Two Foundations, One House

An addition’s slab and the original house’s foundation are different structures with different ages, different depths, and different relationships to the soil, and they do not move together. Forensic engineering guidance on differential settlement lists an addition to an existing house among the typical causes of settlement, and notes that pier-and-beam foundations settle pier by pier while a slab settles as one unit, which means the two sides of a construction joint are always negotiating. The plumbing pays for the negotiation.

Supply lines that cross from the original house into the addition get worked at the seam year after year, and lines cast under the addition slab itself carry the stress of a young foundation finding its final position beside an old one. The same engineering source lists plumbing seepage among settlement causes too, which makes the point in both directions: movement causes leaks, and leaks cause movement, and the way out of that loop is finding the water early.