Definitions

Wood: Fibrous substance forming the trunk, stems and branches of the tree

Lumber: Product of sawing, planing, and preparing Wood. Typically, you are purchasing “lumber”.

Timber: Lumber with a 5” minimum sectional dimension.

• Heavy Timber construction gets its name from this definition.
• Note that heavy timber is different from mass timber.

Rough Carpentry: Structural framing, sheathing, blocking, and miscellaneous pieces necessary to create a structure. Typically, the vast majority (if not all) of rough carpentry is covered up by finish materials.

Framing: The assembled components to construct a building.

• Due to code limitations, most traditional wood framing construction is limited to small to moderately sized buildings, or special construction such as the 5 over 1.
• Mass Timber has grown in acceptance due to its understanding of fireproofing. Modern codes are accommodating this more and more. Read more on mass timber here.

Light Wood Framing

Over the years there have been several different types of light wood framing that have been developed. The main two that evolved in the US were Balloon Framing and then Platform Framing. Light Framing refers to any wood framing consisting of dimensional lumber, normally 2x4s or 2x6s (framing without timber).

• In general, the best way to learn about framing is reviewing diagrams from various (official) published sources and reviewing best practices for framing.
• American Wood Council has a great library under ‘resource hub’

Balloon Framing

The entire exterior wall is one longer stud. The wall structure goes from ground to the underside of the roof. Common in barns, and older homes. Not common in structures over 2 stories tall.

• Generally more expensive as exterior wall studs need to span the entire height of the building, and need to be true.
• Requires more room for staging and setup as the members are longer. Requires more time to complete setup and preparation.

Platform Framing | Western Framing

Platform Framing evolved more recently to be more economical with other benefits during construction. This method uses separate studs for each floor of the building. It’s essentially like stacking tables on top of each other, where the table top is the floor.

• Each floor can be completed and then used for staging the next floor. This allows you to proceed with other areas of interior construction while still working on the exterior envelope and structure.
• Shorter studs cost less money than full length studs.

Framing Components & Definitions

Openings

Openings in wood construction are required for doors, windows, portals, stairs, and similar aspects of construction. Eliminating studs in order to make an opening affects the structural integrity of the building. As a result, framing of openings must be capable of transferring loads from one cut member to another. 

• Framed openings generally consist of double the number of members in order to increase the strength of that section. 
• By using the same exact method of construction and member sizing, rules of thumb have developed which are tried and true. This means that openings can be created without needing to run structural calculations for every condition, and instead leave those for unique construction.

Fire Cut

When wood joists are framed into masonry walls instead of wood walls, it creates a unique construction condition that should be addressed in detailing. There are several approaches to this detail, and all of them should make sure that the floor can collapse without ruining the integrity of the exterior wall.

• Joists rest on metal hangers attached to wood ledger strips anchored to the masonry
• Pocket created in masonry wall to accept joist, the joist should be fire-cut. The fire cut is required to prevent the masonry from being pushed up and away by the wood member should it collapse during a fire.
• Corbeling to accept the joist is another method that acts similar to the joist hangers.

Blocking

Wood framing installed between main structural members such as studs or joists to provide extra rigidity or to provide a base for nailing other materials.

• Sometimes referred to as ‘Solid Bridging’

Bridging

Using dimensional wood framing to span (bridge) between joists that prevents the joists from buckling under loads.

• Installed in intervals not exceeding 8’-0”.
• Cross bridging = an X shape
• Solid bridging = blocking (see before)

Firestops

A Firestop is used to prevent the spread of fire. This is done by creating a barrier, which is typically installed in concealed spaces of construction to prevent the spread of fire caused by drafts.

• In residential construction, allowable materials typically include nominal 2” thick wood members, gypsum board, or mineral wool. Wood is typically used.
• In urban areas, codes may be more strict to fireproof materials and may require engineering judgements. 
• The building code specifies where and when firestops must be installed, but in general, firestopping must be in concealed spaces between spaces (inhabited or not). So, between floors, between a floor/ceiling, floor/attic space, etc.

Sheathing

Sheathing is the part of the exterior envelope that spans across framing. Sheathing is typically a thin panel (sheet) material that is attached to the framing via nails or screws. The sheet of material provides much needed lateral support, increases rigidity, and provides a suitable base for applying exterior cladding/finishes.

• Sheathing is typically either plywood (best) or Oriented Strand Board (OSB)
• You may encounter particle boards used in some sheathing applications, but it really should not be used.
• When lateral bracing is not required via the sheathing material, sometimes rigid insulation is used.
• Further materials may be used such as fiber cement board, backer board, etc. These are typically reserved for special considerations like exterior cladding system requirements, moisture, etc.

Plywood

Plywood consists of thin sheets of veneer glued together to form a rigid panel. In the US, sheets are made in mostly standard 4’x8’ sizes, but other sizes are available (larger and smaller). Plywood may come in different numbers of layers, and the layers are rotated 90 degrees (lapped) about each other to provide more dimensional stability to the material and strength.

• Made in odd numbers of layers (1, 3, 5, etc) so that the grain direction will be the same on top/bottom.
• Exterior plywood should be specified as such (using exterior glue, etc)
• Also non-traditional uses such as furniture and as architectural finishes (growing in popularity)

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Plywood Grading

Plywood is graded in two different ways. The first is strength for quick span rules of thumb, the second is by aesthetics. 

