Ultimate Guide to DIY House-building -- Foundation Floor Sub-bases and Floors

FOUNDATION FLOOR SUB-BASES AND FLOORS

The floor sub-base should consist of a compacted layer of 3/4” stone 4” thick. Plastic sheathing that comes in 4- or 6-mil thick nesses or other suitable vapor barrier materials are placed over the subbase to form an effective insulating moisture barrier before the floor concrete is poured.

The concrete floor should be a minimum of 4” thick, with welded wire mesh and rebar running through it for strength, and the concrete should be sloped to the floor drains. If you desire a smooth finish on a basement floor, it will be necessary to specify that you want the concrete steel-troweled. Should you desire a textured finish, it can be obtained by having brooms dragged across the surface before the concrete is set.

FOUNDATION WALL TOPS

On top of all foundation walls anchor bolts (Figs. 6.22 and 6.23) should be installed or partially embedded at approximately 3’ intervals (check local building code), with protruding bolt lengths long enough to securely fasten the sill plates (wooden planks that join the upper framing structure to the foundation).

Have foundation walls constructed of concrete block capped with a course of solid masonry blocks that will act as an insect barrier and help distribute the weight of the house's upper structure. When solid blocks are not used, the cores or rectangular holes in the top course can be solidly filled with mortar or concrete. To do this, a strip of thin metal lath must be placed in the mortar joint under the top course. The strip, which is just wide enough to cover the block cavities, forms a base for the concrete that fills the top course cavities.

Ill. 21 Types of mortar joints.

Ill. 22 Anchor bolt in a poured concrete wall.

WALL COVERINGS AND INSULATION FOR THE FOUNDATION

Exterior

The foundation will be less susceptible to frost damage, moisture transfer, leaking, and insects if the walls are insulated on the outside (Ill. 24). Concrete block walls should be pargeted (plastered) with a 1/2” of cement mortar.

The half inch of mortar can be applied in two coats for superior holding power.

Apply 1/4” thick layer of portland cement to a moistened masonry wall. The first coating should be put on roughly and allowed to dry for 24 hours.

When ready for the second coating, the first should be moistened, then the second 1/4” coating can be smoothed over the first and kept moist for about 48 hours while it hardens.

For any foundation wall, request two coats of tar or bituminous waterproofing material to be troweled on. Troweled tar is the best, but brushed on or sprayed on waterproofing is better than nothing. If local weather conditions are severe, protection can be further improved with a layer of sheet polyethylene or asphalt-impregnated membrane. Make sure there are no sharp surfaces that could tear or damage the membrane when the membrane is applied. Remove any large rocks or roots from the backfill so the poly membrane won't be damaged during backfilling.

Internal insulation inside the blocks can be accomplished by installing rigid foam insulation inserts that are friction fit into conventional block holes before the block is laid to increase R-values. Block holes that will be concrete filled are skipped. This type of insulation is out of view and protected from the elements. (See Ill. 25.) Exterior insulation can be installed using extruded polystyrene rigid foam that's 1 1/2 or 2” thick tongue and groove for a good seal. It will provide good thermal protection and moisture resistance above and below grade. The joints should be taped with product from the manufacturer. The manufacturer will supply installation instructions.

Above-grade foundation wall exteriors can also be protected with stucco or treated plywood.

Interior

To reduce heat loss, to prevent moisture and water leakage, and to further prevent the possibility of insect penetration, foundation interior walls can be insulated and covered.

A thick waterproofing white paint that's brushed onto the interior surface will reduce moisture penetration and discourage insects from infiltrating interior walls (Ill. 26). Foundation wall interiors can be insulated by first putting up furring strips and then applying blanket insulation in the usual way (Ill. 27).

Exterior of a foundation wall, cutaway. Ill. 24

Concrete block insulation inserts. Ill. 25

Interior of a foundation wall, cutaway. Ill. 26

FLOOR SUPPORT BEAMS OR GIRDERS

To support the house, one or more girders (load-bearing beams that help support the first-floor joists) as determined by the contractor's structural engineer should be installed. Steel I beams make the most reliable girders, but wood girders constructed of planks joined together by bolts or nails are also used (Ill. 28). Steel I beams are rather consistent in quality. The wood girders will vary in quality because they're dependent on the quality of their individual component planks. Girders either rest in pockets formed in the tops of the foundation walls or on top of corresponding masonry pilasters. In any event, the steel girder tops are generally made flush with the top of the wooden sill with wood planking that's laid along the top of the entire steel beam (Ill. 29).

FOUNDATION VENTILATION

Ventilation in foundation walls can be accomplished either with windows or with vents. Basement windows are discussed in the section on windows. If vents are decided upon, have them installed near or at the top course of concrete blocks, or as high as possible in the foundation walls at the rate of one every 50 linear feet, each being about the size of a concrete block: 8 by 16” (Ill. 30). They should be the type that can be closed during cold weather. At least one vent should be positioned at each corner of the house, with cross ventilation arranged for a minimum of two opposite sides. If the house is located in an area experiencing high humidity during much of the year, increase the number of vents to one for every 15 linear feet.

On a crawl space foundation, in addition to vents, there should be at least one access door of not less than 32 by 36” installed. If it's a large crawl space of 2,000 square feet or more, or an unusual shape, more than one access door should be included. Confirm this with local building codes.

