In the United States, there are more than 100 million housing units, and the majority of them are "single family dwellings," or houses. In cities, in the suburbs and in rural communities, houses are a very common sight.

Have you ever wondered how a house is built? What holds up the ceilings and the walls? What keeps the rain out? What parts go into making a house? How many different people are involved when a house goes up? If you would like to know the answers to questions like these, or if you have simply been curious to know all the steps that go into making a new home, then read on! In this edition of HowStuffWorks, you'll learn exactly how houses are built!

Setting the Stage
So let's start at the beginning. What is a house? According to The American Heritage Dictionary of the English Language, a house is "A structure serving as a dwelling for one or more persons, especially for a family." You probably have a very specific mental image of the "typical house." It is a structure on its own piece of land, generally with a lawn and plants outside. It has a pitched roof, walls covered in brick or siding, windows and doors. Inside there are rooms like the kitchen, the living room, bedrooms and bathrooms.

I can show you a hundred pictures of houses and they may all be completely different in their specifics, but they will all share those basic characteristics.

Steps to Building a House
One of the amazing things about American homes is that the huge majority of them are built using completely standardized building practices. One reason for this consistency is a set of uniform building codes that apply across the country. Another reason is cost -- the techniques used to build homes produce reliable housing quickly at a low cost (relatively speaking). If you ever watch any house being built, you will find that it goes through the following steps:

• Grading and site preparation
• Foundation construction
• Framing
• Installation of windows and doors
• Roofing
• Siding
• Rough electrical
• Rough plumbing
• Rough HVAC
• Insulation
• Drywall
• Underlayment
• Trim
• Painting
• Finish electrical
• Bathroom and kitchen counters and cabinets
• Finish plumbing
• Carpet and flooring
• Finish HVAC
• Hookup to water main, or well drilling
• Hookup to sewer or installation of a septic system
• Punch list

Many of these steps are performed by independent crews known as subcontractors. For example, the framing is generally done by one subcontractor specializing in framing, while the roofing is done by a completely different subcontractor specializing in roofing. Each subcontractor is an independent business. All of the subcontractors are coordinated by a contractor who oversees the job and is responsible for completing the house on time and on budget.

We will walk through these different stages so that you can see what is involved, understand all the steps and learn about the different materials used in the construction process. We will use a typical three-bedroom home as our example.

Site Preparation
The first crew on the site handles site preparation. Often, this crew and the foundation crew are the same people, but sometimes not (especially if there are a lot of trees on the lot). Houses are generally built on a foundation that is either a basement, a crawl space or a slab. The site-preparation crew typically arrives on the site with a backhoe and/or bulldozer. The crew's job is to clear the site of any trees, rocks and debris, level the site if necessary and dig as necessary for the foundation being built.

The example house shown here is built on a crawl space. For a crawl space, the site preparation crew digs a set of trenches and holes. Concrete is poured into these trenches and holes and will act as the interface between the foundation wall and the ground. Once the concrete is poured, the house looks like this:

(In these pictures, the bricks for the crawl space have already been moved into position while the concrete cures.) The concrete in the trench is generally about 18 to 24 inches wide (45.72 to 60.96 cm) and 18 to 24 inches deep. Once it hardens, it forms a massive concrete "beam" on which the house rests. The width of this concrete beam is controlled by the compressibility of the soil. In light soils, the beam will be wider to try to spread out the load, while in heavy clay soils it can be narrower.

If the site slopes, the concrete beam is stepped, like this:

Concrete takes approximately two weeks to cure to full strength (depending on the weather), so once the concrete is poured nothing will happen for some period of time while curing takes place.

If this house had been built on a basement, the site-prep crew would have dug a square hole about 8 feet deep. If this house had been built on a slab, the site-prep crew would have trenched around the outside approximately 2 feet deep and then completely leveled the area for the pad.

Foundation
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 formwork 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.

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.

