What’s the Correct Direction for Installing Plywood?


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Plywood has become the go to choice for sheeting floors, walls, and roofs. It is also used in cabinet making and many shelving units.

The location and use of the plywood determine which way the grain should run on the installation. When used for flooring or roof sheathing, it is usually installed with the grain running perpendicular to the joists. When used for walls it can be installed facing either way.

Which Side of Plywood Faces Down?

Plywood has four grades – A, B, C and D – that are determined by the quality, namely the number of knotholes and voids in the top layer. Most plywood has one grade on one side and another grade on the other surface. Normally the side with the highest grade is installed facing toward the installer. The grade indicates the surface quality. The thickness of the plywood is what determines its strength.

How Thick Should Floor Plywood Be?

The weight of some things usually stored in a shed, such as a riding mower or other heavy yard equipment, can put a lot of strain on the flooring. Even smaller things can add up to a lot of weight when piled up on one sheet of plywood. Think of a stack of unused cinder blocks or even cans of paint.

Because of the load the flooring will need to bear, the recommended thickness for flooring plywood is 3/4-inch pressure-treated exterior grade plywood graded CDX. This means that one side of the sheets is grade C and the other is grade D. With all the hard usage a plywood shed floor will take, you don’t need to spend money on fancier surface grades.

Another option is using subfloor plywood. It is waterproof and thicker than most types of plywood. It can also be obtained in the tongue and groove form to keep it attached firmly and prevent squeaking.

This flooring is recommended to be used on 12-inch joist centers. Many people put their joists 16 inches apart, but if you plan to use plywood flooring, it would be much better to space the joists more closely to help the plywood handle the load.

Should I Glue Plywood to Floor Joists?

While most people prefer to nail plywood to floor joists, one of the major complaints of this type of flooring is that the floor squeaks after a while. If you do a lot of yard work, you’ll have a lot of traffic moving over the same spots in the floor over and over, which may tend to loosen nails after a time because of the slight flexibility of the plywood.

To fight the squeaking problem, people are turning to subfloor adhesive. One advantage is that you can apply it all along the joists, creating not just attachment points at the nails but attachments all along the joists. Not only does this decrease the possibility of squeaks, but it increases the stiffness of the flooring overall.

What Types of Plywood are There?

Plywood is made from thin veneers of wood peeled from logs. The binding adhesive is applied to the layers, which are laid at different angles to add strength and prevent warping, cracking, shrinking and breaking. Then the composite is pressed and heated. There are many types of plywood available using different types of materials and construction, as well as the grades.

Veneer core plywood is the most basic type, made of layers of wood bonded together. It is considered to be very strong.

MDF core features layers of wood ply with a center core of multi-density fiber or MDF. Since it’s very stable and more consistent in thickness than the veneer core type, it is often used for doors.

Exterior sheathing plywood is made for outdoor use. It can be used as a base for siding or roofing or as lateral bracing on the exterior of buildings. It is rated CDX, which means that one surface is rated C, the other is rated D, and the X means that it is meant to be used outside when covered with another material. The glue used in bonding the layers is also rated for exterior use.

Subfloor plywood is waterproof. It usually comes in greater thickness than some other types, and usually runs 3/4 to 1-1/8 inches thick. It is also offered in the tongue and groove option to prevent squeaking. Check out my article What Are The Best Types of Plywood For a Shed? for more information on choosing plywood for your shed.

Plywood Sheeting Grades

Grade A plywood means that both sides are nearly free of defects and knots. It features a smooth surface that is meant to be seen, which makes it good for such things as furniture and cabinet doors.

Grade B is sanded smooth, but it does have a few defects, which may have been repaired with wood filler or patches.

Grade C is not sanded. It is allowed to have tight knots up to 1 ½ inches wide. There may be larger areas that have been filled or patched. It is good for use under another covering, as it is not meant to be seen.

Grade D is likewise not meant to be seen, with larger knot holes, patches, filler and perhaps some defects that have not been repaired. When there are two letters in the grade, it means that one side is one grade, with the other side having a different grade. For more information on shed floors read my article What Are the Strongest Shed Floor Materials to Use?

How Far Can You Span ½ Inch Plywood?

The usual span limit for ½ inch plywood is 24 inches. This span measurement is predicated on the plywood being used as a subfloor or sheathing for walls. That means that this measurement is not meant for load-bearing flooring. If you plan to use plywood as your only flooring, ½ inch plywood is really not very suitable and must have the joists placed much more closely. Even ¾ inch plywood is recommended to be used with joists spaced at 12 inches.

What Sheeting Can I Use for Walls?