Ill. 27

Foundation wall furring strips and insulation.

BACKFILLING

Once the foundation work has been completed and before backfilling occurs, you should make a formal inspection. Backfilling is simply the pushing back of excavated soil around the house to fill in the construction ditches. Backfilling should only occur after the first floor is framed and the walls are framed up so the added weight of the structure will stiffen the walls and make them less likely to bulge from the pressure of the backfilled soil.

Ill. 28 Wood girder.

Ill. 29 A steel I-beam girder.

Ill. 30 Foundation wall vents.

To avoid subjecting a "green" foundation that isn't fully cured to pressures that could damage it, plank or timber bracing should be installed inside the crawl space or basement, supporting the walls at about 12’ intervals. The house's central longitudinal support beam or girder, plus the first-floor joists and floor decking should also be erected to help strengthen the foundation walls before backfilling takes place.

As a general rule, backfill height from footer to grade should not exceed about 7’.

The waterproofing must be protected during the backfilling, since rocks and other hard materials in the backfill could scratch and penetrate the waterproofing and allow moisture seepage. If the soil that will be pushed back contains large rocks, the contractor should apply 4 by 8’ or 4 by 10’ sheets of impregnated sheathing or equal material for protection. The final grade must slope away from the foundation.

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STORM SHELTERS

Sometimes referred to as tornado shelters, these units are typically set within the ground and temporarily protect household members who may seek shelter during major storms. A good time to have one installed, of course, is when the home's foundation is being constructed and the back hoe or other piece of heavy equipment is already there anyway to excavate.

The most popular types of storm shelters are manufactured out of fiberglass-a forgiving material that withstands considerable movement and settling of earth, resists corrosion, mold, and mildew, and is very durable (Ill. 31). Units are available that seat up to 10 adults. These shelters are often anchored at the bottom of excavations, in poured concrete foundations that hold the units below ground level. To provide enough air for the inhabitants to breathe, storm shelters need multiple ducts and vents that rise far enough above the ground to prevent surface water and runoff from entering. The vent openings should also be covered with insect screens.

Outside doors should be low profile to permit simple landscaping and to avoid tornado or hurricane winds from affecting the unit.

Some of the more important features found in storm shelters include the following:

+ Prewiring for a television and phone. This would allow shelter users to stay in contact with what's in the news or what's going on out side, and to phone for help in case problems occur.

+ A safe or storage box to hold valuables.

+ Skid-resistant steps with handrails.

+ A lifting jack inside the unit, in case the doors become blocked by a downed tree branch, for instance.

+ A carpeted floor.

Other alternatives for tornado- or hurricane-resistant home protection include "in-residence shelters," which employ more conventional construction methods to encase an interior room of a house with concrete or steel and to then bolt the reinforced room to the foundation. Such rooms can have practical uses as well-as closets, storage areas, or even photographic darkrooms.

Ill. 31 A storm shelter.

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BE SAFE! With any storm shelter, make sure that a number of relatives, friends, or neighbors know of its existence and get them to agree to come looking for you following a major storm if you don't check with them immediately afterward.

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ROOT CELLARS

A root cellar may sound a bit old fashioned, but for households fond of gardening, farming, and cooking, a root cellar can be an interesting and useful addition. Root cellars are still standard fare in Europe and many other parts of the world-small- to medium-sized rooms or portions of cellars carved out of or built into the earth. Their name has evolved from two of their characteristics: root cellars have been used for centuries to store root vegetables such as potatoes, carrots, parsnips, turnips, and onions (as well as canned and pickled foods) that keep longer in dark, cool places; and second, because roots of trees and live plants had often grown directly into the root cellar over the years.

The perfect time to build a root cellar is when the excavation and foundation are under way. Because a root cellar needs to be shaded from sunlight, the basement provides the best location, where it will help resist temperature swings and has a natural ability to keep stored items cool. One way is to take a corner of the basement-a northern corner, if possible-with the greatest exposure to cold outer walls (concrete or concrete block) and frame in a rectangular room (large enough to suit your needs) with insulated studded walls. Another way-if the home will have a concrete porch or patio that rests on top of a full foundation wall-is to simply build the root cellar within that part of the foundation wall. It's typically a large enough area and will need only one insulated wall and door to enclose.

A third option is a root cellar that has three sides carved out of the soil, again, so only one insulated wall and door separates the space from the rest of the basement. Any enclosing wall (or walls) should use 2 by 6 inch studs with fiberglass or rigid foam insulation. The ceiling needs to be insulated as well. Protect both sides of insulation from moisture in the air with polyethylene air-vapor barriers. The ceiling needs the same barriers running up and around each joist, above the insulation, and below the insulation, too. A finish layer of drywall can be placed over the insulation and air-vapor barriers to protect against punctures. For access, an insulated steel door will do a good job here with a tight seal around its edges and extra rigid foam insulation bolted to the inside of the door with large flat washers. Because the door needn't be fancy, contractors often suggest making it out of 2 by 2s, 2 by 4s, and plywood. Wood doors, however, will likely warp over the long haul, and lose their temperature-tight seal.

A root cellar must also be ventilated. A small louvered metal vent the size of a concrete block works well. The louvers can be adjusted or closed as needed throughout the year.

Last, make sure the contractor builds and installs whatever shelving you need to maximize storage space (Ill. 32).

Root cellar shelving. Ill. 32

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