Building Materials
This section describe the common building materials and gives an overview of how they are used. The common building materials are wood, masonry/stone/mud, steel and straw, and each has its advantages and disadvantages. This discussion is about structure; for a summary of the environmental impact of various materials, see the materials section.
Wood
Wood is a renewable resource that is used as a building material in almost all climates. In the US, almost all small buildings are made of wood. Even in many masonry buildings wood is often used for the floors and ceiling. In addition to being fairly common, it is quite strong, has medium insulation ability and fairly easy to work with. The downside is that its subject to rot, insects and fire, and that it has a strong tendency to move seasonally (swelling, warping, bowing twisting).
Lumber comes in various size, various grades and from various species. Most construction lumber is softwood (douglas fir and hemlock on the west coast, and pine or spruce in the east) and the common building grade is called "standard or better", while the lower grade is usually called "stud grade" and is generally limited to 2x4s. Standard and better lumber can have knots, but the are generally tight, meaning they are not likely to pop out: the key being that they don't affect the structural integrity of the board. Standard and Better can also have voids on the edges, but only a limited amount of them. Stud grade lumber can have large voids and many more knots and so its use is generally limited to interior walls, sheds etc.
The most common size used is the "two by" commonly written as 2x, and the common sizes are 2x4, 2x6, 2x8, 2x10 and 2x12. When used in walls they are called studs, when used in floors they're called joists and when used in a roof they're called rafters. The sizes are a bit misleading because if you measure, for example a 2x4, you'll find that its approximately 1-1/2" x 3-1/2". Originally they were actually 2"x4", although the surface wasn't planed smooth (and hence referred to as "rough sawn"). Because wood shrinks significantly when it dries, a board cut to 2"x4" from a freshly felled tree will shrink at least as much as 1/8" per inch. When you buy larger width board, particularly 2x10s and 2x12, but even 2x6, don't be surprised if the width is more then 1/2" less than the specified size: a 2x10 for example is equally likely to be 9-1/4" than it is to be 9-1/2".
Larger sizes are also used, for example 4x is common (it's 3-1/2" wide) sometimes as posts (used vertically) and sometimes as larger beams (used horizontally). Sometimes 6x (its 5-12" wide) are also used. When a size larger than that is called for, most builders use engineered lumber, although natural builders will sometimes use whole logs.
Whole logs and cordwood (short sections of log) are also sometimes used for building (more below on this).
Because so much of our old-growth forests have been liquidated and larger trees are economically more expensive to produce than small ones1, there is price premium for larger lumber, so for example even though a 2x12 is three times bigger than a 2x4, it will cost more than three times as much because has to come from a larger tree. Likewise, a 6x8 will likely cost more three 4x4 even though its the same amount of wood.
Building built of 2x lumber are referred to as stick frame, while buildings build of larger dimension wood are referred to as post-and-beam, or often as timber frame, although the term timber frame usually also implies using traditional joinery and that the beams will be left exposed in the finished building.
Each of the basic building methods are discussed below.
Stick frame - This is the most common construction method used in the U.S. In stick frame construction, the majority of the structural pieces are small and so the building is made up of very many of these small pieces (ie sticks) placed fairly close together which combine to achieve the overall strength. These pieces are usually wood, but sometimes are steel. As the pieces get larger and the distance between them greater, it is no longer called stick framing, and instead becomes post and beam or timber framing system. Of course, various combinations of the two are also possible.
Stick frame construction is popular because the most common technique, platform framing, allows the use of many standardized pieces of wood and can be done relatively easily and rapidly. In platform framing, each floor is built separately, one on top of another, and then the roof is added on the top. The downside of stick framing is that the framing pieces only serve to hold the building up: they don't insulate, keep out rain or resist wind, so other materials must be added to perform those functions. Typically plywood or a comparable sheet good (like OSB) is used to provide stiffness against wind loads, and some insulating material is filled in-between the 2x4s to give heat resistance, and then entire exterior is covered in a water resistant siding.
