46 Construction Materials

This section is designed to give the potential construction worker some knowledge of commonly used construction materials. The trades person should have a reasonable understanding of the material he uses.

Wood

Wood Structure

Just beneath the bark is a spongy layer called the cambium. This is the portion of the tree which builds new wood cells. Beneath the cambium layer is a section called sapwood. It consists of active large wood cells, which convey the sap from the root of the tree to the leaves. The rest of the trunk underneath the sapwood is called heartwood. Heartwood is formed as the tree grows and the sapwood cells mature and become inactive. Heartwood, therefore, is more desirable than sapwood for framing lumber since it is less subject to shrink and warping as its cells are inactive and have lost their former moisture. In the center of the heartwood is the pith, which represents the growth of the tree in it’s first-year.

The series of concentric rings surrounding the pith are formed by the growth of the tree, one ring being added on the outside each year; hence the name annual rings. These annual rings, as shown in the enlarged module Fig. 2A – 2, are composed of fibers or long tubes running parallel to the trunk of the tree.

The annual rings are crossed by fibers or cells that run from the bark of the tree to its center, conveying nourishment from the outer cambium layer to the inner part of the tree. These cells are called medullary rays. When the tubes or cells contain moisture, lumber is said to be green. Lumber is dry, when the cells have collapsed and have been drained of moisture.

Grain and density are important in determining the strength. Wood in which the annual growth rings are narrow is described as close grained. When the annual rings are wider apart the wood is described as coarse gained. In softwoods, close-grained stock is stronger than coarse-grained.

Each annual ring consists of light spring wood and dense In addition to the factor of grain, this density, or proportion o the wood’s strength. The higher the density, the stronger the wood.

Classification of Woods

Lumber may be classified as either hardwood or as softwood. Softwood comes from evergreen or needle bearing trees. These are called conifers cone-bearing. Hardwood comes from broad-leaved trees called deciduous that shed their leaves each fall. This classification is somewhat confusing because some of the softwood species produce stronger materials than some of the hardwoods.

Some woods common to the prairies are:

Softwoods:

1. Spruce
2. Pime
3. Tamarack
4. Fir

Hardwoods:

1. Birch
2. Ash
3. Elm
4. Maple
5. Poplar

Methods of Sawing Lumber

The method of sawing a log has a direct bearing on its durability, quality, and ability to resist wear and to hold its shape.

Plain-sawed lumber, also called slash or flat-sawed, is cut from the log as shown in Fig. 2A -3. The log is first squared by sawing boards off the outside, leaving a rectangular module A-B-C-D, which is then cut up as shown by the vertical lines. This is a common method of sawing framing lumber. Board E and F are shown enlarged. Note that board E shows much closer annual rings as it has been cut from the heartwood portion of the log.

Moisture Content

While a tree is living, both the cells and cell walls are filled with water. As soon as the tree is cut, the water within the cells begins to evaporate. This process continues until practically all of the “free water” has left the wood. When this state is reached, the wood is said to be at the fiber-saturation point; that is, what water contained is mainly in the fiber walls.

There is no change in size during this preliminary drying process and, therefore no shrinkage during the evaporation of the “free water”. Shrinkage begins when water begins to leave the cell walls themselves. These contract, becoming harder and dense causing a general reduction in size of the piece of wood. Kiln drying in great ovens is the modern, scientific system of preconditioning lumber. Under carefully controlled conditions, the moisture content of light framing materials is reduced to an average of 19%.

If a specimen is placed in an oven (kiln) which is maintained at 212ºF (temperature of boiling water) the water will evaporate and the specimen will continue to lose weight for a time. Finally, a point is reached at which the weight remains constant. This is a way of saying that all of the water in the cells and cell walls has been driven off. The piece is then said to be “kiln dry” (K.D.).

If it is now taken out of the oven and allowed to remain in the open air, it gradually takes on weight because of the absorption of moisture from the air. Eventually a point is reached at which the weight of the wood is somewhat constant, expanding and contracting with changing moisture content in the air. At this point wood is described as being “air dry” (A.D.) Ever see a door that fits one season and is tight at other seasons? This is because the door is expanding and contracting due to the change in moisture content.

