How House Construction Works - Foundations
by Marshall Brain

Basements, crawl spaces and slabs are the three main foundation systems used on houses. In wet and coastal areas, it is sometimes common to put houses up on posts as well.

Slab
The slab is probably the easiest foundation to build. It is a flat concrete pad poured directly on the ground. It takes very little site preparation, very little form work for the concrete and very little labor to create. It works well on level sites in warmer climates -- it has problems up north because the ground freezes in the winter and this freezing can shift the slab at worst and at least lead to cold floors in the winter. A cross-section of a typical slab looks like this:

Around the edge of the slab, the concrete forms a beam that is perhaps 2 feet deep. The rest of the slab is 4 or 6 inches thick. A 4- or 6-inch layer of gravel lies beneath the slab. A 4-millimeter sheet of plastic lies between the concrete and the gravel to keep moisture out. Embedded in the concrete is 6-inch by 6-inch wire mesh (shown by the dotted line in the slab) and steel reinforcing bars (shown by the white circles at the bottom of the beams). You will often hear this sort of foundation referred to as a "floating slab" -- it "floats" on the soil, with the deeper concrete around the edge holding it in place. In northern climates, the concrete around the edge has to extend deep enough to remain below the frost line in winter.

One thing about a slab is that the sewer pipe, and sometimes much of the electrical conduit, has to be put in place before the concrete is poured. The sewer pipes are actually embedded in the slab.

Basement
A house with a basement starts with a hole about 8 feet deep. At the bottom of the hole is a concrete slab, and then concrete or cinder-block walls form the outer walls of the basement. Actually, a basement is poured in three pieces in most cases: the "beams," then the walls, and then the slab inside the walls, like this:

This approach helps keep the basement waterproof. The L-shaped piece is a steel reinforcing bar to bind the beam and the wall together.

Crawl Space
A crawl space has several advantages over basements and slabs:

• It gets the house up off the ground (especially important in damp or termite-prone areas).
• It is a lot less expensive than a basement and comparable in price to a slab.
• Duct work and plumbing can run in the crawl space, meaning that they are easy to service and move over the lifetime of the house.

Most of the time, a crawl space is made of cinder block with a brick facing, as shown in the image on the right.
This is exactly how our sample house is put together. Here is how the finished foundation looks:

You might have noticed in the previous pictures that the concrete work for the crawl space was not done with much precision at all. One of the neat things that the mason (bricklayer) does is carefully adjust the height of the cinder blocks and bricks with mortar thickness so that the crawl-space walls end up exactly level all the way around.

One problem that arises in crawl spaces and basements is dampness. In order to keep water out, perforated pipe and gravel are used in a trench around the crawl space to route water away. The drainage system looks like this:

In a house with a basement, this same sort of drainage system is added along the bottom of the walls. The basement walls are then generally insulated with rigid foam board and then heavily waterproofed before dirt is backfilled against the walls.

Working Drawings.
Working drawings and specifications are the main sources of information for supervisors and technicians responsible for the actual construction. The construction working drawing gives a complete graphic description of the structure to be erected and the construction method to be followed. A set of working drawings includes both general and detail drawings. General drawings consist of plans and elevations; detail drawings consist of sections and detail views

ocation of the building in relation to the boundaries, the ground contour, and the roads and walks. It may also show utility lines such as sewer, gas, and water. This type of plan is drawn from a survey of the area by locating the corners of the building at specific distances from the established reference points.

Figure 1-5. Site plan

b. Elevations. Elevations are drawings that are commonly used to show exterior views of a structure from the front, rear, left, and right sides (Figure 1-6). They show a picture-like view as it would actually appear on a vertical plane. You must have a good overall idea of the structure before you examine it in detail. Elevations also show the types of doors and windows (drawn to scale) and how they will appear on the finished structure. Ask yourself does the structure have a simple roof? Is the floor level close to ground level (grade)?

Figure 1-6. Elevation views

Elevations are made more lifelike by accenting certain lines and adding straight lines to represent the types of materials used on the exterior (Figure 1-7). Lines that may be accented are window, door, roof, and building outlines. When accenting lines, you must assume that the light is coming from a certain direction and that accented lines represent shaded areas. Using straight lines to suggest the texture of exterior materials is a form of architectural rendering. Rendering, as applied to architectural drawings, is the use of a pencil, ink, watercolors, or a combination of these to depict (paint) a structure and bring out its form or shape.