1. Plywood Grading x Appearance (Face Veneer)

Plywood is graded based on the quality and appearance of its face and back veneers. The grades range from A to D, with A being the highest quality and D being the lowest. There is a special grade of N as well.

N Grade:

• Intended for natural finish and is made from all heartwood or all sapwood.
• Free from defects.
• Only available typically on special order and typically costs a premium.

A Grade:

• Smooth, sanded surface, and paintable.
• Has no knots or other defects.
• Best commonly available grade.
• Ideal for clear finishing and decorative surfaces where plywood is left exposed.

B Grade:

• Allows for plugged Knotholes but has a smooth surface.
• Amount of knots is typically minimal
• Suitable for painting, staining, or lacquering.

C Grade:

• Allows small knot holes and some splits and defects.
• Has a solid, functional face.
• Typically used in non-decorative surfaces, where appearance doesn’t matter.
• Color and gradient may vary quite a bit across the surface.

D Grade:

• Many flaws and knots will be present, the lowest grade.
• Best used for structural only applications
• Non-appearance grade.
• Color and gradient may vary quite a bit across the surface.

Other Grades and Nomenclature

There are other various grades and blends with nomenclature that have been developed beyond the A-D system. Most plywood manufacturers will have grading guides, and can help you understand what they offer.

S-Grade

• Selected quality that is equal to or better than B-grade

X-Denotation

• X denotes plywood that is moisture resistant. This does not mean marine grade or that it is impermeable, but simply that the plywood will resist moisture. For example this plywood is good for applications that may be exposed to water occasionally during construction until it is (dried and then) covered.

PG-Grade

• Known as “pallet grade” plywood. It’s un-sanded and non-structural. Uses for packaging and other temporary solutions.

2. Plywood Grading x Strength (Span)

Ratings by span are used for structural and sheathing applications. They essentially rate the stiffness and strength of the board. The values presented assume average residential constraints. Typically, panel must have its grain perpendicular to the supports, and the panel must span at least 3 supports.

• R/F (inches) Value is listed as two numbers with one before (or above the other)
  • First number gives maximum spacing in inches for roof supports
  • Second is maximum spacing for floor joists.
• 32/24
  • 32” maximum span at roof
  • 24” maximum span at floor
• It’s important to not get this number confused with the plywood thickness. 15/32 = Thickness, and not the span strength. You can easily tell this if the number on the bottom is larger, it means that is a thickness value, since the floor strength would always be a smaller span due to its structural considerations.

Grades Listed Together

Plywood typically comes with a higher grade on one side than the other, due to the fact that most construction details have one side of the plywood hidden. When plywood is listed, it’s typically two characters. The first character is always the face grade, with the back grade being listed second.

• AA = Two A grade faces
• AC = A grade on the face, C grade on the back
• AAX = Two A grade faces, X = Moisture resistant
  • Preferable choice for outdoor building
• ACX = A grade on the face, C grade on the back, X = Moisture resistant
  • When cost is an issue, cost savings compared to using AAX.
• CDX = C grade on the face, D grade on the back, X = Moisture resistant
  • Very common for construction as it is relatively cheap and has moisture resistant qualities.

Plywood Thickness Chart

Nominal thickness is typically between ¼” and ¾”, but other thicknesses can be special ordered. The actual thickness will be slightly smaller as plywood is sanded during its manufacturing process.

Nominal	Actual
¼”	7/32”
⅜”	11/32”
½”	15/32”
⅝”	19/32”
¾”	23/32”

Oriented Strand Board (OSB):

Engineered panel product manufactured from precision cut wood strands to a maximum of 4” long and .0027” thick. The strands are laid in layers at right angles to each other and bonded with resin waterproof glue under heat and pressure.

• Primary strength of OSB panels is along the orientation of the chips on the face layer, which is generally parallel to the length (longer side) of the panel.
• OSB is more susceptible to delamination than plywood, but is still acceptable to use for sheathing with short-term weather exposure because of the waterproof glue that is used. Long term exposure should be consulted with the manufacturer’s documentation.

Particle Board

Composed of small wood particles, fibers, or chips, of various sizes mixed together in a binder and formed under pressure into a panel.

• Particle Board is generally used for backing and framing of finish carpentry because it is less expensive and more dimensionally stable than plywood. Due to its construction, it is typically lighter than similar plywood sized panels of other types which makes it a nice product to use when weight is a concern.
• However, particle board breaks easily and is not as strong as plywood. So its use in furniture or other areas of construction should be carefully considered.

Medium Density Fiberboard (MDF):

Panel produced from wood particles reduced to fibers. Combined with a resin and bonded together under heat and pressure.

• It is the most dimensionally stable of the mat-formed panel products.
• MDF has a smooth uniform and dense surface that makes it useful for painting, thin overlay materials, veneers, and high-pressure decorative laminates.
• MDF will bubble or welt when wet, and should not be exposed to wet surfaces without a protective layer.

Hardboard (HDF)

• Hardboard is a panel product composed of inner-felted fibers consolidated under heat and pressure.
• Can be made with recycled content which some designers seek out.
• Available sanded or un-sanded on both sides, and tempered or un-tempered.
• Tempered hardboard: Has greater hardness, stiffness, and weight than the un-tempered type.
• Commonly used for protecting finished surfaces in construction
• Also used in furniture and casework as its uniform appearance.