Using Road Base as a Natural Floor Slab
After filling and compacting our floor with 7.5 inches of lava rock we were ready to add 4 inches of road base. The road base has gravel up to 3/4 inches in diameter as well as sand and clay to compact hard. As the name suggests road base is used under paved roads and on gravel roads. This road base layer will act as our construction layer during the framing and hempcrete filling of the walls. Using 4 inches of road base gives us thermal mass for our radiant heat floor and acts as a stable layer to apply the finished earthen floor. We were able to substitute using road base instead of pouring a concrete slab. This allows us to used less concrete and keep the floor more breathable and use a material with less embodied energy. It did mean however that our interior footings for our loft and stairs had to increase 4" in height to give us 12" tall and 12" wide. If we were pouring a 4" slab over the footings we could keep them at 8" tall by 12" wide. This interior footing sits 1 1/2" below finished flor at the top of the road base layer and made a great reference point when raking and screeding the road base.
We wet the road base down before spreading it on top of the lava rock insulation layer. The road base was bone dry when it arrived so I put a sprinkler hose on it right away and by the time we were done compacting the lava rock it had soaked through the whole pile. We raked the 4" of road base over the lava rock and spread it as evenly as possible. I moved a sprinkler over the road base and used a screed to level the layer in the evening before compacting it the next day with a wacker plate. The road base dried hard and smooth and will be a great surface for framing and construction. We will need to go back and fill any potholes or low spots before pouring the finished earthen floor over the road base when the walls, roof, and hempcrete are complete.
Road base is fairly inexpensive but the delivery is not, so make sure you are ordering enough. We ended up using just over 30 tons of road base and had to order another truck load to get 3 more cubic yard. We ended up spreading the rest on our road to keep the dust down after having so many deliveries.