Edge-Thickened Slab

The new slab will be an edge-thickened slab; a 4-inch reinforced concrete slab thickened to 8-inches around the perimeter and extending 1-1/2 feet down, so below Portland’s freeze depth.
It will also extend 6-inches above grade putting an end to the water penetration problems.
The layers are undisturbed soil, 4-inches compacted gravel, 4-inches XPS foam insulation, a moisture/radon barrier, then concrete.


The wet fall weather meant we had to keep the foundation area covered with plastic to prevent the soil getting too saturated. It’s partly a cost issue as you pay for soil disposal by weight. Also mud makes a difficult working environment.

The builder used a mechanical vibrator to compact the gravel and soil. Boards were used to shore up some edges where the plumbing trenches had damaged the soil base. This produce sharp, stable edges that maintained their form all the way through to the foundation pour.

Here’s the compacted soil/gravel base:

Next was 4-inches of XPS insulation in two 2-inch layers alternately oriented to reduce thermal bridging. The seams were taped to help hold the foam in place during the rebar installation and concrete pour.

Next was a 15-mil moisture and radon barrier. This came in a massive roll however I will keep the surplus material for when I eventually replace the main house basement floor.
The five LSTDH8 hold-down straps were mounted onto the formwork. These will be fastened to double studs so it’s necessary to decide exactly where the door and window framing will be located on these three shear walls at this time. The hold-down straps can’t be later moved.

Next the was the rebar. The builder had the L-shaped rods pre-cut and pre-bent off-site so that no rebar bending was needed at the construction site.
He also installed a 20-foot earthing rod (made from rebar). It doesn’t need to be in one long straight length. Instead he had it bent offsite into a compact shape that could be easily transported. It is then tied to the foundation rebar. It will protrude through the mudsill under where the electrical panel will be located.

Here’s a photo of some of the formwork and bracing.

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Bill Dickens

I grew up on a hill country farm in New Zealand, then lived and worked in Hong Kong. I moved to Portland, Oregon in 2009. I'm presently renovating my 1929 "English Cottage" house in the Irvington Historic District.

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