To find the best cure for a damp basement, first determine whether the problem is caused by water outside migrating through foundation walls or by interior water vapor inside condens­ing on the walls. To determine which problem you have, duct tape a 2-ft.-long piece of alu­minum foil to the foundation, sealing the foil on all four sides. Remove the tape after 2 days. The wet side of the foil will provide your answer. Chapter 14 has more about mitigating moisture and mold.

CORRECTING CONDENSATION

If the problem is condensation, start by insulating cold-water pipes, air-conditioning ducts, and other cool surfaces on which water vapor might con­dense. Wrap pipes with preformed foam pipe insu­lation. Wrap ducts and larger objects with sheets of vinyl-faced fiberglass insulation, which is well suited to the task because vinyl is a vapor barrier. Use duct tape or insulation tape to seal seams.

Next install a dehumidifier to remove excess humidity. For best results, install a model that can run continuously during periods of peak humidi­ty; place it in the dampest part of the basement at least 12 in. away from walls or obstructions. To prevent mold from growing in the unit’s collection reservoir, drain it daily and scrub it periodically.

Survey the basement for other sources of humidity. An unvented clothes dryer pumps gal­lons of water into living spaces; vent it outdoors. Excessive moisture from undervented kitchens and bathrooms on other floors can also migrate to the basement; add exhaust fans to vent them properly. Finally, weatherstrip exterior doors and keep them closed in hot, humid weather.

DAMPNESS DUE TO EXTERIOR WATER

Position and maintain gutters and downspouts so they direct water away from the house. And, if possible near affected walls, slope soil away from the house.

Besides those two factors, water that migrates through foundation walls or floors is more elu­sive and expensive to correct. Basically, you have three remedial options: (1) remove water once it gets in; (2) fill interior cracks, seal interior sur­faces, and install a vapor barrier; and (3) exca­vate foundation walls, apply waterproofing and improve drainage.

Option one: Remove water. Sump pumps are the best means of removing water once it gets
into a basement. If you don’t have a sump pump, you’ll probably need to break through the base­ment floor at a low point where water collects, and dig a sump pit 18 in. to 24 in. across. Line the pit with a permeable liner that allows water to seep in while keeping soil out, and put 4 in. of gravel in the bottom.

There are two types of sump pumps. Pedestal sump pumps stand upright in the pit. They are water cooled and have ball floats that turn the pump on and off. Submersible sump pumps, on the other hand, have sealed, oil-cooled motors, so they tend to be quieter, more durable, and more expensive. And because they are submerged, they allow you to cover the pit so nothing falls in. A й-hp pump of either type should suffice.

The type of discharge pipe depends on whether the pump is a permanent fixture or a sometime thing. Permanent pumps should have 112-in. rigid PVC discharge pipes with a check valve near the bottom to prevent expelled water from siphoning back down into the pit. If the water problem is seasonal, many people simply attach a 50-ft. gar­den hose and run it out a basement window. In either case, discharge the water at least 20 ft. from the house, preferably downhill and not directly into a neighbor’s property.

Option two: Interior solutions. If basement walls are damp, try filling cracks as suggested on p. 205 and applying damp-proofing coatings. (However, this approach won’t work if the walls are periodically wet. After scrubbing the base­ment walls, parge (trowel on) a cementitious coating such as Thoroseal® Foundation Coating or Sto Watertight Coat® or a polymer-modified system such as Bonsal’s Surewall®. These coat­ings can withstand higher hydrostatic pressures than elastomeric paints or gels. Epoxy-based coatings also adhere well but are so expensive that they’re usually reserved for problem areas such as wall to floor joints.

To further control moisture diffusion through the walls, install a vapor barrier. After parging the walls with a damp-proofing material, use con­struction adhesive to attach heavy (6-mil at a minimum) sheet polyethylene to the foundation walls. Then place sheets of rigid foam insulation over the plastic sheeting. Caulk and tape the foam seams to seal them. If the seams are airtight, the insulation layer becomes a second vapor barrier and makes condensation less likely because it iso­lates cool basement walls from warm air.

Option three: Exterior solutions. To water­proof exterior foundation walls, first excavate them. At that time, you should also upgrade the perimeter drains, as shown in "Foundation Drainage,” on p. 203. Then, after backfilling the excavation, slope the soil away from the house. That is, no waterproofing material will succeed if water stands against the foundation. Before applying waterproofing membranes, scrub the foundation walls clean and rinse them well.

