Plywood or Drywall?

Since the 1950s or 1960s, most homes built have the interior walls covered with drywall, more correctly known as gypsum board. This relatively inexpensive engineered material was developed as a replacement to plaster and lath walls. It provides a smooth, paintable wall for a fraction of the effort of plaster and lath and once properly finished, textured and painted it really can’t be distinguished from plaster.

Benefits of Drywall

The major impetus for developing drywall was saving cost by reducing the skilled labor needed to finish walls. While taping and finishing drywall still requires a fair degree of skill, it is much less time consuming that nailing up lath strips, applying the cement inter-coating and then floating the plaster on top of that. In the beginning, drywall was slow to gain popularity, seen as a cheapening of the workmanship that was going into the home, but it eventually took hold. Today, it is pretty much the standard means of covering interior walls, although it is not the only way that interior walls of homes and offices can be covered.

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OSB and Drywall, Mindy Georges

Benefits of Plywood

Plywood offers some distinct advantages of drywall and is a good alternative to be considered, especially if one is looking for a warm, natural look. The sterility of drywall doesn’t provide much warmth, whereas wood grain naturally does. So, what are these examples?

Lower weight – Plywood is actually lower weight than a comparable sheet of drywall. A 5/8″ sheet of drywall weighs 2.75 pounds per square feet, while a 5/8″ sheet of plywood weighs from 1.8 to 2.1 pounds, depending on the type of plywood.

Easier installation – Plywood can be used naturally, without the need for taping and finishing. The only finish most people apply is a couple of coats of varnish. If hardwood plywood is used, the grain patterns provide a distinctive and attractive finish, without any fancy painting techniques. Plywood can be attached to the wall with a number of different fasteners, depending on the “look” one is trying to create.

Resisting damage – Plywood is considerably stronger than drywall. You can easily break a drywall wall, either from simple horseplay or from hitting it with a piece of furniture while moving in. Granted, damage to drywall is easier to repair than damage to plywood, but it’s better to not have the damage to worry about. This is especially good for laundry rooms, garages and other “utilitarian” spaces within the home.

Structural strength – If you’ve ever tried hanging a heavy picture or mirror on the wall, you understand how structurally weak drywall is. You really can’t count on the drywall to carry the weight, but must instead find a stud to nail the hanger into. With plywood, you can attach pictures, mirrors or even shelves anywhere, without a risk of the wall breaking or the attachment falling off.

Advantages of Drywall over Plywood

There are two disadvantages to plywood over drywall. The first is cost. Plywood is considerably more expensive than drywall, especially if you are using a cabinet grade hardwood plywood. This is a large part of what has kept plywood from being used more consistently as an interior wall treatment. The other disadvantage is that plywood may not meet the fire-resistance requirements of local building codes. If the building code requires that an interior wall have a one hour fire resistance, as is common between garages and living areas or between adjacent living areas, plywood is not able to provide that. However, a wall built with 5/8″ drywall on both sides does meet that building code requirement. So, if you wanted to use plywood for aesthetic reasons, you would need to mount the plywood over a layer of drywall.

Strengthening Plywood

Plywood is used in a lot of situations where strength is required. Generally speaking, if the strength of the plywood is not required, then other engineered materials are used. However, there are times when plywood alone isn’t strong enough; or, although it might be strong enough, it isn’t stiff enough to meet the need. In those cases, the plywood needs to be reinforced.

Adding structure

The most common way of reinforcing plywood is by adding structure to it. This can either be an edge piece, that drops below the level of the plywood, such as a edge that is used to support the outer edge of a plywood shelf, or it can be structural ribs that are run underneath the plywood. As a general rule of thumb, the farther these ribs extend from the surface of the plywood, the more strength and stiffness they offer to support it. But what do you do in a situation where there isn’t room to put edges and lips below or behind the plywood to support it? Is there another way?

Fiberglass

Yes, plywood can be strengthened by making a plywood and fiberglass composite. This is fairly common for boat decks, where the fiberglass outer shell is backed up by a plywood core or at times even a balsa wood core.

Fiberglass adds a lot of stiffness to the plywood, especially when it coats both sides. The fiberglass resists stretching, which is technically called “tension” that would try to pull the wood fibers slightly apart, allowing the sheet of plywood to bend. In this, it acts in the same way as rebar does, when used in concrete structures.

To reinforce plywood with fiberglass, start with clean wood. It should not be painted or prepped in any way. A slightly rough, unsanded surface is best, as that allows greater adhesion for the fiberglass resin.

Paint a heavy but even coating of fiberglass resin onto the surface of the plywood. This needs to be heavy enough so that it will soak into the fiberglass cloth, but not heavy enough to run and drip. You will need to work quickly, as the fiberglass resin is a two-part, thermoset plastic, that starts to set as soon as you mix it.

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Bottom side of bench-seat, Sarah D.