Figure 1-7. Accent lines

c. Floor Plan. A floor plan is a cross-sectional view of a building. The horizontal cut crosses all openings, regardless of their height from the floor. The development of a floor plan is shown in Figure 1-8. Note that a floor plan shows the outside shape of the building the arrangement, size, and shape of the rooms; the type of materials; and the length, thickness, and character of the building walls at a particular floor. A floor plan also includes the type, width, and location of the doors and windows; the types and locations of utility installations; and the location of stairways. A typical floor plan is shown in Figure 1-9.
(1) Drawings and Specifications. Drawings and specifications inform the contractor, owner, material dealers, and tradespeople of decisions made by the architect and owner of the structure. Floor plans are usually drawn to scale (1/4" = 1' or 3/16" = 1'). Symbols are used to Indicate different types o fixtures and materials.

NOTE: Electrical, heating, and plumbing layouts are either on the floor plan or on separate drawings attached to the floor plan.

(2) Floor Plan Details. Detailed drawings may appear on the plan or on separate sheets attached to the plan. When detailed drawings are on separate sheets, a reference symbol is drawn on the floor plan. A door and window schedule is presented on the plan (see sample on Table 1-2 is a sample showing the information given on the schedule.

Figure 1-8. Floor-plan development

Table 1-2. Door and window schedule

Figure 1-9. Typical floor plan

d. Detail Drawings (Sections and Details). Detail drawings are drawn to a larger scale than plans and elevations to give more elaborate information, dimensions, and details. For example, they may give the size of materials and show the placement of parts in relation to each other.
(1) Sections. Sections are drawn to a large scale showing details of a particular construction feature that cannot be given in a general drawing. They show-

• Height.
• Materials.
• Fastening and support systems.
• Any concealed features.

(a) Wall section. A typical section, with parts identified by name and/or size, is illustrated in Figure 1-10. This figure shows how a structure looks when cut vertically by a cutting plane. Wall sections are very important to construction supervisors and to the craftsmen who do the actual building. They show the construction of the wall, as well as the way in which structural members and other features are joined to it. Wall sections extend vertically from the foundation bed to the roof. Sections are classified typical and specific. Figure 1-11 shows a typical window section.
(b) Typical sections. Typical sections are used to show construction features that are repeated many times throughout a structure.
(c) Specific sections. When a particular construction feature occurs only once and is not shown clearly in the general drawing, a cutting plane is passed through that portion.

Figure 1-10. Typical wall section

Figure 1-11. Window section

(2) Details. Details are large-scale drawings which show features that do not appear (or appear on too small a scale) on the plans, elevations, and sections. Sections show the builder how various parts are connected and placed. Details do not have a cutting-plane indication but are simply noted by a code. The construction of doors, windows, and eaves is usually shown in detail drawings. Figure 1-12 shows some typical door-framing details, window wood-framing details, and an eave detail for a simple type of cornice. Other details that are customarily shown are sills, girder and joint connections, and stairways.
Figure 1-13 shows how a stairway is drawn in a plan and how riser-tread information is given. For example, on the plan, DOWN 17 RISERS followed by an arrow means that there are 17 risers in the run of stairs going from the first floor to the floor below, in the direction indicated by the arrow. The riser-tread diagram provides height and width information. The standard for the riser, or height from the bottom of the tread to the bottom of the next tread, ranges from 6 1/2 to 7 1/2 inches. The tread width is usually such that the sum of riser and tread is about 18 inches (a 7-inch riser and 11-inch tread is standard). On the plan, the distance between the riser lines is the width of the tread.