The amount of water contained by wood in the green condition varies. As a general average, at the fiber-saturation point most woods contain from 30% water as compared with the oven-dry weight of the wood. When air dry, woods contain from 9% to12% depending on the climate.

You can check moisture content by drying and then weighing a sample or the more common method today is with a moisture meter.

Defects and Blemishes

The Canadian Wood Council refers to a defect as “any irregularity occurring in or on wood that may lower some of its strength, durability, or utility values”. A blemish is defined as “anything marring the appearance of the wood, but not classified as a defect”. There are specified sizes and characteristics of defects in lumber that should not be overlooked.

A knot in lumber is caused by the growth of a branch, the inner end of which is embedded in the main stem of the tree. The location of a knot in a board may seriously affect the structural strength of the board. If the knot is solid and small, it may not do any particular harm. The knot itself is as strong as the rest of the wood, but the cross grain which develops around the knot weakens the lumber. When the lumber is being dried, checks and cracks often develop in this irregular grain. Knots can be tight or loose. Loose knots, which are formed when the wood grows around a dead branch, are apt to fall out when the log is cut into lumber. Round knots are produced when the limb is crosscut, and spike knots when the limb is sawed lengthwise.

A shake is the separation of the wood between the annual rings lengthwise of the board (Fig. 2A-4). This defect greatly weakens the board when it is subjected to a load.

A check is a lengthwise separation along the grain and across the annual rings. They are commonly seen on the ends of lumber and are caused by too rapid and uneven drying. They not only weaken the lumber but make it difficult to nail the board, as the nailing may cause additional splitting.

Warp is a bending of the lumber from a flat plane. As a board dries, it shrinks more along the long annual rings than along the short ones. Then the board will tend to curl or cup. The cupped face is on the same side as the longest annual rings. This is why green lumber should not be used for framing.

Dry rot in lumber is caused by a fungus. The term dry rot is misleading, as it occurs only in the presence of moisture where the free circulation of air is prevented. Wet or green lumber used in a building, and so enclosed as to partially cut off air circulation, is very likely to be affected by dry rot. The fungi are sometimes found in dry wood but can draw their needed moisture long distances. Wood in the advanced stage of dry rot is shrunken, discolored, brittle, and powdery.

Some fungi do not rot the wood but only cause stain or mold which affects the wood’s appearance. Wood which is blue-stained can be painted; or if the stain is not too deep, it can be planed to restore its original appearance.

Care of Lumber

The care that lumber is given after being delivered to the building site is very important. Green or partially dry lumber, when not properly piled, will twist and warp in drying and will retain this twisted and warped shape. The lumber should be protected from the rain and should be piled in such a manner that air can freely circulate through the pile. If the lumber is piled tightly together and is allowed to get wet, an infection may start and continue in the lumber after it has been placed in the building.

Lumber that has been used for concrete forms, scaffolds, and staging should properly cleaned and inspected for defects caused by rough handling before it is used for permanent parts of the building.

Surfacing of Framing Lumber

Framing lumber is classified as rough lumber and dressed lumber. Rough lumber is lumber as it comes from the saw in the mill.

Dressed lumber is lumber that has surfaced by running it through the planer. It may be surfaced on one side (S1S), sides (S2S), or a combination of sides and edges (S4S). Most framing lumber is (S4S) that it has been planed at the mill on all sides of the material.

Standard Sizes of Framing Lumber

Dimensions of milled lumber are always smaller than the full size rough lumber, since the wood must be seasoned and smoothed after it is measured and cut. For instance, a board nominally 1″ thick is actually 3/4″, while a 2″ x 4″ measures about 1/2″ less each way, or 1 1/2″ x 3 1/2″. Boards nominally 8″ or more wide lose as much as 3/4″ in seasoning and milling. Metric sizes for a 2″ x 4″ is 38 x 89 mm.