(picture: typical stick frame wall and roof section)
Traditionally, stick frame houses were made out of 2x4s, but newer houses are often made with 2x6's instead using a modified framing method often called "advanced framing" or "optimum value engineering", which is an attempt to use less lumber and allow for more insulation. Taking this concept even further, some houses are built with double 2x4 walls (in various configurations), allowing for even more insulation to be added, but at the cost of using more wood (and labor).
Recently, some builders have been substituting light gauge steel framing for 2x4 in non-load bearing walls. This can be done by continuing to use wood stick framing for the load bearing parts or going to a post and beam structure. Steel has the advantage of being recyclable (as well as typically having a high recycled content), but comes with the problem of "cold bridging" because steel is a good conductor of heat. See more under "steel", below.
Post and Beam and Timber Frame - A post and beam structure differs from a standard "stick frame" structure in that rather than using smaller size lumber like 2x4s or 2x6s spaced 16 or 24 inches apart, it uses more sizable lumber like 4x4s and 6x6s or larger spaced 4 feet or more apart. Structurally, timber framing is a post and beam method, and although traditional timber framing didn't use nails, there is not a significant difference between the two. The biggest difference in in implication: in timber framing the large dimensional pieces of lumber are usually left exposed, while a in a post and beam they may or may not be exposed.
Timber frame or post and beam structures are very commonly used in strawbale, cob and light clay buildings. Timber framing is also combined with SIPs, although this is obviously overkill since building with SIPs generally requires few structural members.
There are various claims out there that timber framing uses less wood than stick framing, but it does not appear that this happens very often in practice. Likewise, it does not appear that there is any significant difference between post and beam and stick frame construction in terms of wood use. When the wall in-fill is straw, cob or equivalent, at least the plywood wall sheathing is eliminated--a significant savings in wood.
Although there has been a move away from larger beams (or at least toward engineered wood) in order to reduce pressure to cut old growth forests, this conventional wisdom may be less relevant when applied to FSC certified beams. By buying larger beams, you're encouraging the woodlot owners to cut some trees on a longer rotation, promoting a forest that is more ecological robust.
Engineered Wood - Larger diameter piece of wood are more frequently being replaced by "engineered" wood2, in which a larger piece of wood is made from smaller ones glued together. While this may sound inferior, in fact engineered wood is sometimes stronger for the same size lumber piece and is almost always more dimensionally stable. The additional strength comes from the fact that the wood fibers are organized the way they are needed, often either all parallel (eg LVL, PSL), or in many directions (OSB, plywood). Both organizations create stable end-products because either the grain is all in the same direction (hence no warping) or the layer are bonded to each other so they resist each others desire to warp.
The disadvantage of engineered wood is (1) more glue means more chance of exposure to potential toxins (2) if the additional strength results in smaller pieces (ie TGI instead of 2x10) it will burn faster (3) the embodied energy is generally higher (4) some engineered wood (OSB in particular, soaks up water at a much faster rate than lumber. This is because the "strands" of wood in OSB are often partial end grain, which exposes the capillary channel in the wood. Plywood does not have this issue. (5) the glue is generally not vapor permeable, so while a wall sheathed in 1x8 shiplap can dry to the exterior much better than one sheathed in OSB.
Bamboo - While technically a grass, bamboo is made of the same building blocks as wood, ie cellulose, and has many properties of wood. In some places in the world bamboo is a common construction material, and a handful of green builders have adopted is a building materials of choice, however there is no standard code3 approval route for using structural bamboo. Bamboo grows much faster than trees4, but the largest pieces are still quite small and are hollow, so building techniques for bamboo are inherently different than for wood. The two most intriguing are the use of bamboo trusses, and engineered bamboo lumber.
Logs/Cordwood - These building methods make walls that are essentially solid wood (while stick frame and post and beam walls have a cavity that is filled with insulation). While they can be crafted with very little machinery, they use more material and have a significantly lower R-value than any type of frame wall in-filled with insulation.
