It wasn’t easy.
We are putting a lower level under our existing structure so knowing where the four corners are should be easy, right? The existing structure is being supported by concrete columns holding a double I-beam, which the existing East and West walls are bolted to. These walls are anchored laterally to the ground, holding them solidly in place. The support system was designed to withstand wind gusts to 100 mph.
The structure was surveyed prior to us starting any work at all, so the four corners of the existing house were identified and the coordinates established by Dunbar and Craig, licensed surveyors. These four corners are still in place today. The four corners of the house that were surveyed were the outside corners of the wood, which had 1/4 inch plywood and 1/2 inch moulding strips on top of. These corners, which were probably not perfectly square to start with, have moved a bit in the almost 30 years since the existing house was built in 1969. Plus, the house was rocked vigorously by the the 1989 Loma Prieta earthquake, which measured 6.9, as the epicenter was only 14.7 miles from 17509 Via Sereno.
In hindsight, we should have exposed the corners of the concrete slab-on-grade and surveyed those points.
A beautiful, blue-sky California day with snow on the peak of Mt. Hamilton.
Projecting the Four Corners Onto the Working Slab
Our surveyers took the existing four corners, which have not moved, and established offset marks on the working slab. When we measured the diagonal distance between the north-east and south-west offset points, and the north-west and south-east points, we found the lengths to be different.