Floor
The framing crew is the next group of people on the site. They start by building the floor (unless the house uses a slab foundation, in which case the slab is the floor). The floor framing looks like this:

The floor starts with a sill-plate made of pressure-treated lumber in direct contact with the bricks of the crawl space wall. One interesting thing to note is that this house literally "sits" on the foundation -- it is not held on or bolted on in any way. Then the floor is constructed on the sill with 2x10 lumber:

You may have noticed the brick posts when you saw the picture of the foundation. They hold a beam that runs down the center of the house. The beam is also built from 2x10 lumber (three pieces thick):

All of the "joists" (as the 2x10s in the floor are called) meet on this center beam:

(In many houses the meeting of the joists is somewhat better organized!)

This funny little cantilevered section of the frame will eventually hold the fireplace:

Once the floor framing is complete, it is covered with 1/2-inch or 5/8-inch plywood or OSB (oriented strand board).

And the floor is finished!

Walls
The framing crew next starts on the walls. Walls are assembled on the floor...

...and then raised into place. Here's what the first wall looks like once it is up:

You can see that this wall is made of 2x4 lumber and covered on the outside with an OSB sheathing. Using plywood or OSB as the sheathing gives the wall rigidity -- you may have seen diagonal pieces used at the corners of older homes (homes built before plywood was widely available). The plywood does the same thing, but it provides much more strength.

The 2x4s are placed on "16-inch centers," meaning that the center of one 2x4 is 16 inches away from the center of the next. In this wall, two things interrupt the consistent 16-inch pattern:

• Windows
• Special framing that will accommodate internal walls once they are built

The plywood will be cut out of the window openings as construction proceeds. Above the window is a 2x10 header, which is actually two 2x10s with a piece of 1/2-inch-thick plywood sandwiched in between and a 2x4 along the bottom:

A cross section of a header is shown at the right. The reason why the header has plywood in the middle is simply to make the header as wide as the rest of the wall. A 2x4 is really 1-1/2 inches by 3-1/2 inches, and a 2x10 is really 1-1/2 inches by 9-1/2 inches. When you sandwich two 2x10s together they are only 3 inches wide. Adding the piece of plywood in between makes the sandwich 3-1/2 inches wide.

A 2x10 header is a beam. You see these headers over all windows and doors -- they give the wall enough strength over the window or door to support the roof. When a header spans more than 5 feet, you find double full-length studs on either side of the header instead of the single studs seen here.

All of the exterior walls go up following this same basic pattern. In the corners, the top plate on one wall overlaps the top plate of the next, and the walls are nailed together to bind the corner. Then the interior walls go up, fitting into the top plates of the exterior walls as shown above.

This house has a garage and a breezeway connecting the garage to the house.

The walls of the garage are built slightly differently (because the garage will have a slab floor). The walls are bolted directly to the brick foundation walls:

Here's how the house looks right before the roof framing starts.

You can see that the framers have covered the outside walls in pink house wrap.

Roof
This house uses trusses for the roof framing. Trusses are pre-fabricated, triangulated wooden structures used to support the roof. The alternative is to build up the roof's frame with 2x8s and 2x10s. Trusses are quite common these days because they have five big advantages from the builder's standpoint:

• Trusses are incredibly strong.
• Because they are built strictly from shorter lengths of 2x4 lumber, they are generally a lot less expensive than the alternative.
• You can have just about any shape custom-built, and this allows interesting features like cathedral ceilings at low cost.
• You can span a large distance with a truss and the truss transmits all of the weight to the exterior walls. Therefore, none of the interior walls are "load-bearing," so they can go anywhere and are easily moved later.
• Trusses go up quickly!

Trusses come in several standard configurations:

"W" truss

"M" truss

"Scissors" truss

"Gable" truss

Gable trusses are used at the ends of the roof (the outermost trusses on either end). The vertical pieces are 16 inches on center so that siding can be nailed on. Our sample house uses a custom truss in the main part of the house that looks like this:

The left-hand side will provide a cathedral ceiling over the living room. Scissors trusses are used for the front room, and M trusses are used over the garage. Gable trusses are used at the ends of the three rooflines.

