When carpenters are masons...

I knew from the early going that the ground floor of the house would have to be elevated in order to achieve the necessary slope to the septic system. The trouble was that the rock prevented digging down and the elevated floor wouldn’t be high enough to create a basement or even a useful crawl space without creating grading problems around the house. It was the realization that there would be no below-grade level that led me to the idea of pouring an elevated concrete slab - great for radiant heat and solar storage... but how to support it?

The options were really to pour a concrete stem wall or use some form of block that would support the slab edges while containing the stone backfill necessary to support the slab itself. Ultimately I decided to use block instead of pouring concrete because I wanted more precision than I was likely to get from a concrete foundation contractor. In my line of work - panelized buildings - close foundation tolerances are key to ensuring a fast and pain-free panel setting process. Ideally I wanted to use something the carpenters could install, that way I would be assured of precise work and I wouldn’t have to find, schedule and pay for a mason. For a while I considered using a polystyrene ICF (Insulated Concrete Formwork) but the less than ideal environmental profile of the product, along with the need to finish the exterior surface left me lukewarm.

Instead I chose “Durisol” (www.durisol.com), a concrete block made of cement and waste wood fibre. I’d used the product for a foundation a few years earlier and had been very pleased with its ease of installation and appearance. Durisol blocks are meant to be laid up dry - no mortar joints - then reinforced with re-bar and poured full of concrete, much like a styrofoam ICF. The blocks can be cut with a circular saw, and are generally friendly to all types of woodworking tools and fasteners - and thus to carpenters. They come with semi-dense rockwool inserts to increase their thermal resistance and the exterior offers an attractive textured surface that the manufacturer assures me won’t suffer from exposure to the elements.

































In keeping with my desire to perfect the layout I called Tim Rowe, my favorite surveyor, to come and lay out the building corners on the footing. If the footing had been level - i.e. the top surface all at the same elevation - and the footprint less complicated (fewer corners) - I would have been comfortable having George and Jesse lay it out because it’s easy to confirm square by checking diagonal dimensions on a flat surface. But this footing had several 12” steps in it (12” is the height of a Durisol block) as it marched up and down the site, so the potential for error was higher. Tim defined his starting point by driving a nail through a piece of orange surveying tape at the corner of the garage footing closest to the lot line, and set up his “Total Station” (a computerized surveying instrument) on that point. An hour later all the corners of the house were bang-on and we were ready to start laying block.

































While part of the plan was to avoid the hassle of finding a mason to do the foundation, George thought it a good idea to set the first course in mortar to iron out any discrepancies in the top of the footing. Luckily my first call produced results. Nick Deritis, a mason I’d worked with twenty years earlier on the very first house we built in Ottawa, was available and keen to help. Nick, now retired and without a truck, needed some help getting his mixer and other tools to the site but once we got over that he got down to business. Being of the old school, Nick was pretty sceptical about our wood-fibre concrete blocks but ultimately the light weight and ease of cutting won him over. A solid twelve hour day of mixing mortar and laying block finished off the first course for both the house and garage. The stage was now set for Jesse and George to install the horizontal and vertical re-bar and lay up the remaining courses.

























































At the lowest corner of the building the height of the foundation wall would be 6’ - or six courses of block. This scheme of backfilling behind the block with stone worried me because of the potential for shifting the block wall during the process of compacting the stone. I actually hired an engineer to think about this, and their recommendation was to do what I’d been planning all along, which was to simply brace the tallest and longest lengths of wall prior to backfilling - advice that scarcely warranted the five hundred bucks they charged me... They did also recommend tying the top of the Durisol wall into the edge of the concrete slab with pieces of 90 deg. angled rebar to permanently connect wall to slab and eliminate the possibility of the foundation wall ever shifting - a good idea that made me feel slightly better about their invoice.

The folks at Durisol also suggested that a thicker wall would be helpful in resisting backfill pressure so we used 12” block for all but the top course, where we switched to 10”. The narrower top course allowed us to move the slab edge, and the 1-12” thick strip of polystyrene that serves as a thermal break between foundation wall and slab, closer to the interior surface of the wood-frame wall where the joint between block, polystyrene and slab edge will be hidden by baseboard and won’t interfere with the final floor finish. Actually the 10” block wasn’t even quite narrow enough to achieve the desired detail so I had George and Jesse cut back the top inside wall of this top course a couple of inches to get the slab edge where it needed to be.