Masonry/Stone/Clay
These materials are used worldwide, and are probably the single most common building materials. In almost every case, building with these materials is a matter of combining the "strong" materials (stone and sand) with some kind of binder (clay or cement), and a stiffener (straw or rebar). The one exception to this is dry stack stone, where large stones are fit into a wall course by course with no binder. These materials are found essentially everywhere, and can be reused (the material is long lasting, although reusing it can be very difficult), and other than systems containing Portland cement5, they have fairly low embodied energy. They also don't rot, are immune to bugs, and don't burn. Anything using clay as a binder, however will disintegrate in the rain. The main drawbacks of masonry are (1) it wicks water, so you need additional moisture protection (2) its brittle, so the only "bending" strength is in the stiffener which means that in earthquake zones the limitation on the use is the (3) its insulation value is generally terrible.
Concrete is a mixture of stone, sand and Portland cement. Some of the Portland cement can be replaced by various pozzolans, which are cement like material, including blast furnace slag and some coal fly ash. Concrete comes in a wide variety of mixes which specify the ratio of Portland cement to sand to stone as well as the types of sand, the type and sizes of stone and various admixtures (other ingredients that change the properties of the concrete). A concrete batch plant might have hundreds of mixtures in its repertoire. A standard mix is called "five sack" which refers to the number of bags of Portland cement in the mix, in this case five, which is about 500 pounds (out of about 3200) total per cubic yard, which is the unit you buy concrete by. Concrete has a very high compressive strength, but a very low bending strength and a low strength in tension (pulling), and hence it is usually reinforced with steel. Concrete does not achieve its full strength overnight--in fact although it hardens in a couple of hours, its not close to its fully strength for weeks, so the strength of concrete is specified at a time many days out--28 days being the standard, but the concrete will continue to get stronger past then--but for typical mixtures not enough to make a difference. The typical compressive strength of a five-sack concrete mix is 3000psi--very strong indeed!
Concrete is usually reinforced with steel, because steel is extremely strong, but steel also rusts, so failure of the reinforcing is often a major cause of failure of a concrete structure. For light-duty reinforcing, you can get a concrete mix with fiberglass fibers in it, which although not as strong as steel reinforcing, results in less cracking. Other reinforcing materials have been suggested but none are readily available.
Concrete is by far the most common material used for foundations, although in terms of total use, commercial buildings have much more concrete in them. Although a concrete structure with rebar in it will resist breaking due to bending forces, it will still crack when subject to those forces, and those cracks are subject to leaking. Concrete is a lousy insulator: only about R1 per foot. However it does have significant thermal mass (see R-value discussion). The one exception to this is Autoclaved Aerated Concrete (AAC), which are concrete panels made in a factory using a special mix and then cooked in an autoclave, and is about 1/3 to 1/6 the weight of regular concrete; but it also has only 1/3 to 1/10 the compressive strength. Its R-value6 is between .8 and 1.25/inch.
Concrete block/brick - Concrete block is a hollow block made of concrete, sometimes also filled with concrete. Brick is made with sand and clay, fired at high temperature to make the clay bind to itself chemically so that is no longer disintegrates in water. Brick also typically has holes in it. In both cases the holes create channels where rebar can be placed. If the air gaps in the blocks remain, the R-value of the resulting wall is better than a concrete wall, but still quite bad. Block and brick walls are two of the most common building methods in the world, and unfortunately also the cause of the most earthquake deaths due to lack of proper reinforcement.
Adobe/Cob - these are mixtures of sand, straw and clay, where the amount of sand and clay is relatively high compared to the amount of straw, so that the material generally still has enough compressive strength to hold up a roof or even a second floor. Because clay absorbs water readily, an adobe or cob building subject to regular rain will disintegrate. If there is concern that it can't be kept dry, some builders replace some of the clay with Portland cement --enough so that the clay is protected from the rain. Both of these systems produce a wall which is similar to lightweight concrete, although since they're denser than AAC, they have a lower R-value, and hence are not very good insulators. They do have high mass, so will exhibit the thermal flywheel behavior (see the discussion in R-values for an explanation), but are otherwise not very good insulators. The lure of these materials is that they are readily available, low-cost, low embodied energy, non-toxic and decompose easily.