Kinds of Lumber Used for Framing

There are four main varieties of wood generally used for framing lumber in western Canada: Spruce, Pine, Fir, and Hemlock.

The Firs – Douglas, Balsam, Alpine and Amabilis are found largely in British Columbia and the U.S. states of Oregon and Washington. They are strong and have straight, stringy, and tough grain. They are used where considerable strength is needed in a framework, as well as for ordinary structural members. Douglas Fir is costly. Often using a larger size of common lumber (Spruce or Pine) is cost effective.

Spruce may be classified as white, red, black, and western. The western spruce is strong and light in weight with a straight, even grain. Red spruce is nearly white with a reddish tinge. It is light in weight and has an even grain with a fine texture. White spruce has a coarser grain than the others. Spruce is frequently used for scaffolds, studs, and lath.

Pines are classified as Jack, Lodgepole, Northern Pine. All of these species have the same strength characteristics as the firs and spruces.

Grades of Lumber Used for Framing

The primary component of wood-frame construction is dimension lumber. It forms the structural shell which encloses and divides spaces and to which finishes are applied. In addition to lumber, other wood products are used in the construction of the shell and in the interior and exterior finishes. All of these products are intended for specific uses and are manufactured to meet certain standards.

The lumber that is commonly used for framing is 1-1/2 inches by 3-1/2 inches thick, (38mm x 89mm) dimension lumber. There are also decking, boards and finish lumber groupings. Table 6 presents the grades, common grade mixes, principal uses and grade categories for the various sizes of dimension lumber.

Dimension lumber is judged primarily on the basis of strength. There are two grade groups, common dimension and structural dimension. Common dimension lumber is 2″ or more in thickness and from 2″ to 12″ wide. The lengths are in multiples of 2 feet from 6′ to 20′ long. The structural dimension lumber is always 4″ or more wide. Any lumber more than 5″ in width and thickness is called timber.

The easiest way to learn about lumber grading is to study the span tables in the Building Code of Canada and compare the strengths and usage of different grades and species of construction wood.

Grade Marks

Lumber for construction uses is grade stamped in Canada with identifying markings to show that it conforms to the National Lumber Grades Authority (NLGA) grading rules for Canadian lumber. The grading and grade marking of lumber must also conform to C.S.A. Standard 0141, “Softwood Lumber.” Grade stamps usually show the name or symbol (or both) of the grading agency, the species combination designation, the grade, the moisture content at the time of manufacture and the mill number.

“S-GRN” in the grade mark signifies that the lumber was surfaced at a moisture content higher than 19 per cent to a size which would allow for natural shrinkage during seasoning. “S-DRY” in the mark indicates the lumber was surfaced at a moisture content not exceeding 19 per cent while “MC 15” indicates a moisture content not exceeding 15 per cent.

Lumber Grades

Each piece of lumber is examined and assigned a grade depending on its physical characteristics. In addition to visually-graded lumber, machine-stress-rated (MSR) lumber is available in Canada. MSR lumber is identified in grade stamps by its structural properties and, for wood-frame construction purposes, is independent of species.

Many softwood lumber species in Canada are harvested, manufactured and marketed together. Those having similar properties so that they can be used together easily are combined into a single species combination and marketed under a group designation.

The top grade of most species is Select Structural, which is used only where high strength, stiffness and good appearance are all required. Lumber marked No. 1 grade often contains some percent of Select Structural material, but permitted knots are slightly larger.

No. 2 lumber may have larger knots than No. 1, but they are tight knots. The grade is excellent for floor and roof framing members. No. 3 lumber has still more and larger defects; it is adequate for sills, plates or on non-load bearing studs.

The lowest grade is Economy, is used for non-structural purposes such as stakes and temporary bracing.

Four other grades also appear on the lumber stamps of 2 X 4 (38 x 89 mm) lumber. Stud grade is stiff, straight lumber suitable for vertical wall members. Construction grade falls somewhere between No. 1 and No. 2.

