Wood as a building material

Wood as a Building Material Sawn lumber is the most common material used to build house frames. It is relatively light for its’ strength, due to its’ structure. Wood can be compared to a bundle of straws (tracheid cells) held together by glue (lignin) Wood cells are produced in the roots, leaves, and under the bark (cambium)

Wood as a Building Material Wood in the cambium layer grows at varying rates, depending on the season. Springwood produced fast, and has a low density Summerwood produced more slowly, and has a higher density.

Wood as a Building Material Alternating layers of springwood and summerwood form “rings” of a tree The orientation of the rings relative to the wood surface affects how the wood behaves Plane-sawn (flat-grained) and Quarter-sawn (edge or vertical grained) are the two classifications

Wood as a Building Material When wood dries, the rings tend to “straighten”, causing one of the most common lumber defects, “cup” Vertical grain is more expensive than flat grain lumber, but will not cup as it dries.

Wood as a Building Material The structure of the lumber also depends on the type of tree that is harvested. Softwoods, Evergreens, Conifers, or Angiosperms have enclosed seeds Hardwoods, Broadleaves, Deciduous, or Gymnosperms have exposed seeds

Wood as a Building Material Hardwoods are not necessarily harder than Softwoods (Balsa is a hardwood, botanically speaking). But Softwoods are much more plentiful in large acreages and therefore less expensive to convert to a building material.

Wood as a Building Material Softwood lumber is classified (and graded) according to use: Boards (up to one inch thick) used for appearance, e.g. window trim and fascia – Graded based on size and type of knots, amount of wane, blue stain Dimensions lumber ( from two to four inches thick) used for strength e.g. studs and joists – Graded based on splits, wane, shake, and knot holes.

Wood as a Building Material

Wood as a Building Material Wood grades – Divided into boards, dimension lumber, and timbers (based on different usages) Grading done visually or mechanically Boards graded for appearance Dimension lumber graded for strength Timbers graded for strength

Wood as a Building Material Nominal vs Actual size Lumber is sawn at a target size, and then dried and planed down to the actual size. The difference between the Nominal (Name or what the lumber is called) and Actual size depend on the thickness of the lumber If you measure a 2 x 4, you will find that it measures 1 ½” by 3 ½”, but a 1 x 8 measures ¾” by 7 ¼”.

Wood as a Building Material Dimensions up to 1 inch nominal are – ¼” actual Dimensions 2 to 6 inch nominal are – ½” actual Dimensions over 6 inch nominal are – ¾” actual

Wood as a Building Material Green wood is filled with water when it is harvested. The weight of the water is about equal to the weight of the wood in the tree. Lumber is dried depending on the use and final location. Wood will eventually balance its’ moisture to the humidity in the air (about 8% m.c. in the northeast) Construction lumber in the northeast is dried to about 19% moisture content (below 20% decay is negligible). Furniture lumber is dried to about 8% moisture content. Construction lumber loses moisture and shrinks during the first heating season.

Wood as a Building Material The moisture content of the lumber is determined by: Green weight – Oven dry weight x 100 = % moisture content Oven dry weight

Wood as a Building Material The behavior of wood drying is similar to what happens to a sponge as it dries From Green to fiber saturation point (approx 30% m.c.) water is lost from the cell cavity (free water) and no shrinkage occurs From Fiber Saturation Point to Oven Dry (0% m.c.) the bound water in the cell walls is removed, and the wood shrinks

Wood as a Building Material Board Foot Measure – Softwood lumber is commonly priced by the stick (individual piece) and by the board foot. Determining the cost for all of the lumber in a project can be done quickly using board foot e.g. Price the following: 15 – 2x10s – 14’ @ 8.15 22 – 2x8s – 12’ @ 5.60 or 1286 board feet at .35 bf 73 – 2x6s – 8’ @ 2.80

Wood as a Building Material What is a board foot? It is a measure of lumber volume, equal to the volume of a board 1” thick by 12” wide by 12” long = 144 cubic inches The 144 cubic inches can come in many different shapes

Wood as a Building Material The most common way to calculate board feet is the following: # of pieces x width (in) x thickness (in) x Length (ft) 12 N.B - The dimensions used are the nominal dimensions How many board feet in 17 – 2x4s 10 ft long? 17 x 2 x 4 x10 = 113.3

Wood as a Building Material Sheathing- APA rates sheathing for their structural use- Plywood – Crossbanded veneers give the material high dimensional stability and a definite strength axis. Still manufacturers suggest it be installed with a 1/8” gap at edges (“sized for spacing”) Veneer quality on the faces is graded with a letter designation A – No knots B – Any knots replaced with plugs C – Knots over ¾” plugged D – Knots over ¾” present

Wood as a Building Material OSB – Oriented Strand Board – Made from wood chips (instead of full sheets of veneer) and therefore less expensive than plywood. Can be used for same structural applications as plywood if the span rating is the same Strands are oriented for strength on long axis, giving it a strength axis like plywood. Span rating classifies the maximum spacing for supporting members when used on roof/floor.

Wood as a Building Material Wood Preservative Treatments- Designed to resist insect attack and fungal decay. Not designed to change wood/water relationship. If you let water lay on wood, it will absorb the water and shrink and swell. Creosote, Pentachlorophenol, Copper Cromated Arsenate (CCA) have all been used and discarded for health concerns ACQ – Ammoniacal Copper Quaternary is the current pressure treatment method, Most commonly with an amine carrier. Very corrosive to fasteners.

Wood as a Building Material Pressure treated wood- The treatment salts tend to corrode fasteners, thus hot-dipped galvanized, stainless steel, or plastic coated fasteners are recommended. The treatment chemicals should be bound to the cell walls, and thus not leach out in contact with water. Avoid contacting the chemical if released by: Cutting – wear dust mask. Handling – wear gloves. Burning – don’t burn in open fires.

Wood as a Building Material Engineered Wood Products To span distances ( 16’-24’) not easily handled with solid sawn lumber, or for loads too great for solid sawn, Laminated Veneer Lumber (LVL), Parallel Strand Lumber (PSL), Laminated Strand Lumber (LSL) or I joists may be used.

Wood as a Building Material

Wood as a Building Material Nails Smooth shank – Most common Spiral shank – Used to increase withdrawal resistance Annular ring shank – Used to increase withdrawal resistance, shock resistance Galvanized – Used to resist rusting Cement coated – Used to aid driving and increase withdrawal resistance Stainless steel – Used to resist severe corrosion

Wood as a Building Material