Foundation over bedrock dilemma
Our Tobermory cottage building site is on existing gravel fill (approximately 2 feet) over bedrock. Having spoken with builders; researched on-line; and seen neighbouring builds – we just can’t decide whether to go with the usual crawl space approach, or, keep our finish floor elevation as close to grade as possible (with a suspended slab) – in order to maximize our precious views of the lake.
Keeping the cottage footprint behind the setback for the 100 year flood line we can (I’m assuming) have our finish floor where we want- free of the flood constraint regulations. This would then have our sightlines as close to grade as possible to minimize having to limb-up the trees to be able to see the water. One main down side to this is that the septic inlet is approx. 2′-6″ above grade and would require a pump. I have heard warnings about failure/power outages but am thinking a generator would take care of that.
Seeing as the bedrock is fairly close to the surface, I’m guessing that we would technically have a “suspended” slab, and not a “slab-on-grade” as there would be a poured stemwall tied into the bedrock. I found a discussion about this on this website and a detail that has an inward facing ledge on the stem wall with 2 inches of rigid insulation as a thermal break from the slab (refer to attachments). My builder friend thinks that this is not enough insulation. I’ve seen an EcoHome video where they are putting 4 layers (8″) of Rockwool Comfort Board under slab and on the exterior side of the vertical formwork – is that overkill? (note: this was true slab-on-grade construction).
The alternative is the standard crawlspace approach. Upsides: gravity feed for septic and pipes/wiring accessible; some storage; (cheaper build?). Assuming a minimum 4 foot crawlspace clearance, (depending on the basement slab displacement on the bedrock: Q: can you pour slab directly on bedrock?) with 12″ TGI floor joists – I’m going to guess that the floor elevation is at least 4′-6″ above grade. This puts your eye level almost 10 feet above grade smack dab in the middle of all the shoreline cedars!
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