished and slight chamfered. All the other surfaces were hand split faces.

Soapstone cutting and measuring

Before drilling each stone and cutting them top and bottom for spline kerfs, we rechecked lengths and heights and where necessary trimmed them to length, resquared them and sanded down any slight height excesses with a belt sander.

We calculated each course from our original drawings and from a story pole reflecting the layout of the critical height and opening measures in the core already built. Each course was numbered in order and carried upstairs for stone by stone construction and assembly. Surplus mortar was sponged and scrubbed off. Each course was leveled, plumbed, pinned, and splined. Occasionally, a slightly short stone had to be shimmed up with a short cut section of spline hidden in the base joint. For the gaps between the veneer stone and the mineral wool covered core at the loading door and oven door openings, Arthur designed and cut shaped, half round stones to trim the openings.

Downstairs, in the basement garage, he also designed and laid out a gorgeous split faced jack arch with a polished shaped keystone. Laying cardboard templates on a rough face raw stone requires great skill to transfer the two-dimensional measures of the template to a three dimensional surface. Together with Albie’s help, Arthur got all the stones to fit tight to one another in a dry stack layout. We built a temporary arch form support to lay the jack arch on and removed it a day or two after it was laid and secure. Arthur also cut out a curved split faced arch series to ride over the oven door already mounted with its cast iron surround on the mineral wool gasketed core face.

We used the stone we had manufactured, starting with l2″ high courses, but changing to other dimensions when arch construction oven door heights and supply and other considerations entered in. Our smaller 7″ and 5″ stone courses were, once combined, equal to a single l2″ course. We then created some patterns in the heater and chimney that utilized the large quantity of smaller height stones in a repeating and alternating manner with the taller stones.

Arthur laid out and cut a beautiful curved 2″ stone shelf beneath the oven and also polished and trimmed the cap stones for the heater, which we set above the last masonry heater course with rollers and straps. We covered the cap and bench stones with construction paper and plywood and then protected the front of the bench stones with plywood attached to a 2″ by 4″ framework screwed to the floor. We did this because as we continued with the chimney we had to erect pipeBuilding the soapstone wood burning heater staging at the chimney end with a “j” davet bar and pulley wheel to raise blocks, flue tiles, mortar, water, and soapstone course stone. On the masonry heater end we blocked up and built plank staging supported by the heater cap and tied into the second floor deck and leveled with planking in the pipe staging assembly. This gave us 360-degree access to the chimney.

All materials were typically carried up by rope, or rope and bucket, to the current working level of the pipe staging and then shifted horizontally to wherever we needed them. With rough faced stone, it is not possible to lay corners to a plumb line, so we sighted each corner carefully with levels as the work progressed slowly upwards. Passing the damper location on the heater we carefully cut slots in the veneer stone for the damper to pass through and then Arthur meticulously carved a flat surface in the rough face stone for the flat damper frame cover plate to rest and bolt on to the frame. Arthur had carved a similar smaller flat in the rough surface on the face of the heater where the by-pass damper rod and cover flange came through. Near the ceiling, we scribe fitted one end of the chimney to keep a two inch curved distance from a large log purlin beam supporting the roof.

Sharon and Chris decided that they wanted us to carry stone through the roof rather than brick, so we purchased some hand cut rectangular granite cobble stones from our local masonry supply year and brought enough stone to the project to finish the chimney, bringing it first inside to raise the temperature above freezing.

On a sunny day, Albie and Chris Pellitier built a wonderful 8′ by 8′ saddle hung wooden framed hut above the chimney hole and diagonally braced it and secured it by tying down two corners to the deck level with heavy nylon rope. We double skinned the roof with plastic tarps and installed a hinged door at the end of our roof staging and sealed all the gaps along the roof with plastic and tarp scraps and insulation. The hut was built off two pairs of saddle planks secured in custom made adjustable (to any roof pitch) steel hangers that Albie had designed years ago and reuses on most jobs.

Masonry heater chimney construction

Below the hut on all four corners we let the planks extend and added little “chicken coop” steps so that we could work around the hut in sheathing it and taking it apart. We set double sets of 2″ by 6″ planks horizontally across the bottom on both sides for similar access and to create a wrap capability for tarps within the hut that lay on the pitched roof floor of the hut so we can catch mortar and stone chip debris and contain it within the hut. Dave and Albie installed two electric heaters in lower corners of the hut and Dave and Arthur fed Albie chimney blocks and tiles through the old chimney exit hole while Albie raised the chimney. Once the blocks and tiles were set, we started laying the stone veneer, adding liberal metal flashing as the veneer passed through and up the roof. We set the flashing extra high on the stone and did not tap it down tightly on the corners to allow for patchwork after our departure on the old roof hole and to also allow for a possible additional layer of insulation and sheathing on the roof.

On a day that Albie was away, Dave and Arthur formed and poured a charcoaled colored and slightly shaped concrete cap with the spaces over each tile exit protected under the slab with a thin stainless steel sheet. Because the stone surfaces were uneven, we laid up the form and transitioned from the stone tops to the oiled form with a narrow strip of lead. Once the cap had formed and set we removed the form, cut the lead back to the edge of the stone with utility knives, saved the lead scraps, and then resealed the joint with a high temp clear silicone bead. Arthur and David also mixed a sandy concrete mix and laid in the sloped “wash” from the chimney edge up to the gasketed and lead sealed flue tiles to keep the core of the chimney waterproof from storms. Dave and Arthur washed the stones and cleaned the roof with a mild acid wash and water rinse.

After a two or three day cure, Albie and Dave picked a sunny Friday and disassembled the hut and all the roof staging, cutting the tarps apart from the safety of the inside of the hut. The top was disassembled in reverse order from its assembly, stud-by-stud, brace by brace and plank by plank with each piece carried down the ladder. Last to go were the saddle planks, steel hangers and safety ropes and then the horizontal planks and roof jacks that had led us from our ladder to the hut position.

Masonry heater chimney constructionIndoors, carpenters took over our staging under the peak of the roof and completed a patch of the old chimney hole and then replaced a couple of short beams to frame the new chimney and built a lovely new very small false ceiling section of tongue and groove pine boards scribe fitted around our last course of split soapstone veneer. Above that ceiling line, but below the roof, Albie had cantilevered and doubled the chimney thickness in concrete solid blocks to eight inches to support the heavy granite cobblestone veneer above.

Once the carpenters departed, we proceeded to wash down the soapstone veneer and to simultaneously sponge, mop and vacuum all the stone dust from many weeks of work off the surrounding beams and board panels. Course be course we descended, filling any little gaps with fresh soapstone mortar and took down each level of staging as we went, trucking the pipe staging and planking home as it was freed up. With all staging down and the beams and wood work cleaned we mounted the trim pieces around the loading door, mounted all the soot doors and ash box door with high temp silicone and tap con screws and then carefully mounted the large glass loading door with its new through the door cam lock handle.

Shortly after, Scott and Chris took a first round of finished photos while removing some hardwood flooring that needed to give way for a radiant floor pour in the living room. A day or two after that Scott and Albie came together and met with Chris and Sharon for a two hour break in burn and photo session and Nova Scotia wine toast. Albie sat with Sharon and Chris on the bench smiling and happy with the completion of a challenging and wonderful project. We returned for more photos after the family moved into the house a few months later.

Finished soapstone wood burning masonry heater