How To – Selecting the right plywood for residential construction

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    There are a variety of locations within your home where plywood is used.  Plywood can be constructed of either layers of wood that are glued together or chips of wood that are oriented to obtain a specific structural strength.

    Standard plywood is made of layers that are glued together.  When logs go through a plywood mill a thin layer of wood is stripped from the logs in a similar method to a pencil sharpener.  These long rolls of wood are then cut so the grain runs in the opposite directions.  There then glued together and pressed under heat.

    A special type of standard plywood his cabinet plywood.  This type of plywood has an exterior surface made of hardwood that is decorative.

    The second half of plywood is OSB plywood which stands for oriented strand board.  This type of plywood is made out of scrap lumber where the chips of wood are aligned for increased strength.

    Both of these types of plywood use a moisture resistant glue however they can not be used in high moisture conditions.     Pressure treated plywood is available for some high moisture conditions however its use is limited.

    You are likely to find a mixture of standard plywood and OSB plywood used throughout your home proper application is necessary for best performance.

    Exterior sheathing

    You’ll find that most new homes make use of OSB plywood for exterior sheathing.  Some contractors have found that the additional glue used in OSB plywood increases its performance.  However you may find that only the walls are sheathed in OSB and the roof is sheathed with standard plywood.

    When applying exterior sheathing it is important to install it in its proper orientation.  Exterior wall sheathing is applied vertically however some contractors install it horizontally.  This is especially important when using OSB plywood which is designed with a specific structural direction.

    When applying roofing sheathing it is important to apply mid span clips that will increase its ability to support live loads.  Roof trusses are often spaced at a distance of 24 inches inches wider than wall Spans of 16 inches so clips are always required.

     

    Plywood for flooring

    There are two types of plywood that are commonly used in flooring.  The first type is decking which is used to create a structural floor of your home.  The second is luan subfloor material that is used to prepare the decking to receive vinyl or tile floor.

    Decking material is 1 inch plywood that has a tongue and groove edge that allows it to interlock when applied to the joists.  Decking material should also be glued and ring nailed to your joists to reduce possibility of squeaks due to friction of the boards.

    Lauan plywood is 1/8 inch plywood that is applied over the decking with glue and screws.  This material allows the contractors to install their flooring over level surface.

    Plywood Siding

    Plywood siding is more commonly used on out buildings and sheds than on residential homes.

    Plywood siding is plywood that has been cut and textured to give the appearance of standard siding.  They have been many variations plywood siding however the visual appearance is lacking in comparison with other products.  The main advantage of plywood siding is that it is quick to install.  The cost difference of the material is also a factor when selecting that for structures that are not residential.

    Higher quality plywood siding will have a tongue and groove on all edges this reduces warping.

    Finish grade plywood

    Finished grade plywood allows furniture and cabinet makers to produce high quality items from a lower cost.  From a script plywood has an exterior surface that is made of hardwood.  The most common plywood of this type is oak and then birch however this material can be ordered in almost any hardwood that is readily available.

    Normally finish grade plywood comes in an A/C grade which means the higher quality A surface is the hardwood surface and the C grade surface is the interior of the cabinet or furniture.

    Finish grade A/C plywood is also available for painted finishes.  This product is made of pine and the Finnish surface is clear of all knots and defects.

    Lesser quality plywood is also available from grade A to D at the lower end.

    Understanding plywood markings

    Each piece of plywood should be marked by certifying agency showing that it was inspected and is rated for its stated use.

    When plywood is used in framing of exterior surfaces is important that it passes the inspection and is labeled for exterior sheathing.  Exterior shooting is tested to withstand storm forces and impacts of materials.

    The standard piece of plywood will have three markings that you want to look for.  The first marking is that designation of interior and exterior.  The second marking is the face and back rating of grade finish.  The final marking is the test and inspection label.

    Installation of plywood for framing

    When plywood is used for framing the thickness of the plywood and its structural rating will determine how far apart the studs support joists can be placed for adequate support.

    Normally plywood is installed so that its longest length runs along a joist or stud.

    When standard sizes are used during the design process materials are designed so that nailing surfaces align this means the material does not need to be cut as often and there is less waste.

    Plywood is normally produced in 4×8 sheets which allow the ends to align with a 16 Inch on center wall and joists or a 24 inch on center roof truss.

    Because this material is glued up at manufacturer it can be ordering nonstandard sizes.  This is more common in commercial construction.

    A variety of different fasteners are used to attach plywood to structural members.  Normally standard 16 penny nails are used for fastening.  The number and distance of nails to be used is set in standard building code.

    When using a nail gun many clips of nails have what is called it clipped head which allows the gun to hold more nails however this type of nail may be rejected by your local building official.

    A glued nail is preferred when framing.  The glue coating on the nail will heat up when the nail is shot by the nail gun.  This will reduce squeaks in sub floors.

    Final note

    Selecting that correct plywood is not difficult if you follow your architects recommendation.  Always look for markings on your plywood that indicate its use and grade.

    Remember plywood is not waterproof unless it is specifically rated for moisture contact.  Only pressure treated lumber can come in direct contact with water or high moisture conditions.  However even in these instances waterproof plywood is not normally recommended.  An example would be a framed wall that is below grade.  Although this is possible the lifespan of the wall will be very short.

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