Standard and Utility are still lower grades, but somewhat better than economy.

Minimum grades for various uses of lumber in wood-frame construction, such as stud wall framing, plank frame construction, posts and beams, sheathing and subflooring are set forth in the National Building Code of Canada. Tables giving maximum allowable spans for visually-graded lumber and for MSR lumber when used as joists and rafters are available from the Canadian Wood Council. For this module, refer to Tables 11 through 19 in Canadian Wood Frame House Construction which presents the maximum allowable spans for ceiling, floor and roof joists as well as roof rafters using Nos. 1, 2 and 3 grades of lumber.

Metric sizes of softwood lumber are the same as those in use in Canada under the imperial system of measurement, but their sizes are expressed in millimeters of actual thickness and width after surfacing. The concept of “nominal size” is not used. Table 10 in Canadian Wood Frame House Construction relates the current metric dimensions to the imperial equivalents actual and nominal.

Composite Structural Members

Dimension lumber and other wood products are often combined in the manufacture of composite structural members by the use of glue or mechanical fasteners or both. The most common example is the engineered pitched roof truss. Less common, but of increasing application, is the parallel-chord truss with metal, wood, plywood or wafer board webs.

All these products provide greater flexibility in design by virtue of their larger spans and their capability to house services – HVAC pipes, plumbing etc. In addition, when used for the roof structure, they can accommodate higher insulation levels.

Sheet or Panel Products

In addition to dimension lumber, other wood products in the form of sheets and panels are used in wood-frame construction. Plywood and wafer board, for example, are used to add stiffness to the structural components of the roof, wall and floor, apart from forming a uniform surface for the application of other materials. Fibreboard, particle board and hardboard are also used in interior and exterior finishing.

Plywood

Plywood is made of thin layers or plies of wood glued together with the grain of each ply running in opposite directions. Common thicknesses range from 1/4 to 3/4 fl. (6.0 to 19mm). Like dimension lumber, plywood is graded for particular uses. Douglas fir plywood (DFP) and Canadian softwood plywood (CSP) are the two most common softwood plywood produced. All sheathing grade plywood, wafer board and oriented strand board is made with an exterior grade glue. Pre-oiled or overlaid plywood is used for concrete formwork.

Wafer Board

Wafer board has the same use as plywood: subflooring, roof sheathing and wall sheathing. Wafer board is made of wide wood shavings and glued together.

OSB

Oriented strand board (OSB) is similar in appearance to wafer board, but uses strands instead of wafers. The strands are mechanically oriented in layers, with the outer layers running parallel to the long dimension of the sheet, and the inner layers having a random or cross alignment. OSB is primarily used for roof or wall sheathing, sub-flooring, siding and the webs of wood I-joists bonded together under pressure. It is available both in a plain and an asphalt-impregnated form. The impregnated version is used primarily for wall sheathing.

Particle Board

Particle board is generally used in underlay or interior finishing such as shelving and other cabinetry. Often covered with a plastic laminate or other protective and decorative material, it is used to manufacture cabinet doors. The same material is often used as the base for kitchen countertops.

Hardboard

Hardboard is made of wood fibre, like fiberboard, but is denser and harder. It is present in many furniture and cabinetry products. Hardboard siding with a prefinished colour is an alternative to wood, vinyl or aluminum siding. Large panels with prefinished and textured surfaces are often used to create special effects inside or outside the building.

Nails and Common Fasteners

All construction projects are held together with fasteners. This unit will introduce you to the most commonly used fasteners. You will develop:

Knowledge of different fasteners and their applications in the construction industry.

Awareness of the impact of new technologies in the construction industries.

Commonly Used Fasteners

1. Common B. Wire C. Ardox D. Finish
2. Roofing F. Masonry (hardened) G. Blued
3. Siding I. Air Gun Staple J. Air Gun T-Nail

Nails

Nails and spikes come in various lengths, diameters, heads, points and shank styles. They are usually specified by the length and diameter of wire, although in the United

States, they are commonly specified in sizes by the “penny weight” system. A “penny” size could refer to up to four nails of different lengths and diameters.

