A COMPLETE GUIDE
Plywood is an assembly of wood veneers (Thin layers of wood peeled from trunk logs) bonded together using resin(glue) to produce a flat sheet that is dimensionally and structurally stronger than solid wood. An extremely versatile product, plywood has its usage in a wide range of structural, interior, and exterior applications – from formwork to internal paneling.
While plywood can be tailored/manufactured to suit an extensive range of applications, a normal ply panel consists of at least 3 plies and can extend up to 15 plies or more, with the wood grain in alternate plies running at right angles to each other.
Plywood can be sub-divided into four groups, depending upon specific applications :
• Structural (BWP | IS 10701)
• Exterior (BWR/BWP)
• Interior (MR | IS 303)
• Marine. (BWP | IS 710)
In the following section, we talk about the construction of plywood, how its made, its various grades, its applications, and the various benefits of using plywood as a building material.
BENEFITS OF PLYWOOD
1. Increased stability
Additional to all the inherent advantages of the parent wood, plywood exhibits enhanced properties in its laminated structure.
2. High impact resistance
Plywood being a wood-based panel, can tolerate temporary short-term overloads; up to twice the design load. This is specifically useful wherever there are chances of seismic activity or cyclonic winds. Plywood is also extensively used as construction flooring or as concrete formwork. Plywood’s laminated structure is designed to distribute loads from impact over a larger area on the opposite face, which effectively reduces the overall tensile stress.
3. Surface dimensional stability
The 90 Degree cross laminated construction of plywood ensures that plywood panels remain relatively stable under changes in temperature & moisture whereas normal solid wood would easily expand and contract. This is especially important in flooring & formwork construction where moisture exposure is high.
4. High strength-to-weight ratio
Owing to its cross-laminated construction, Plywood also exhibits a high strength-to-weight ratio making it very cost-effective to use plywood in structural applications such as flooring, shearwalls, formwork, and webbed beams.
5. Panel shear
Plywood exhibits high Panel shear strength, nearly double that of solid timber, due to its cross-laminated structure again. This allows the plywood to be highly effective when used in gussets for portal frames, webs of fabricated beams, and bracing panels.
6. Chemical resistance
Since most of the manufactured plywood panels are treated chemically, Plywood does not corrode and can be used in chemical works and cooling towers as a cost-effective, durable material.
SUSTAINABILITY
Sustainability is one of the guiding principles behind everything Avron Ply manufactures. Plywood being a wood-based product recognizes the potential of wood as a truly renewable and sustainable building material.
What makes building with plywood sustainable?
Plywood’s low embodied energy, which essentially means it consumes minimal non-renewable energy in its production and construction, majorly answers the question. Plywood structures, due to wood’s inherent thermal properties, rely much less on carbon-emitting heating or cooling appliances. Many Indian hardwoods (ex-Eucalyptus) have a natural durability of 25+ years, indicating less energy consumption in maintenance, demolition, and new construction. Besides these characteristics, wood’s recyclability and its ability to store carbon for life add to the benefits. Trees absorb a large portion of carbon dioxide from the environment, storing the carbon directly in the form of wood and thus reducing free carbon in the environment. The more trees we grow and harvest to use as wood, the more carbon we manage to eliminate from the atmosphere.
DURABILITY
The durability of plywood majorly depends on the bonding quality that the manufacturing process uses, which, in turn, depends on the resin (glue) used to stack wood veneers on top of each other. Manufacturers generally use two primary adhesives to manufacture plywood. They use a phenolic-based formaldehyde resin to manufacture the exterior grade marine ply, while they typically use melamine-fortified urea formaldehyde resin for the interior grades.
Manufacturers produce structural plywood from a variety of hardwood and softwood species, which might not be durable in exposed weather situations. Therefore, they must treat it with a preservative to ensure it can reach its full-service life.
SIZES
In India, plywood is available in several dimensions but a few standard sizes are as follows :
Length: 2440 mm (8 ft), 2135 mm (7 ft), 1830 mm (6 ft)
Width: 1220 mm. (4ft), 915 mm (3ft)
Thickness: Standard plywood panels are available in 4, 6, 9, 12, 16, 18, 19, 25 (mm) thicknesses.
GRADES & THEIR APPLICATIONS
Structural plywood – The manufacturer uses hardwood timber species to manufacture structural grade plywood, which is suitable for all permanent structural applications. It is commonly used for formwork, flooring, bracing, and manufactured beams.
Interior plywood: Manufacturers use interior-grade plywood for non-structural interior applications where aesthetic finish is prioritized and high load is not a concern. It is used for furniture construction such as cabinets, tables, shelves, internal wall paneling, furniture, ceiling linings, and interior door skins. In the Indian market, the interior grade is referred to as MR Grade or Commercial Grade and meets the specifications of IS-303.
Exterior plywood: The intended use for exterior-grade plywood is in exterior applications subject to the elements occasionally, where a high-quality aesthetic finish is also required. In India, exterior-grade plywood generally refers to marine plywood. Applications for this type of plywood include outdoor furniture, flooring, exterior door skins, hoarding, signs, and non-structural cladding.
Marine plywood: exterior- Exterior grade plywood is a subset of marine plywood. It is structural plywood intended for use in boat and yacht hulls, as well as other furniture applications such as washrooms, kitchens, or outdoor furniture that need to withstand water exposure. It is also utilized in aircraft construction. Manufacturers of marine plywood focus on minimizing water penetration and use a phenolic-bonded formaldehyde adhesive bond.
ADHESIVES USED IN MAKING PLYWOOD
Plywood is primarily manufactured using two types of guidelines:
- Type A – Phenol Formaldehyde Resin
- Type B – Melamine – Urea Formaldehyde Resin
Type A Bond – Phenol Formaldehyde bond is a permanent bond required for all structural and marine plywood. The Phenolic bonded resin is recognizable by its black color, which is made to not deteriorate under wet conditions, heat, or cold.