► Liquid membranes are usually sprayed on, to a uniform thickness specified by the manufacturer, usually 40 mil. That takes train­ing, so hire a manufacturer-certified installer. Liquid membranes are either solvent based or water based. Modified asphalt is one popular solvent-based membrane that contains rubber­like additives to make it more flexible and durable. Asphalt emulsions are water based and widely used because, unlike modified asphalt, they don’t smell strong, aren’t flammable, and won’t degrade rigid foam insulation panels placed along foundation walls. Synthetic – rubber and polymer-based membranes are also water based; they’re popular because their inherent elasticity allows them to stay flexible and span small cracks. Note: Water-based membranes dry more slowly than solvent-based ones and can wash off if rained on before they are cured and backfilled.

► Peel-and-stick membranes are typically sheet or roll materials of rubberized asphalt fused to polyethylene. They adhere best on pre­primed walls. To install these membranes, peel off the release sheet and press the sticky side of the material to foundations. Roll the seams to make them adhere better. Peel-and-stick costs more and takes longer to install than sprayed – on membranes, but they’re thicker (60 mil, on average) and more durable. Though not widely used on residences, these materials seem justified on sites with chronic water problems. They’re often called Bituthene®, after a popular W. R. Grace Construction product.

► Air-gap membranes aren’t true mem­branes because they don’t conform to the surface of the foundation. Rather, they are
rigid plastic (polyethylene) sheets held out from the foundation by an array of tiny dim­ples, which creates an air-drainage gap. Water that gets behind the sheets condenses on the dimples and drips free, down to foundation drains. (For this system to work, you must coat the foundation walls first.) Air-gap sheets are attached with molding strips, clips, and nails; caulk the sheet seams.

► Until technology transformed water­proofing compounds, cementitious coatings rivaled unmodified asphalt as the most com­mon stuff smeared onto foundations. These days, acrylic additives make cement-based coatings a bit more flexible, but they will still crack if the foundation flexes. Bentonite, a vol­canic clay sheathed in cardboard panels, swells 10 times to 15 times its original volume when wet, keeping water away from foundation walls. Use construction adhesive or nails to attach the panels. These panels are costly and not widely available, and they can be ruined if rained on before the foundation is backfilled.

I Pedestal Sump Pump

slabs consist of 4 in. of concrete poured over 4 in. of crushed rock, with a plastic moisture barrier between. In addition, garage floors are often reinforced with steel mesh or rebar to support greater loads and forestall cracking.

PREP STEPS

As with any concrete work, get plenty of help. Concrete weighs about 2 tons per cubic yard, so if your slab requires 10 cu. yd., you’ll need to move and smooth 40,000 lb. of concrete before it sets into a monolithic mass. Time is of the essence, so make sure all the prep work is done before the truck arrives: Tamp the crushed stone, spread the plastic barrier (minimum of 6 mil), and elevate the steel reinforcement (if any) on
dobie blocks or wire high chairs so it will ride in the middle of the poured slab. Finally, snap level chalklines on the basement walls or concrete forms to indicate the final height of the slab— you’ll screed to that level.

To pour concrete with a minimum of wasted energy, use a 2-in. (interior diameter) concrete – pump hose. A hose of that diameter is much lighter to move around than a 3-in. hose. Another advantage: It disgorges less concrete at a time, allowing you to control the thickness of the pour better. And a 2-in. hose gives easier access to distant or confined locations. Important: As you place concrete around the perimeter of the slab, be careful not to cover up the chalklines you snapped to mark the slab height. And as you

place concrete in the slab footings, drive out the air pockets by using a concrete vibrator.

Establishing screed levels. If the slab is only 10 ft. or 12 ft. wide, you can level the concrete by pulling a screed rail across the top of form boards. Otherwise, create wet screeds (leveled columns of wet concrete) around the perimeter of the slab and one in the middle of the slab to guide the screed rails. The wet screeds around the perimeter are the same height as the chalk­lines; pump concrete near those lines and level it with a trowel. This technique is very much like that used to level tile mortar beds, as explained in Chapter 16.

The wet screed(s) in the middle should be more or less parallel to the long dimension of the slab. There are several ways to establish its height, but the quickest way is to drive 18-in. lengths of rebar into the ground every 6 ft. or so, and then use a laser level or taut strings out from perime­ter chalklines to establish the height of the rebar. In other words, the top of the rebar becomes the top of the middle wet screed. When you’ve troweled that wet screed level, hammer the rebar below the surface, and fill the holes later.