Nails are available in lengths from 3/8” (9.5 mm) to 6” (153 mm), and Spikes, from 4” (102 mm) to 14” (350 mm). Their diameters and patterns will vary according to each particular use.

Nails that are designed for a special purpose are more costly than the same size of common nail but are usually well suited to their specific purpose.

Different Nail Types

Nails sizes are determined according to their length and gauge. Common nails and spikes have a heavy gauge. Wire or box nails and finish nails are of a lesser gauge.

They also come in many heads and points. Generally work could involve the use of flat heads, and double heads nails for construction, brad heads for concealment type jobs, and specialty heads for roofing (larger flat heads), drywall, siding etc..

Ardox Nails

Ardox nails are heavy gauge nails with a four-sided twist that increases holding power. These are also available with phosphate coating. Ardox are stronger nails that hold better, but are more expensive. They are commonly used in renovations where they are easier to drive into old, dry fir studs.

Common Nails

Common nails have a medium-sized head, and are smooth steel. Sometimes called “brights” because of their shininess, they have good holding power but may split the wood due to their thicker shank.

Wire Nails

Wire nails are usually in the form of a “coated” nail (phosphate coating). This grey slender nail with the chalky finish has excellent holding power but may be harder to drive into hard woods because they are somewhat softer and have a tendency to bend.

Finish Nails

Finish nails are slender and have small heads. The head is sunk below the surface of the wood by its own width and the nail hole can be filled for a better finish.

Special Nails

Corrosion may reduce the longevity of a nail or cause staining of a finished wood surface. Corrosion-resistant nails are used for wet and exterior applications. Coated or plated steel and aluminum, or copper nails were developed for these applications. Nails made of brass, bronze, and stainless steel can be specially ordered.

Double-headed nails are especially effective for use on scaffolding, form work, and other temporary construction work. The lower head acts the same as a normal nail, leaving the upper head exposed. They save tear down time and prevent undue damage to wood.

Nailing Tips

Be sure to use the proper nail for the job.

A couple of sharp taps, not blows, will help to start nails more easily (and save your fingers).

Keep the face of your hammer clean and avoid hitting it against brick or concrete.

Hardwoods are less likely to split if you drill a pilot hole first. The pilot hole should be smaller than the nails’s shank diameter and to a depth of ½ to 1/3 of its length.

To avoid splitting work, blunt nail points when nailing into hardwood near the end of a board.

When driving nails, keep your eye on the nail head. You will drive it straighter and bend less nails.

Screws

Because of their greater holding power, wood screws are more widely used in furniture and cabinet work than are nails or staples. Screws can be removed without damaging the work-piece. Although screws are somewhat costlier, their neatness and ease of application for certain jobs make them a wise option.

Types of Screws

Screws come in many different types, each designed for a specific use.

The four basic parts of a screw are shown in the following diagram.

The “head” of a screw comes in several shapes. Those shown are only the most common variations used in construction.

There are three common types of driver that a worker will likely use.

The slotted screw is used less and less due to its poor performance and inability to be used in a power driver. Phillips screws are commonly drywall screws and are the common head pattern used in the US and Europe. The Robertson head is a Canadian invention and is common here.

Screws also vary in the number, and the shape of threads, diameter, gauge of shank, and finish. There are several finishes available, including chromed, coated, and solid brass.

The four (4) commonly used types of screws used are: wood screws (flat-head), sheet metal screws, drywall or framing screw, and self-threading concrete screw

Wood Screws (Flat – Head)

These have a flat countersunk head and threads extend only part way up to the head. These can range from 9 mm to more than 80 mm in length and are available in different gauges of thickness. The smaller the gauge number, the smaller the diameter of the shank. Screw gauges run from #2 to #14 A deck screw is a #8.

Sheet Metal Screws

These usually have a pan head and are threaded right up to the head. Sheet metal screws are sometimes self-tapping – there is a cutter on the point of the screw to cut start the hole.

