Cutting into a Concrete Floor

To enlarge an existing load-bearing pad or create one where none was, you may need to cut through a concrete floor. Depending on the condition and thickness of that floor, the job will range from nasty to horrible. Cutting concrete is noisy, dirty, and dangerous; and the tools are heavy and unpredictable. Wear safety glasses, gloves, hearing pro­tectors, and a respirator mask. Adequate ventilation and lighting are a must.

If the floor was poured before the 1950s, you’ll likely find that it is only 3 in. or 4 in. thick and is without steel reinforcing. The floor may also be badly cracked.

In this case, you can probably break through it with a pickax, but to minimize floor patching later, rent an electric concrete-cutting saw with a diamond blade to score around the opening. Then finish the cut (the saw — blade rarely cuts all the way through) with a hand sledge and a chisel.

Be advised, however, that a concrete-cutting saw cuts dry and thus throws up an extraordinary amount of dust. Therefore, you may need to seal off the basement with plastic barriers and then spend an hour vacuum­ing afterward. Alternatively, you can rent a gasoline — powered wet-cut saw, which keeps down the dust but fills the basement with exhaust fumes. And, if the con­crete floor is a modern slab, 5 in. thick and reinforced

with rebar, you can spend a day accomplishing very little. Well. . . you get the picture.

Fortunately, for a few hundred bucks you can hire a concrete­cutting subcontractor to cut out a pad opening in about an hour. (Don’t forget to allow for the thickness of the form boards when siz­ing the opening.) The subcontractor can also bore holes needed for drainpipes and such.

under the post (a dab of silicone caulking will hold it in place while you plumb and position the post); then lower the jack so the new post bears the load. Or replace the wooden post with a preprimed metal column. However, if basement floors are wet periodically—suggested by sedi­ment lines along the base of walls—build up or replace the existing pad with a taller one to ele­vate the base of the post. Add a sump pump, too, as explained on p. 224.

Replacing pads. Replace concrete pads that are tilting or sinking because they are undersize for the loads they bear. Likewise, you’ll need to pour a new pad, if there was no pad originally and an overloaded post punched through the concrete floor. Pads for load-bearing columns should always be separated from floors by isola­tion joints.

Load-bearing pads should be 24 in. by 24 in. by 12 in. deep, reinforced with a single layer of No. 4 (12-in.) rebar arranged in a tic-tac-toe grid. Pads supporting a greater load (such as a two — story house) should be 30 in. by 30 in. by 18 in. deep, with two layers of no. 4 rebar; in each layer, run three pieces of rebar perpendicular to three other pieces. In either configuration, keep the rebar back 3 in. from the edges of the pad.

Line the forms with sheet plastic so that the water in the concrete won’t drain into the soil and weaken the pad. (Plastic also prevents soil moisture from later migrating through the pad and rotting the post.) If you carefully level the tops of the form boards and screed off the con­crete to them, the top of the pad will be level as well. Allow the concrete to cure before putting weight on it: 3 days minimum; 7 days recommended.

ADDING A GIRDER

Adding a girder beneath of a run of joists short­ens the distance they span, stiffens a springy floor, and reduces some loading on perimeter foundations. If your floors are springy and joists exceed the following rule-of-thumb lengths, con­sider adding a girder.

JOIST SIZE____ TYPICAL SPAN (ft.)

2 x6 8

2 x8 10

2×10 12 2×12

An engineer can size the girder for you.

"Beam Span Comparison,” on the facing page, shows maximum spans for built-up girders in two-story houses.

Ideally, the new girder should run beneath the midpoint of the joist span, but if existing ducts or drainpipes obstruct that route, avoid them by shifting the girder location a foot or two. Once you locate the girder, snap a chalkline to mark its center, and plumb down from that to mark posi­tions for pads and posts. Place posts at each end of the girder and approximately every 6 ft. along its length. If you create a girder by laminating several 2xs, keep at least one member of the "beam sandwich” continuous over each post.

Size and reinforce pads as described in the preceding section. After the pad’s concrete has cured 1 week, bring in the girder or laminate it on site from 2-in. stock. Prescribed widths for built-up girders are usually three 2x boards (4И in. thick when nailed together). For built-up girders and beams, the Uniform Building Code recommends the following nailing schedule:

20d nails at 32 in. on center at the top and bot­tom and two 20d nails staggered at the ends and at each splice.

Whether solid or laminated, if the girder has a crown, install it crown up. Installing a new girder is essentially the same as positioning a temporary shoring beam, except that the girder will stay in place. Have helpers to raise the girder and sup­port it till permanent support posts are in place. Properly sized, the pad will have more than enough room for jacks and posts, so place jack­ing posts as close as possible to the permanent post’s location. Raise the girder approximately 18 in. higher than its final position, to facilitate insertion of the new posts.

STEEL BEAMS

If there’s limited headroom or clearance under the house, steel beams provide more strength per equivalent depth than wood beams. For steel beams, hire an engineer to size them and a spe­cialist to install them: Steel I-beams are expen­sive and heavy, and they can be problematic to attach to wood framing, without special equip­ment to drill holes, spot-weld connectors, and so on. For commonly available sizes and some sense of the weights involved, see "Steel I-Beams,” on p. 53.