The trusses are fist stacked on top of the walls, either by hand or with a crane.

These trusses went up in about four hours. They are on 24-inch centers.

The trusses are tied to the walls with small metal plates.

Once the trusses are up, the roof is covered in plywood or OSB, which gives the roof tremendous rigidity.

There are two small custom roofs in this house: the roof over the porch (see next section) and the roof over the breezeway.

Roof framing without trusses is actually fairly complicated. The angles found in anything but the simplest roof become intricate. A book like Do It Yourself Housebuilding by George Nash goes into roof framing in incredible detail.

Porch
The crew built the porch, starting with the frame.

Then a floor goes on top of the frame.

Here are the trusses of the roof over the porch:

Windows and Doors
Inside the house, things are now beginning to look enclosed:

The next step in enclosure is windows and doors. The windows and doors arrive in one shipment and are unloaded from the truck into a stack:

Plastic stripping is stapled to the inside of all window and door openings, like this:

The windows used here are standard vinyl windows. They are placed in each rough opening and stapled in place on the outside.

The front-most window is the feature window of this house:

Roofing
This house uses standard asphalt shingles for the roof. The first step is to cover the roof with building paper (tar paper):

The shingles then go on very quickly (on this house, in less than a day):

In the following shot you can barely detect the ridge vent that runs along the peak of the breezeway roof. There is a vent like this along the peak of all the roofs.

This vent replaces the triangular "gable-end vents" found in older homes. Ridge vents give better circulation (especially when cathedral ceilings are used) and also prevent bats and squirrels from getting into the attic.

In the following shot, you can see the aluminum flashing that keeps water away from the walls at the points where the shingles touch the walls.

At the edge of the roof, the shingles are cut off with about 2 inches of overhang:

Siding
This house uses standard vinyl siding. The siding is made from thin, flexible sheets of plastic about 2 millimeters thick, pre-colored and bent into shape during manufacturing. The sheets are 12 feet long and about a foot high. You start at the bottom and the sheets interlock into each other as you go up.

Because vinyl expands and contracts due to temperature and sunlight, it fits into deep channels at the corners and around windows and doors. The channels are deep enough that as the siding contracts it remains within the channel. The following shot shows a channel nailed to a corner of the house and a piece of siding fitting into it. It is nailed in place and ready for the next sheet to be interlocked and installed above it:

This is the back wall of the house, showing the scaffolding used to install the siding:

This shot shows roofline detail. The area extending out from the house under the roof is known as the soffit (parallel to the roof). The fascia boards are perpendicular to the roof. The soffit is perforated so that air can flow into the attic and up through the ridge vents to ventilate the attic. In this shot, part of the soffit is in place, while part is awaiting installation. Note that all exposed fascia wood is capped with a sheet of painted aluminum that was bent into shape on the site:

At this point, the house is "dried in," meaning that it is completely protected from rain. Now interior work can begin.

Rough Plumbing
Let's say you want to put a toilet in a house. Two-hundred or 300 years ago this was not an option -- everyone used outhouses. If you visit the governor's mansion in Williamsburg, VA, you will see that in the 1700s even England's high colonial governor used a pair of three-holer outhouses located at the back of the formal garden. Eventually, public water supplies and pressurized well systems allowed people to have indoor plumbing, and this allowed for the addition of indoor toilets. A toilet has to flush somewhere, so sewer systems evolved.

Why can't you run the sewer line from a toilet or a sink out of the side of the house so it spills on the ground? That certainly would be easy and inexpensive, but people learned fairly quickly that human waste spilled on the ground smells bad and leads to incredible disease problems. Septic tanks and sewer systems take care of this. The uniform plumbing code lists hundreds of rules for septic-tank installation. These rules ensure that tanks work properly over many years.