Figure 1-12. Typical eave, door, and window details

Figure 1-13. Stairway and steps

e. Wood-Framing Drawing. Framing plans show the size, number, and location of the structural members constituting the building framework. Separate framing plans may be drawn for the floors, walls, and roof. The floor-framing plan must specify the sizes and spacing of joists, girders, and columns used to support the floor. Detail drawings are added, if necessary, to show the methods of anchoring joists and girders to the columns and foundation walls or footings. Wall-framing plans show the location and method of framing openings and ceiling heights so that studs and post can be cut. Roof-framing plans show the construction of the rafters used to span the building and support the roof. Size, spacing, roof slope, and all necessary details are shown. Working prints for theater of operation (TO) buildings usually show details of all framing.
f. Light Wood Framing. Light framing is used in barracks, bathhouses, administration buildings, light shops, hospitals, and similar structures. Detailed drawings of foundation walls, footings, posts, and girder details normally used in standard TO construction are shown in Figure 1-14.

Figure 1-14. Typical foundation wall, post, footing, and girder details

The various details for overall framing of a 20-foot-wide building (including ground level, window openings, brace, splices, and nomenclature of framing) are shown in Figure 1-15.

Figure 1-15. Light framing details (20-foot-wide building)

A construction drawing shows the type of footings and size of the various members. Some drawings give the various lengths, while others specify the required lengths on the accompanying BOM. Figure 1-16 shows floor-framing details showing footings, posts, girders, joists, reinforced sections of floor for heavy loads, section views covering makeup of certain sections, scabs for joint girders to posts, and post-bracing details as placed for cross sections and longitudinal sections.

Figure 1-16. Floor-framing details (20-foot-wide building)

Wall framing for end panels is shown in view A in Figure 1-17. Wall-framing plans are detail drawings showing the locations of studs, plates, sills, and bracing. They show one wall at a time. The height for panels is usually shown. From this height, the length of wall studs is determined by deducting the thickness of the top or rafter plate and the bottom plate. Studs placed next to window openings may be placed either on edge or flat, depending on the type of windows used. Details for side panels (view B) cover the same type of information as listed for end panels. The space between studs is given in the wall-framing detail drawing, as well as height of girt from bottom plate and types of door and window openings, if any. For window openings the details specify whether the window is hinged to swing in or out, or whether it is to be a sliding panel.

Figure 1-17. Typical wall-panel framing details

Examples of drawings showing the makeup of various trussed rafters are given in Figure 1-18. A 40-foot trussed rafter showing a partition bearing in the center is shown in view A. The drawing shows the splices required, bracing details, the stud and top plate at one end of the rafter, and the size of the members.

Figure 1-18. Trussed-rafter details

A typical detail drawing of a 20-foot truss rafter is shown in view B. Use filler blocks to keep the brace members in a vertical plane, since the rafter and bottom chord are nailed together rather than spliced. The drawing shows placement of the rafter tie on the opposite side from the vertical brace. Usually the splice plate for the bottom chord (if one is needed) is placed on the side where the rafters are to be nailed so that it can also serve as a filler block.
Use a modified truss, shown in view C, only when specified in plans for certain construction. It should not be used in areas subject to high wind velocities or moderate to heavy snowfall. In this type of trussed rafter, the bottom chord is placed on the rafters above the top plate. The construction plans specify the best type of trussed rafter for the purpose. The drawings must show, in detail, the construction features of the rafter selected.
g. Heavy Wood Framing. Heavy wood framing consists of framing members (timber construction) at least 6 inches in dimension (for example, 2 by 6 inches or 4 by 12 inches). Examples of this type framing are heavy roof trusses, timber-trestle bridges, and wharves. The major differences between light and heavy framing are the size of timber used and the types of fasteners used.
h. Foundation Plan. Figure 1-19 shows a foundation plan. The foundation is the starting point of the construction. Detail drawings and specifications for a plan are usually attached on a separate sheet.

Figure 1-19. Foundation plan

A foundation-plan and footings

Here's a foundation-plan example for a small home plan. The floor needs a girder in the middle to help with the span of the floor joists.

The girder plan is in the middle of theto show how it is built. Girders are boards nailed 

together resting upon concrete piers that run the length of the foundationFloor joists are

to the girder crosswise to make the floor more secure.

This illustration shows how the foundation footings are made. The cement forms can be made from boards or plywood. The rebar is then placed into the forms before the cement is poured.

The footing and foundation are usually two separate pours so that the footing can dry completely. If done correctly, you can avoid future