You can use 3/8 inch CDX plywood for wall sheathing. It is not waterproof but is resistant to moisture and rot. It can be painted or stained or covered with another kind of siding. If it will be the only wall material, you might want to consider using a plywood a little thicker.

What Direction Does Wall Sheeting Go?

The strongest way to install wall plywood is to have the grain follow the studs, so installing with the grain going vertically is the way to go.

What Type of Plywood Do I Use for Shed Roof?

The anticipated load on your roof helps determine the plywood thickness for roof sheathing. Usually, 3/8-inch plywood is used for roofing with light loads, installed on rafters or trusses at 16-inch centers. If your rafters are spaced at 24 inches, use ½ inch with h-clips or 5/8-inch plywood. If your roof will be subjected to heavier loads, such as with heavier roofing material or in areas prone to heavy snows, ¾ inch plywood is recommended, regardless of the rafter spacing.

How to Install Floor Plywood?

An easy way to start out installing your plywood floor is to lay out all the plywood before starting to attach it. If the shed will be larger than 4 x 8′, the seams should be offset to add stability. Once you have the plywood laid down, use chalk lines to identify the placement of the joists. Using wood screws for the floor seems to work better than using nails, as they are more secure. The screws should be around eight inches apart.

You can use screws all the way through or use subfloor adhesive. If using glue on joists under the middles of the plywood sheets is awkward, you can use screws on the joists toward the center and adhesive on the joists closer to the edges of the sheets. Be aware that subfloor adhesive takes two to seven days to fully cure, depending on the temperature and humidity.

How to Install Wall Plywood?

You might want to investigate using tongue and groove plywood for the outer walls. This will add stability and make a smoother finish, making an even seal at the seams. Be aware that since this plywood will be subject to the weather, it makes sense to leave a tiny space between sheets to allow for expansion and contraction caused by weather conditions. Some prefer to overlap the plywood panels.

The tools you’ll need are a jigsaw, #9 deck screws two inches long or eight-penny galvanized nails, a drill with a bit for screws, a three-foot T-square, and a pencil. A couple of sawhorses will also help. If you are using foam insulation board, of course, this will be installed first.

You’ll need to figure on starting the sheets at half an inch below the top of the wall. The bottom edges of the plywood should extend at least ½ inche below the wall framing’s bottom plate. The space at the top keeps the siding away from the rafter seat, while the bottom extension keeps rain running down the shed wall from getting into the shed.

Set your first piece on nails placed into the bottom frame to use as temporary props. Slide the panel so it is even with the corner of the shed. Attach one screw or nail to the lower corner, then another at the top, making sure that the top corner is also even with the shed corner. Attach a screw in the center of the corner stud.

Use a tape measure and pencil to mark the rest of the stud locations on the plywood sheet. Finish attaching the sheet using a nail or screw on center around the edges every six inches on center. You only need fasteners every twelve inches in the center part of the sheet. Once you install the first piece of siding you can use that sheet to align the rest. Install the rest of the siding on the walls. In the front of the shed, cut your plywood around the door opening to expose the door opening frame an inch on top and sides if you are building your own door. If you have a pre-hung door, attach the plywood all the way to the edge of the door frame.

If you have a window in your shed, it may be easier to just attach the plywood over the whole wall and then cut out the window opening later, then fastening the edges around the window cutout frame.

How to Install Roof Plywood?

Check the level of your nailing surface first, using a long level or a straight piece of lumber. You can use shims on the rafters or trusses to level out the surface. Position your first panel at the top. Install your fasteners at one end, then use chalk lines to indicate rafter or truss centers.

Install your nails the width of the sheet, then move down to your next row 12 inches down to keep stress from your sheets. Nails should be flush with the sheet surfaces. Nails should be 3/8 inches from all the sheet edges. While nailing, stand on the panels over the framing. If you stand in between the framing, it can cause the plywood sheet to shift slightly.

As with the wall siding, leave a bit of space between the sheets for expansion and contraction. Once you have the plywood attached, you’re ready for your underlayment felt material.

When planning for ventilation if you plan for an enclosed attic, many people use a wood block between some of the rafters on the end plate with holes cut for airflow. In order to keep any insulation or other material from blocking the holes, baffles are available to insert between the truss chords or rafters to keep materials from blocking the ventilation holes.

Conclusion

Plywood is a popular material for shed building, whether using it for flooring or outside walls. Of course, it makes good roof sheathing. In fact, it’s the most popular material for roof sheathing on houses as well as sheds. When planning for your shed build, you’ll need to take load bearing in the various places you intend to use plywood to figure out the thickness and grade of the plywood you’ll need, and perhaps joist placement as well. You may find my article Do I Use Nails or Screws for Attaching Plywood? helpful for choosing the right fasteners.

With proper planning, plywood should serve you well.

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