SCREEDING AND FLOATING

Screeding is usually a three-person operation: two to move the screed rail back and forth, strik­ing off the excess concrete, and a third person behind them, constantly in motion, using a stiff rake or a square-nose shovel to scrape down high spots or to add concrete to low ones. You can use a magnesium screed rail or a straight 2×4 to strike off, but the key to success is the raker’s maintaining a good level of concrete behind the screeders, so the screed rail can just skim the crest of the concrete, without getting hung up or bowed by trying to move too much material.

Screeding levels the concrete but leaves a fairly rough surface, which is then smoothed out with a magnesium bull float, a long-handled float that also brings up the concrete’s cream (a watery cement paste) and pushes down any gravel that’s near the surface. This creates a smooth, stone-free surface that can be troweled and compacted later.

A bull float should float lightly on the surface. As you push it across the concrete, lower the han­dle, thereby raising the far edge of the float. Then, as you pull the float back toward you, raise the handle, raising the near edge. In this manner, the leading edge of the bull float will glide and not dig into the wet concrete.

FINISHING THE SLAB

After the bull float raises water to the surface, you must wait for the water to evaporate before finishing the concrete. The wait depends on the weather. On a hot, sunny day, you may need to wait less than an hour. On a cool and overcast day, you might need to wait for hours. Once the water’s evaporated, you have roughly 1 hour to trowel and compact the surface. When you think the surface is firm enough, put a test knee board atop the concrete and stand on it. If the board sinks % in. or more, wait a bit. If the board leaves only a slight indent that you can easily hand float further and then trowel smooth, get to work.

As the photos show, knee boards distribute your weight and provide a mobile station from which to work. You’ll need two knee boards in order to move across the surface, moving one board at a time. Then, kneeling on both boards, begin sweeping with a magnesium hand float or with a wood float, if you prefer a rougher finish. Sweep back and forth in 3-ft. arcs, raising the far edge of the float slightly as you sweep away and raising the near edge on the return sweep. The “mag” float levels the concrete.

After you’ve worked the whole slab, it’s time for the steel trowel, which smooths and compacts the concrete, creating a hard, durable finish.

As the concrete dries, it becomes harder to work, so it’s acceptable to sprinkle very small amounts of water on the surface to keep it work­able. Troweling is hard work, especially on the back. When the concrete’s no longer responding to the steel trowel, edge the corners and then cover the concrete with damp burlap before call­ing it a day. If the weather’s hot and dry, hose down the burlap periodically—every hour, at

least—and keep the slab under cover for 4 or 5 days. At the end of that time, you can remove the forms. Concrete takes a month to cure fully.

Capping an old foundation with new con­crete is relatively rare but is done when the existing foundation is in good condition and needs to be raised because the house’s framing is too close to the ground, allowing surface water to rot sills and siding.

To raise wood members sufficiently, the new cap must be 8 in. above grade. At the very least, that means shoring up the structure, removing the existing mudsill, shortening the pony-wall studs, drilling the old foundation, epoxying in rebar pins to tie the new concrete to the old, and pouring new concrete atop or around some part of the existing foundation. That’s a lot of work. So if the existing founda­tion is crumbling or lacks steel reinforcement, you should replace it altogether.

On the other hand, if the house lacks pony walls and the joists rest directly on the founda­tion, you have basically two options: (1) grade the soil away from the house to gain the neces­sary height, which may not be possible if the foundation is shallow, or (2) jack up the house at least 8 in., which means hiring a house mover. Here again, replacing the foundation is usually more cost effective.

Otherwise, cut the tops of the stakes off and leave the rest embedded in the new concrete.

Hammer the outside of the form boards and then use a concrete vibrator, to drive out the air pockets. For this, insert the hose-like vibrator into the forms. As the concrete approaches the tops of forms, signal the pump to shut off so that the concrete doesn’t spill over the sides. When the forms are full and vibrated, use a trowel to flatten the top of the wall and sponge off any globs on the stakes and forms. Allow the concrete to cure 3 days at a minimum and 7 days for the optimum, before removing the forms and shoring, replacing the siding, and tightening down the washered anchor bolts. For further pro­tection against moisture, apply below-grade waterproofing to the outside wall and footing before backfilling.

Concrete Work

Concrete is a mixture of portland cement, water, and aggregate (sand and gravel). When water is added to cement, a chemical reaction, called hydration, takes place, and the mixture hardens around the aggregate, binding it fast. Water makes concrete workable, and cement makes it strong. The lower the water to cement ratio (w/c), the stronger the concrete.

POURING A CONCRETE SLAB

Pouring (or placing) a concrete slab is pretty much the same procedure, whether for patios, driveways, basements, or garage floors. Most