Once the surface is wetted, roll out the fiberglass cloth and cover the surface of the plywood, allowing the cloth to extend beyond the edges of the wood. Be careful to avoid any wrinkles in the fiberglass cloth, as well as to keeping it as straight as possible. As you roll it, work out any air pockets that form, so that it lays smooth. Once the resin is “cured” (not necessary dry, but has reached hardness) according to the directions on the can, a second coat of resin should be mixed and painted on to cover the top side of the fiberglass cloth. Alternating layers of resin and cloth can be added until the desired strength is reached.

It is best to coat both sides of the plywood with fiberglass, rather than just one side. While only one side may be seen as the finish side, the opposite side needs to be coated for both strength and to prevent warping. The resin shrinks slightly as it cures, which can cause the plywood to warp towards the side that is being worked on. By alternating sides in the application of the fiberglass, this warping can be avoided. Keep in mind that the true strength in this composite will come from the fiberglass and not the plywood. Therefore, you want to be sure to use enough layers of fiberglass cloth to ensure the strength you need.

Plywood Scarf and Butt Joints

Plywood is useful as both a building material as well as making a wide variety of projects. But what do you do when the project you are working on is larger than a sheet of plywood? In homebuilding, this is dealt with by nailing sheets next to each other, so that the edges of both sheets are on a structural element. But not all projects allow that. Some things, like boat building, require joining the plywood between structural elements (just like slats). They also require that the joint be made in such a way as to turn the plywood structurally into one continuous sheet, as well as providing a smooth surface.

Difference between Scarf and Butt Joints

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Woodworking frame and box joints, Jordanhill School D&T Dept

There are two ways of doing this, both of which are commonly used in boat building; scarf joints and butt joints. The basic difference is that a butt joint allows the edges of the two pieces of plywood to meet each other and be held together by a third piece of plywood, called a scab. In scarf joining, the ends of the plywood sheets are tapered and then glued together, creating a single piece of plywood that is consistent in thickness. It also retains most of the flexibility of the original plywood.

Making a Butt Joint

Butt joints are considerably easier to do than scarf joints, but have two distinct disadvantages. The first of these is that they reduce the flexibility of the plywood. So, if you are attaching the plywood around a curve, such as might be done on a boat’s hull or when making a receptionist’s desk for a large office, the joint would cause a flat spot. The other disadvantage is that screws are usually used in the butt joint, which will be visible.

To make a butt joint, a butt block or scarf is cut out of the same thickness of plywood that is being attached. It needs to be wide enough to extend a minimum of four inches on either side of the joint. If thin plywood is being joined, a thicker scarf is usually used.

It is only possible to make a butt joint without screws if epoxy adhesives are used. Epoxy is also useful when the plywood does not have a smooth surface finish, as it is excellent for gap filling. However, most butt joints are screwed, as well as glued.

When fasteners are used for butt joints, they are placed about two inches apart and about one inch from both edges. That means at every two inch spacing, across the width of the joint, there are four screws. Two which are one inch from the edges of each piece of plywood being joined, backed up by another two which are one inch from the edge of the scarf block.

In the case of thinner pieces of plywood, especially 1/4″ thick plywood, screws should be used which are long enough so that the major diameter of the screw pierces the back of the plywood. The excess screw length is then cut off with a grinder.

Making a Scarf Joint

Scarf joints require tapering the edges of both pieces of plywood equally and evenly. This makes the scarf joint much harder to accomplish, but it provides a joint that is virtually invisible. The angle of the tapering is 1:12 or 1:10 for plywood under 1/2 inch thick and 1:8 for plywood over 1/2 inch thick. That means that a 1/4″ thick piece of plywood would have a three inch scarf joint.

To cut the taper for the scarf joint, the two pieces of plywood are marked to show the depth of the taper. They are then stacked, with the edge of the top piece aligned with the marking on the bottom piece. Screw or clamp the pieces down to hold them in place while cutting.

The actual cutting of the taper can be accomplished with a hand or power plane or belt sander. Extreme care must be used to keep the cut and angle consistent. Gaps in the finished joint will weaken it. The final cuts should be made with a jointer plane that has a long base, to ensure that the cut is straight and even.

Once the taper is cut, the sheets of plywood can be unclamped and the top one flipped over for gluing. It is a good idea to put a piece of waxed paper under the glue joint, while gluing and clamping, to prevent the plywood from sticking to the bench top. Either epoxy or resorcinol can be used; however, epoxy is preferred for its gap filling properties.

Coat both sides of the joint with the adhesive and overlay them, being careful to align the two pieces of plywood. The joint needs to be clamped while the adhesive is setting. This can be done by clamping it to the bench top with a thick board on top of the joint (use another piece of waxed paper here) or by screwing through the joint, screwing the plywood to the workbench. It is extremely important that equal pressure be applied all the way across the joint. The holes caused by the fasteners can be filled after the adhesive cures.