Ultimate Guide to DIY House-building -- Foundation Construction

Ill. 16 Foundation block pilaster.

Reinforcement of a foundation's floor and walls is a critical part of the house building process that can be easily slighted by marginal builders. Most home buyers don't realize what's involved and depend solely on the recommendations of contractors who can, at times, underemphasize the specs that are needed for a sturdy foundation.

This cost-cutting philosophy can lead to big problems later, at the homeowner's expense.

Here are some guidelines for the reinforcement of foundation floors and walls:

1. A concrete floor should be strengthened with reinforcement rebar. The rebar should be elevated to the middle of the floor's thickness on steel bridges or chairs set so they're positioned evenly across the floor before the concrete is poured with ends overlapped.

If welded wire fabric is used along with the rebar, its joints should over lap. Material overlap should meet local building code.

2. If a foundation has long walls or walls subjected to above-average stresses, they can be strengthened with pilasters (Ill. 16). A pilaster is a vertical block or concrete column poured or constructed adjacent to or adjoining a foundation wall, located at about the middle of the wall's length. More pilasters are often needed along unusually long walls. By having thicker walls at selected points, this extra support lessens the over all stress on the walls and helps prevent cracking. Steel rebar at least 5/8” thick, with four rebars to a set should be installed in and extend the full height of the concrete-filled pilaster voids.

3. Another modern and commonly used method of reinforcing the walls of a foundation is to place long pieces of 5/8”-thick rebar the length of the wall, in sets of four, through the rectangular openings of every other concrete block, vertically, and then to fill in those reinforced holes or "cores" solidly with concrete (Ill. 17).

The same rebar can also be embedded in the footing pour as a tie from wall to base, especially when high walls must withstand considerable pressures from slopes, water, or backfilling. All house foundations should have either the pilasters or the reinforcement rebar for vertical support. For a stronger foundation wall, use both pilasters and vertical reinforcement rebar throughout the walls.

Vertical concrete block wall reinforcement. (Ill. 17)

Horizontal concrete block wall reinforcement. ( Ill. 18 )

A steel tie bar. ( Ill. 19 )

4. For horizontal support, reinforcement wire should be placed in the mortar bed joint of every other course of blocks (Ill. 18).

5. Block walls that are quickly erected and backfilled require bracing within the crawl space or basement for temporary support until the concrete and mortar dries, to make sure the structure is tightly knit before stresses are applied. Leave temporary wood sup ports up in the basement as long as possible, especially during and after backfilling takes place.

6. Load-bearing foundation walls should not be joined or tied together with a masonry bond unless the walls join at a corner. Instead, steel tie bars vertically spaced not further than 4’ apart (check local building code) will form a strong bond (Ill. 19). If the walls are concrete block, strips of lath or steel mesh can be laid across the common joints in alternate layers or courses (Ill. 20). If a nonbearing wall will be constructed at a later date, ties should be incorporated into the first wall, to be left half-exposed so they'll be available when needed for the second adjoining wall.

Masonry Joints

If using brick or block for foundation walls, you have an option to select any of the following mortar joint types that would best go with your style of construction: flush, struck, V, con cave, raked, beaded, extruded, and weathered (Ill. 21). The V and concave versions are the most popular since they look neat and don't form a miniledge that could accumulate water. Beneath the grade, or the ground level, where appearance does not count, the joints are typically left flush.

Concrete Forms

If you're using poured concrete walls, make sure you realize that the quality of the concrete forms directly affects the finished appearance of the walls. Concrete forms must be tight, smooth, defect-free, properly aligned, and well-braced to resist lateral pressures created by the poured concrete.

Steel mesh reinforcement. ( Ill. 20 )