Monolithic slab capillary break
saundersjordan
| Posted in General Questions on
Building a Frost Protected Shallow Foundation in zone 6 BC Canada. Subsoil is very well draining gravel. My slab will have 4” of reclaimed XPS, vapour barrier, concrete.
My questions is whether I need to add 4-6” of free draining gravel under the XPS or can I just add say 2” of pea gravel to help with leveling and for sewer pipe bedding to avoid excess excavation, purchased materials and needing two different bedding and draining materials?
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Replies
saundersjordan,
Pea gravel has no fines. I would consider it free draining gravel.
Most places in the US use a version of the IRC model code, which requires at least 4" of clean sand or gravel passing a 2" sieve under slabs-on-grade: https://codes.iccsafe.org/content/IRC2021P1/chapter-5-floors#IRC2021P1_Pt03_Ch05_SecR506.2.2. The important thing is for the material to allow drainage, which means aggregate of roughly similar size and no finer material which will clog the "pores." I'm not sure if Canadian code has the same requirement but Malcolm would know.
Are you in a radon risk zone? I am, and always specify 4"+ of gas-permeable material (generally similar to freely draining material) and a 4" radon loop under the slab.