Type B Bond – Recognisable by its light glue line color, Melamine Fortified Urea Formaldehyde is appropriate for interior applications, especially in tropical areas marked by high humidity free from wet conditions.
TREATMENTS OF PLYWOOD
Plywood is treated with a range of preservatives depending on the application. The most widely used preservatives include CCA (copper chrome arsenate), LOSPs (light organic solvent preservatives) and veneers, Ruply, and ACQ (Alkaline Copper Quaternary Compounds).
For residential or commercial construction purposes, the most suitable treatments are Copper Chromium Arsenate (CCA) and Light Organic Solvent Preservative (LOSP).
HANDLING & STORAGE OF PLYWOOD DONE RIGHT
The plywood needs to be properly handled, stored, and protected from the elements as much as possible to extend its service life:
- The storage area in particular should be protected from the sun & rain that could cause any rapid changes in temperature & humidity. To avoid staining & surface checking, the plywood sheets should not be exposed to the weather while awaiting usage.
- During storage, support for the panel sheets should be provided at both ends along with support at 600mm wide intermediate centers to avoid distortion of ply.
- The plywood stacks should be stacked flat on its face and not on its edge. Additionally, the stack should be kept in dry conditions and clear of ground contact to avoid Termite Infestation.
PROTECTING PLYWOOD
Plywood, if mishandled, may get subjected to decay or termite attack. Specifically, in-ground applications run a high risk of causing both decay conditions and termite attacks. For plywood sheets that are permanently exposed to the weather, it’s essential to treat them with the necessary preservatives against fungal attack (rot), either painted or unpainted. Termite Infestation is not a problem for plywood when used in above-ground applications.
Plywood can be treated through either Veneer Impregnation, before pressing OR Preservative treatment after manufacture(Envelope Treatment).
Lastly, but most importantly, Plywood is originally made from wood and therefore is susceptible to responding to elements as wood does. We highly recommend avoiding water contact with plywood at all times to protect against any adhesive breakdown, wood swelling, or bending responses. Unless plywood is of Marine Grade (IS – 710), it is susceptible to reacting negatively in contact with water.
WORKABILITY – CUTTING HOLES & NOTCHES
Regular plywood sheets can be cut, sawn, and drilled using any ordinary carpenter’s tools or woodworking tools.
The placement of screwholes or nailing in particular plywood elements should be restricted in size and location to maintain the structural integrity of the plywood panel. A minimum of a 3-inch gap should be provided between two holes or between the screw hole and the edge.
Since Plywood is based on timber, it has the same practical workability attributes as solid structural timber with the added advantage of its flexibility. Bending or molding plywood is relatively easy. Manufactured plywood panels are generally very stable, however occasionally, warping, bowing or twisting can occur due to unsupervised handlings such as moisture imbalances, unbalanced laminates, paint coats, or unbalanced stresses.
FINISHES
Exterior Plywood
Exterior Grade Plywood used for exterior applications may be finished in either of the following ways :
- Painting
- Coating with water repellents
- Overlaying with medium-density phenolic-impregnated papers.
Please Note: While not necessary, it is advised to edge seal plywood post any cuts or sawing as it minimizes moisture uptake. For best paint performance, use 100% acrylic latex paint systems on plywood. Oil-based or alkyd enamel paint systems are not recommended for usage in weather-exposed applications of plywood.
Interior Plywood
In the case of Interior Grade Plywood, Clear finishing, French polishing, staining, and painting of plywood are all viable options. ( Paint manufacturer’s directions are strictly advised )
Before initiating any finish work please ensure that the plywood is dry (below 12 – 15% moisture content) and that the surface of the ply panel to be painted is clean, smooth, and wax-free. We recommend satin or matt finishes and paint finishes on interior grade plywood as they lend a high-quality aesthetically pleasing surface that is more uniform with a timber’s characteristics. We advise against using high gloss finishes and paints as they would highlight beat marks from sanding, knots, grain variations, patches, or even open defects.
COMMON ISSUES IN PLYWOOD
Plywood is made by gluing together thin sheets of wood veneers sourced from logs of softwood or hardwood tree species. Since it’s made from a naturally occurring element – wood, it also entails that the plywood is highly responsive to the surrounding it’s kept in and is therefore at risk of degradation if not cared for properly. We’ll try to highlight some of the most frequent issues which affect plywood and how to tackle them.
Thickness Variation – When plywood is made, the alternating layers of core veneers are glued together with heat and pressure. During this process, there is some degree of compression in the core layers. Different parts of a layer may compress more than others, and each layer will pick up inconsistencies from adjacent layers and this compression is largely unpredictable. The rules are written such that hardwood plywood cannot be thicker than its nominal size, but it can be up to 0.5 – 1mm thinner. The reasoning is that a below-thickness panel will still fit in a groove cut at the nominal size, but an over-thick panel would not.
Bending or Warping – It is a common occurrence in plywood panels for it to warp or bend due to differences in humidity/moisture and temperature. We explain why it does so and how to tackle it if it does.
Splitting – Splitting generally is referred to a split in the layers of plies in the plywood panel when being cut or when a nail or screw is being driven into the edge of a panel. This is largely a construction error where the layers may have failed to bond properly while pressing or if there were too many overlappings or voids during the construction/composition of the multiple layers of a plywood panel. On average for Indian factories, the Splitting problem should not exceed 2% of the total sheet production. Ask for a replacement if you stumble across this issue.
We list here several quality checks that can be performed while purchasing plywood to ensure you buy genuine and good quality plywood. Click here.