How This Old House lowered a basement floor without undermining a fieldstone foundation

The sequence, as the video presents it: dig down, form and pour the reinforced buttress against three of the walls, then lay a vapor barrier, a layer of sand and 6x6 welded wire mesh, and pour the slab. The buttress was cast with a ledge that will later hold the framing for finished walls.

Builders have described this risk for years. Stone walls built without footings rest on the soil under them, and once the soil beside them is disturbed, moving water can carry away fines and weaken that support, a builder explained in a Fine Homebuilding forum thread on deepening a stone basement. Another contributor described the local practice of leaving 6 to 10 inches of undisturbed soil against the wall and pouring a kneewall to hold it in place. A third reported that the weight of fresh concrete pushed in a section of rubble wall about 4 feet wide and 5 feet high under a basement window.

For the slab itself, check which code edition is in force. The 2021 International Residential Code raised the under-slab vapor retarder from 6-mil polyethylene to a 10-mil product meeting ASTM E1745 Class A, with laps of at least 6 inches, according to vapor barrier maker Stego Industries. Stego adds that generic poly sheeting does not meet that standard at any thickness.