My wife and I are restoring/un-renovating a 1912 two story four-square. Part of the project invoves fixing a single story addition added sometime in the ’80s. Unfortunately the addition has an unheated crawl space with a questionable foundation. The original structure has a full basement with concret footings at about five feet below grade. The foundation walls are concrete block. The addition is 25 feet wide (same width as the original sturcture) and ten feet in depth. The foundation consists of five 10 inch dianeter concrete piles of unkown depth. They are placed at each corner of the addition with the fifth in the middle of the long, outside wall. The floor is constructed of fir 2×10 joists running parralel to the long wall. The joists are 12 foot 3 inch in length. Rim joists (no hangers) support the floor joists at either end of the structure. The rim joists are supported by the corner piles. There is a centre joist header (double 2×10’s, no hangers) which supports the other end of the floor joists. The header is supported by a 4×6 beam. One end of the beam is sitting on the centre pile and the other is supported by a telepost inside the basement of the original sturcture. There are also 8 inch x 12 inch poured concrete grade beams between each of the piles (not sure if the grade beams are pinned to the piles). Partition walls of concrete blocks sits on the grade beams (between the piles) but do not support the rim joists (2 inch gap to the rim joists). The crawl space is currently unheated and contains the spill material from the piles. There appears to be a poured concrete slab under the spill piles, probably a padio slab existing before the additon was constructed. Local building codes call for footings at least four feet below grade (frost line). I would like to seal the crawl space and heat it and also improve the foundation for a possible second story additon.
The solution I came up with involves replacing the piles and grade beam with a rubble trench foundation complete with a poured concrete footing at grade and a 2×6 PWF foundation wall. I would like to complete the work myself with as little expense as possible and am prepared to hand dig the trench if necessary. I plan to support the structure durring the excavation work with timber supports laid out on the concrete slab. The new foundation wall would be sheeted with PWF plywood, parged with concrete to match the original structure and insulated with fibreglass batts.
I am interested in comments or suggestions regarding the suitability of my solution as well as tips for the construction. I’m particularly interested in knowing whether a rubble trench foundation is suitable for an additon to a struction with a different foundation system.