products Resources: Message Board/Forum, Books, Glossary How To & DIY guides & articles

Complex Ground-Level Additions: Building the Foundation (How to Build Additions Guide)

Building a large foundation uses the same techniques as building a small foundation, except that much more work is involved (See former section for basic foundation techniques.).

A large excavation will produce a great amount of soil, so hire professionals to haul it away. You will also need help with building forms, set ting rebar, and pouring concrete. If you build the forms yourself, rent or buy a builder’s level or transit to make the process easier. If you are pouring concrete against existing wood framing, install metal flashing against the wood to protect it from the concrete. If you are pouring a slab against the existing house foundation, you should install expansion joint material first.

On a large project you will need to involve certain subcontractors early in the foundation work—a plumber, an electrician, and perhaps a heating specialist. The plumber will need to locate sleeves in foundation forms for pipes and install plumbing under any slabs. If all house wiring is being upgraded, the electrician may need to set a secondary grounding rebar (underground) in place before concrete is poured. A heating contractor may need to install radiant heating pipes. If you are subcontracting the framing, have the framer inspect the foundation forms before the pour.

If the foundation has an engineered design, it may require special engineering inspections before the building inspector will pass the work. The engineer will need to see the steel rebar and the size of excavations, and may also want the concrete tested for strength by an independent testing service.

A large concrete pour, of 20 yards or more, takes planning and coordination. If you are in charge, have at least four helpers lined up. Rent a vibrator to consolidate the concrete. Start early in the day. If the ready- mixed concrete will be pumped, try to hire a large boom truck with a 4-inch-diameter trunk rather than a smaller pumper with only a long hose. However, this may not be possible for a backyard addition.

Lay out all anchor bolt locations a day or two before the pour. If weather is unusually hot, arrange for additives in the concrete mix to retard setup time. Provide a place for concrete delivery trucks and pumper truck to clean out their hoppers rather than having them dump the spill in the street. If no space is avail able in front of the house, have a wheelbarrow handy to haul cleanup overflow to the backyard.


Laying out a large foundation: L-shaped addition as a variation of the simple rectangle shown here. Equal measurements assure that new foundation will be parallel to existing building; Equal diagonals assure square corners; Corner stakes beneath string crossings

Next: Building the Addition

Prev: Preconstruction

Top of Page  All Related Articles  Home