Drywall or Framing Screw

They have a fully threaded slender built shaft and a flat bugle head. The shank is black (usually), and very smooth and sharp.

Concrete Screw

• easy to use
• quick
• good holding capacity
• economical
• comes with a variety of head shapes

Screw-Driving Tips

Two thirds of the screw should penetrate into the piece it is screwed into.

Pilot hole should be the same diameter as the root of the screw being used. It can be drilled smaller or even eliminated when fastening into soft woods. In hard woods, you may have to drill a second hole for the unthreaded portion of the screw shank.

Flat head screws require using a counter-sink, to make the head flush with the surface. A counter-bore hole can be used to recess the screw heads so they may be covered with a plug or dowel.

Screw-Driving Tips

The screw driver should fit snugly into the slot, no matter what type of screw is being used and should be the same width as the screw in the case of slotted screws.

A variable speed drill with an appropriate bit and holder can be used for quicker placement and removal of screws. Care must be taken not to over-torque when the screw becomes seated at the desired depth.

Hollow Wall Fasteners

When choosing a fastener, consider the wall it will be inserted into: a hollow wall or a solid wall. Hollow applications require an anchor that passes through the surface and spreads out to anchor itself against the back. Solid walls require a fastener that holds by expanding against the hole it’s inserted into.

Other considerations are; the weight involved, and the kind of load it represents. Shear loads exert a straight down pressure type of load, while a combination load also pulls outward. Shock absorbing loads must hold up under sudden impacts and increased weight. When there are vibration loads involved you must consider a fastener that you can tighten up periodically and perhaps some type shock absorbing washer (thick rubber).

Shear capacity is the ability of the anchor or fastener to withstand an applied load at right angles to the anchor body. “The shear values (strength) are increased with deeper embedment”.

The strength of the bolt across the threads is also a factor in shear applications.

Plastic Anchor

Dual purpose: hollow and solid wall

Screw sizes from 4 -16 will hold from 3 – 50kg in concrete (shear loads).

Best for:

Mounting any item normally held with wood screws (pictures, mirrors, brackets, drapery hardware, kitchen and bath accessories).

Advantages:

1. Rust proof which makes it is fine for bathroom fixtures and outdoor applications.
2. A smaller hole results in less wall damage and the screws can be removed and reinserted.

Disadvantages:

1. Caution in drilling the correct size is crucial because insert must fit snug.
2. It is only as good as the material it’s inserted into.
3. For light shear loads only.
4. Won’t hold against much horizontal pull.
5. Not very good in crumbly material.

Toggle Bolt

Single Purposes (Hollow only)

May be “butterfly” or gravity type, “tumble head.” These have a great deal of holding power, and are constructed so that the head is not lost when the bolt is removed from the hole.

Recommended uses

Hanging cabinets, heavy -shelving, and bathroom grip bars.

Advantages:

1. Simple, immediate action installation, great strength for combination loads.
2. Alignment of mounting holes with wall holes isn’t critical.

Disadvantages:

1. It needs a larger hole to pass through, so bolt has a rather a sloppier fit, resulting in a fixture not precisely positioned. The attached fixture must be large enough to hide the hole.
2. Once removed, the wings are lost in the wall.
3. Installation is awkward where fixtures requires several bolts, since all must be inserted through fixture before positioning.

Expansion Bolt

Holds up to 200 kg, but does not exceed 50 mm in length.

Recommended uses

Mirrors, wall cabinets, lamps and clocks, adjustable brackets, shades and blinds.

Advantages:

1. Uses smaller hole.
2. Centers the bolt precisely.
3. Remains in place for reuse if fixture is removed.
4. Bolt is easily inserted through fixture after positioning on the wall.
5. Resistant to corrosion because they are made of aluminum.

Disadvantages

1. Slower installation (unless a special kind of tool is used).
2. Uses a smaller bolt and a thinner anchor metal resulting in less strength.
3. Mounting holes must exactly align with the fastener.