Adobe is typically constructed in blocks, dried in the sun, creating a home that is usually fairly conventional looking, while cob is generally applied by hand until the wall is tall enough, often built into whimsical curving structures. In each case, sand provides compressive strength, straw provides the "breaking" strength, and clay (possibly with some Portland cement added) acts as the glue to hold the whole mess together.
Rammed Earth - this is essentially concrete made with a clay binder instead of Portland cement, although it often has some Portland cement in it as well. The R-value is similar to concrete, although somewhat better since it is less dense. Rammed earth walls are generally structural (ie they need no help to hold up the roof), but have only approximately 1/4 the compressive strength of concrete. Like cob and adobe, the appeal is using low-embodied energy local materials, the possibility of a very low cost building. However, unless you do it yourself, Rammed earth is labor intensive, so it can be more expensive than conventional building methods.
Rammed earth is built in forms, and installed in "lifts". In each lift, a layer of wet material (typically around a foot thick) is put in the forms, and then compressed by "ramming", in a way similar to a soil compactor until its more like 6" deep, then the next layer is put in. As a result, rammed earth walls tend to look like sedimentary rock, only the layers are wavy.
Steel
There are three main varieties of steel buildings: heavy gauge, light gauge and pole. Heavy gauge ("I beams etc) is used on large commercial buildings and is structural. Light gauge, particularly steel C channel "studs" are somewhat drop in replacements for wood 2x4s, but they are generally not structural, so they can only be used on non-load bearing walls or in conjunction with other load bearing materials. Poles are essentially pipes, or sometimes steel square channel, typically cut and welded on site and is used largely for agricultural buildings and simple storage sheds.
On wood framed residential buildings, steel is mostly used for connectors: bolts, structural screws, brackets and of course nails.
These days steel commonly has a high recycled content and is recyclable. Of all building materials it has the lowest R-value, which although not actually zero, in the context of buildings is essentially zero. Steel also conducts sound very well, so extra sound insulation may also be needed. Although it does not burn, it will warp and potentially melt in a fire.
Straw
Straw is the largely inedible stalk of a grass, usually the residue left over after harvesting either wheat or rice. Like wood and bamboo it is largely made of cellulose, but it not as stiff as either. Straw is used to supply bending strength to adobe and cob, combined with a small amount of clay to form a filler material called straw-clay, or by itself in compressed bales. Straw can also be formed into panels by compressing it with heat; the resulting panels are similar to MDF, but have better bending strength.
Like bamboo, its a rapidly renewable resource, and in many cases is currently considered a waste product from industrial farming. It rots quite a bit faster than wood or bamboo, but may not be susceptible to insect damage since neither termites nor carpenter ants typically attack it. Although lose straw burns very rapidly, compressed straw burns more like wood due to the much lower availability of air.
In general straw is not structural, although you can compress straw bales enough for them to be structural in situations where the load is not that great.
Notes
1: At least this is what the timber industry claims.
2: The web already has places that describe engineered wood fairly well, for example, wikipedia: http://en.wikipedia.org/wiki/Engineered_wood
3: apparently the ICBO 2000 standard does contain some language that would allow its use, but even if that makes it into your local jurisdictions code, you'll still have to do some convincing of the local code officials.
4: this is oft-repeated, but with little to no data. Whether plant really produces significantly more useable mass per acre than trees is still not clear.
5: The energy used to make Portland cement could be reduced by 50% if all manufactures upgraded to the newer style plant (dry kiln versus wet). Further, in most cases a good deal of the Portland cement can be replaced by pozzolans, although the most common one, coal fly ash, is not exactly sustainable.
6: the data is mostly taken from cement.org.