Once you have a septic tank in place, you can add sewer lines from the sink or toilet to the septic tank. Say you tried this approach:

The problem with this approach is that as the septic tank fills up with stuff, it produces a rather malodorous cloud of fumes. These fumes float from the septic tank up the sewer line to the sink and into the bathroom. Therefore, plumbing codes require a "P-trap" at every drain opening, as shown here:

You may have wondered why you find these funny loops of pipe under every sink in your house. The idea is that water gets trapped in the "P." This water blocks the fumes from the septic tank and keeps them from entering the bathroom. Unfortunately, a P-trap alone does not solve the problem because it turns out that the fumes in a septic tank are under pressure. The fumes simply bubble through the water in the trap and cause the same problem. Therefore, there is the concept of a vent pipe, which allows the pressure to escape, as shown here:

You may have wondered why houses have pipes sticking up out of the roof. They are vent pipes to relieve the pressure so that P-traps can do their jobs. It turns out that vents also break vacuums so water flows down the pipes faster.

Besides covering P-traps and vent pipes, the uniform plumbing code specifies all sorts of other things:

• The required diameters for pipes
• The allowed materials for pipes
• The types of joints you can use
• The necessary supports for pipes
• The angle at which pipes must fall
• The longest distance for lateral pipes
• And on and on and on through hundreds of pages

When plumbers follow all the rules, they are able to create extremely reliable and safe plumbing systems. Over time, new rules get added as people realize funny little quirks and nuances. These new rules prevent problems in the future, and each one makes the code a little bigger and better.

This is all a nice way to say that, even though plumbing looks simple in this section, there are many subtleties and nuances dictated by code that plumbers know and neophytes generally do not. (The same holds true for electrical systems, by the way.)

Rough plumbing involves installing all of the water lines, sewer lines and bathtubs. Tubs are normally installed early because:

• One-piece shower-and-tub units are big and often cannot be maneuvered into place later in the construction cycle. They also frequently "change size" -- that is, the size drawn on the plans and the size delivered often differ significantly.
• A full tub is heavy. Therefore, the tub is installed and filled so that the frame can settle quickly. This step prevents cracked walls and tile the first time someone uses the tub.

Typically, rough plumbing involves installing all sewer lines and vents as well as all water supply lines for each fixture. Here's a typical sink fixture:

Here's the fixture for a washer:

The tub is put in place and filled. Note the framing problem being corrected on the left side of the tub because the tub changed size:

Here are the lines for the tub:

In the crawl space, the supply lines all branch off from common pipes running the length of the house:

The sewer lines all join together...

... and then exit out the back of the house, ready for connection to the septic tank:

Rough Electrical
The article How Power Distribution Grids Work goes into extensive detail on how power gets from the power plant to your house. The purpose of the electrical system in a house is to distribute the power safely to all of the different rooms and appliances.

The electrician for this house first placed all of the boxes for electrical outlets, lights and switches:

Then he ran wires from the fuse box to each box and between boxes. Here's what the fuse box looked like once he got done:

Wires were first run through to the boxes. A lot of drilling is necessary, both down into the crawl space and up into the ceiling, as well as through studs to run wires between boxes:

Wires are then pulled through the boxes, clipped and capped:

This shot shows the water line for the refrigerator's ice maker as well as the refrigerator's electrical line. Note the use of metal reinforcing plates at all holes (also visible in the shot of the fuse box above):

Insulation
The purpose of insulation is to lower the heating and cooling costs for the house by limiting heat transfer through the walls and the ceiling. The insulation process starts by installing foam channels in the eaves:

These channels guarantee that air will be able to flow from the soffit vents to the ridge vents. Without these channels, insulation tends to expand into the eaves and block the soffit vents. This house uses standard fiberglass insulation throughout:

Notice that over the insulation is a thin plastic vapor barrier. The idea behind the vapor barrier is to keep moisture that develops inside the house inside. Without the barrier, here is what happens inside the wall in winter: Warm, moist air moves through the drywall and into the insulated wall cavity; at some point inside the cavity it becomes cold enough for the moisture to condense, soaking the insulation. The vapor barrier prevents this process. In older homes, the siding and sheathing were so loose that air easily migrated out before the moisture condensed, but that is no longer the case so the barrier is essential.

