Building on bedrock with 2/12 slope
I want to build a workshop on granite bedrock with a slope of 2″ per foot. Shop will be 20′ wide by 28′ long. I want the shop to have a wood floor (to save wear and tear on feet and knees). I was thinking of 1′ by 8″ by 28′ concrete footing on high side stepping it down over 20′ to an 18″ by 8″ by 28′ footing on the low side. I will use pressure-treated 2″by 6″ to fill in stepped footing to bring up to level for floor. Floor will be 2″by 8″ joists supported by beam running at 10′ on center. I want the floor to be well insulated. I live in a 6a climate zone. Do I create a dead air space between bedrock and joists? Do i ventilate?
I would appriciate any ideas about ways to approach this. I am open about changing the design.
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Replies
Mike,
You are describing a crawl space foundation. Crawl space foundations are routinely used; any experienced concrete contractor should have no problems stepping down the concrete stem walls.
Crawl spaces are best insulated by sealing the space (no vents) and installing rigid foam insulation on the walls. Be sure to insulate the rim-joist area; closed-cell spray foam is the best product for insulating rim joists.
Of course, before proceeding, it's important to be sure that what you call "bedrock" isn't subject to frost action.
Read more about crawl space foundations here:
https://www.greenbuildingadvisor.com/green-basics/crawl-spaces
Mike,
Be sure to pin your foundation to the granite with rebar epoxied into drilled holes in the rock.
You can use a sealed, insulated crawl space (as Martin suggests) as long as there is no chance of ground or surface water entering the space and the rock is not in contact with high ground water. IECC requries an R-10 crawlspace and an R-30 floor.
If water is a problem, you can build a vented crawlspace, insulate between the floor framing and apply a radiant foam thermal barrier (foil-faced polyiso) under the framing for a thermal break, to increase the R-value and to decouple the floor from the ground radiant temperature and humidity.
Depending on what kind of equipment and weight you intend to place on the shop floor, I would recommend increasing the joists to 2x10 minimum.