Basement Foundations

Of the three types of house foundations (slab, crawlspace, basement), full basements take the most time and work during construction. The construction of a full-height basement foundation can take two or three days. They consist of footings, a slab floor and reinforced vertical walls. The footings and slab are poured before the walls, mostly to make sure the concrete in these parts completely fills the formwork through the settling process. If wall, slab and footings are poured all at the same time, tamping equipment and mechanical vibration machinery are used to move the wet concrete into position.

In addition, because full basements require more concrete than other foundation types, more time is needed for curing before actual framing of the house can begin. Depending on outside temperatures, there can be a wait of a week or even more. One alternative that is used by some contractors is to construct the walls from concrete masonry units (CMUs). In this case, masons lay concrete blocks for the walls, including reinforcement bars and stabilizer braces. Mortar must be added between the blocks, and the cure times for the mortar are often just as long as for poured concrete walls.

Consideration should also be given to cut-out framing if windows or doors are to be installed, as in a building on a sloping grade. Window bucks are what the framing for windows are called; these are structural lumber which provide a space and nailing surface for installation of windows. Other insertions are also typically added, such as plastic pipe sections for accommodating utility and other service lines.

Concrete can be poured and cured properly in a wide range of temperatures, but care must be taken as it can be tricky at extremes. With hot weather, concrete may cure too quickly and unevenly, creating large cracks. One way of dealing with this is to keep the concrete’s surface wet, by hosing it down periodically for example. In cold weather, concrete can cure too slowly, putting projects behind schedule. There are ways of speeding up the cure, such as mixing concrete using hot water, or using blankets, straw or portable heaters to keep the concrete warm.

Waterproofing

After the concrete has cured and the formwork has been removed, some patching of the wall surface is usually required. Reinforcing tie wires are cut flush to the wall surface. Rough pitted surface areas, caused by incomplete filling of concrete in the formwork, are patched and filled in with special mastic. A properly smoothed wall surface is needed to provide good adhesion for waterproofing membranes.

Sprayed on waterproofing membrane is used for blocking moisture, resisting water penetration and water pressure against foundation walls; it also provides a more even temperature. There are different types of basement waterproofing membranes, including solvent-based rubber and non-flammable asphalt polymer membranes. Using a compressed air driven hose and spray nozzle, the membrane is applied in a .6 thick layer to the prepared wall surface.

Note that basement waterproofing is different from dampproofing. Prior to the widespread use of polymer membrane waterproofing systems, enabled by new lower cost technologies, below grade moisture protection relied on dampproofing, which is application of a layer of black asphalt. Although the old system did slow moisture and water penetration, it would degrade over time, weakening to the point where it would fall away from the wall entirely. For a basement that is to be used as a finished living space, there is no substitute for a good waterproofing membrane.

After the waterproofing membrane has been applied, it dries rapidly, and typically within one hour, work can start on filling in the excavated foundation trench. Thin panels of fiberglass or rigid foam are installed over the membrane to protect it from backfill debris, help shed any water which may get between the panels and the wall, and provide additional insulation to lessen inside wall condensation. At this time, perimeter drainage is installed also before final backfilling.