Drywall
On the outside, the house now looks complete; but inside, it won't look "like a house" until the drywall goes up. Drywall (also known as "plaster board" and by the trade name "Sheetrock") is a half-inch layer of plaster or gypsum sandwiched between two thick sheets of paper. It is remarkably solid, and also remarkably heavy.

To finish this house, 134 sheets of drywall measuring 4 feet wide by 12 feet tall were delivered to the site and stacked in the living room:

A 4x12 sheet of drywall weighs about 50 pounds (23 kg). So this room has about 6,700 pounds (about 3,000 kg) of drywall stacked in it!

The drywallers put up all of the drywall in a day and taped it the next day:

To "tape" the drywall means to cover all of the cracks and nails with drywall mud (spackling compound) so that the walls are completely smooth. You can see that the cracks and nails are all covered in the pictures above.

Garage Slab
The floor of the garage is a 4-inch-thick concrete slab poured very late in the process. Four inches of gravel were placed on the ground and covered with plastic and reinforcing wire. Around the edges, half-inch thick homosote will allow the slab to contract and expand with temperature changes. Once the prep work was finished, a concrete truck came and poured about 7 cubic yards of concrete to create the slab.

Finishing Up
At this point, the steps that remain are all "finishing steps" and are things you can see in your own home by opening doors and removing cover plates. The finishing steps include:

• Putting down underlayment - The plywood put down during the initial framing is called subflooring. Generally it is covered by tar paper or 4-mil-thick plastic as a vapor barrier and then by 5/8-inch particle board. Alternatively, it may be covered with rigid concrete wonder board for tile installations.

• HVAC - Once the underlayment is down, the HVAC contractor will install the heating and air conditioning unit (aka the furnace), the vents and all duct work. If this had been a two-story home or a home built on a slab, the HVAC people might have had to install some ductwork in the walls or between floors. However, in a single-story home on a crawl space or basement, the furnace and all ducting can go in very late in the process because everything goes under the house.

• Finish electrical - The electrician will return and install all light fixtures, wall outlets, switches and cover plates.

• Kitchen and bathroom cabinets and counters - The cabinet company will install kitchen and bathroom cabinets. They are simply aligned on the wall and screwed into the wall studs. Countertops are screwed on top of the cabinets.

• Finish plumbing - Once the cabinets are in, the plumber will return and install sinks, toilets and faucets. The plumber will also install the water heater if it was not installed during rough plumbing.

• Installation of well and septic system or hook-up to city water and sewer - Depending on where the house is located, it will either have a private well and septic system or it will hook up to municipal water and sewer lines. If a private well and septic tank are needed, the contractor will bring in a well-drilling subcontractor and a septic-tank subcontractor. Otherwise, the contractor will call someone (either the municipality or a subcontractor) to extend the municipal lines to the house. The plumber will hook up the water and sewer lines.

• Wall trim - Once the cabinets are in, the interior doors are installed and the molding around the doors, windows and baseboards goes in.

• Paint - Once the molding is on, it is time to paint and wallpaper the interior of the house.

• Carpeting and tile - Once the paint is done, carpet and tile goes down.

• The final punch list - At this point, the builder inspects the house, noting any problems. All problems are tabulated on a punch list. The different contractors return to fix all of the problems.

This article has shown you the basic steps involved in house construction. Obviously there are a lot of details that we have omitted here, as well as a huge number of rules embodied in the building codes. If you want to learn more, I would recommend the excellent book titled Do It Yourself Housebuilding, by George Nash.

The next time you walk into a house or start poking around under it, I hope you do so with a much better understanding and appreciation for everything that goes into making it work!

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More Great Links!

• Open Directory: Links on home designs
• Open Directory: Links for do-it-yourself builders
• HomeToolz - for do-it-yourself projects
• Sound Home Resource Center - a source for information on home construction and other topics
• HouseNet - advice, projects and tips
• Today's Homeowner: Expert Advice On Improving Your Home
• Home Building Manual
• E-Plans - a resource for house building
• B4UBuild
• COOLhouseplans.com
• House Plans and Home Designs by Rick Garner