How to Put Plywood Flooring in an Attic

The average home today doesn’t have enough closet space for the things we own. Many people like to make up for this by using their home’s attic to provide a little extra storage space, especially if the house doesn’t have a basement that can be used for storage. While attics aren’t designed for storage, some work can transform many into valuable storage space. The trick is to do it without causing any damage to the home.

How can this damage the home? There are two ways you might damage your home. The first is by overloading the ceiling of the floor below, causing it to sag; the second is compressing the insulation, reducing your attic insulation value, and causing an increase in your home energy cost. We don’t want to do either of these, so the method we discuss will avoid those problems.

If you do it yourself, adding flooring to your attic is a relatively low-cost project, running about $5.12 per square foot, about $190 for a 10’x 10′ area. Hiring a contractor to do the job will roughly triple the cost, as you have to pay for their time and labor.

Extreme care should be used when working in an unfloored attic. While the joists can support your weight, the space between them cannot. That space is just ½” drywall, covered with insulation, and mounted to the ceiling below. Accidentally stepping into the space between the joists will guarantee that your foot will go through the drywall, adding a ceiling repair to your project.

Can you put that stuff above your ceiling?

Before doing anything, you must determine if you can floor your attic. Not all attics can be floored easily; it depends on how the home’s roof and roof structure are constructed. The 2″x 4″ structural members used for the ceiling joist part of the truss aren’t normally considered strong enough to support the weight of a floor, the weight of whatever is going to be stored in the attic, and people walking around up there. If the supporting structure is constructed from trusses, it’s much harder to floor it.

On the other hand, if the roof structure consists of a ridge board with rafters, it is ideal for flooring over, providing extra storage space in the home. Typically, the ceiling joists for the floor below, which will be the floor joists in your attic, are made of 2″ x 8″ or 2″ x 10″ dimensional lumber, depending on how great a distance it has to span.

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On-going attic repair, Alex Cockroach

If the floor underneath the attic has a load-bearing dividing wall that runs perpendicular to the direction of the ceiling joists, as most houses do, so much better.

That wall will help support the weight in the attic. If you have this, it will most likely be one of the walls for the hallway, which will run the length of the house, including doorways and archways. The doorways and archways will have headers hidden inside the wall to support the weight of the ceiling and whatever is in the attic.

Other considerations

Most people only floor the central part of an attic, as there isn’t enough headroom to make it worth flooring to the eaves. Sculptured ceilings, with inset areas located in the floor below, may also limit the area that can be floored, as it is impractical to floor over those insets since they are above the floor height in the attic area.

Those areas can still be used for storage if platforms or shelves are built above them. Any time you put shelves around the floored area, such as creating a perimeter for the floored-over area, make sure to extend the flooring under those shelves.

Depending on the design of the house, the HVAC unit may be installed in the attic, often off to one side, so it doesn’t get in the way of flooring the attic. In that case, always leave access to it for maintenance and repair.

How to install plywood flooring in an attic

Before putting the flooring in, take a look at your insulation. Heat rises, so the insulation in your attic is the most important part of your home’s insulation. According to the US Department of Energy, your home’s attic should have an R-38 insulation value for homes in the Southern part of the country and an R-49 insulation value for homes in the North. That’s somewhere between 12″ and 15″ of fiberglass cellulose insulation. Those who talk about energy-efficient homes will come up to R-60, which is even thicker.

When you inspect your attic, compare the current insulation to the ceiling joists for the floor below (floor joists for the attic). Those joists are probably 2″ x 8″, which are 7.5″ high. If the top of the insulation is flush with the tops of the joists, you have R-21, which is what most homes are built with. But if your home has been around for a while, chances are there’s only 4″ to 5″ of insulation because it has crushed down. In that case, you only have R-15 coverage. In either case, you need more insulation.

Since you’re putting in a floor, you won’t be able to insulate your entire attic to R-49; the best you will manage under the floor is about R-38.

Insulating your attic

Now that you’ve inspected your attic’s current insulation and determined how much it has, you want to do what you can to bring it up to par. This is especially important in the area where you want to install the new flooring, as you won’t be able to add any further insulation later.

To start with, add insulation, as needed, to bring the level of the existing insulation up flush with the top of the joists. This can either be done with unfaced batts or blown-in insulation. In either case, the idea is to make up for the crushing of the existing insulation before adding more.

Suppose you are doing this with blown-in insulation. In that case, you will probably want to take the opportunity to add additional insulation around the perimeter of the attic in the areas which are not going to be covered by the new flooring.

To ensure that you have enough to make it even, once the floor is installed, pile up some extra blown-in insulation around the edges, which can later be smoothed out to the level of the new flooring. You have to buy at least 10 bags of the insulation anyway to get free use of the machine for blowing it in, so you may as well go to town with it.

One precaution, though; your attic should have soffit vents installed around the edges. These are necessary to prevent moisture from building up in the attic and provide proper ventilation to prevent excess heat from building up in your attic area. Therefore, you want to make sure you don’t cover these vents. If necessary, block around them with plywood or corrugated cardboard to prevent the insulation from piling up on the vent.

Ensuring adequate insulation under the flooring

To give yourself enough space to ensure adequate insulation under the flooring, construct a riser of 2″x 4″s in the attic over the existing joists. The joists for this riser should run perpendicular to the existing ceiling joists.

This will make installing these new joists easier while spreading the weight across several joists. The riser should be built on 16″ centers, and the ends should be capped with a single cap later. Some people put in a floor without this step and crush their insulation, but that’s not a good idea.

Don’t use nails when building and installing the riser; use screws. The shock and vibration of nailing the joists can cause cracking in the drywall ceiling and the texture to fall off. By using screws, you eliminate this shock and vibration.

With the riser secured, install a layer of unfaced R-13 fiberglass insulation. This layer will help deal with the crushing of the existing insulation and add additional insulation to your attic, helping reduce your heating bills. This is your only opportunity to do this, as having the floor in place will eliminate any future opportunity to add insulation in this part of the attic.

You may end up with too much insulation under your flooring. When the flooring goes down, it will crush that insulation, making it effectively equal in R-value to whatever space you have. So, you are better off moving the excess insulation out of that area and applying it to someplace you are not flooring, allowing it to add to the overall insulation value of your home. This is not a problem, other than it means that you are paying for insulation that isn’t doing you any good.

Flooring the attic

If used only for storage, an attic can be floored with 1/2″ CDX plywood. However, the same can’t be said if you plan to use the space as a living space. In that case, you will need to use 3/4″ thick plywood. As an alternative, 3/4″ OSB can be used. Don’t use 7/16″ OSB, as it is intended for external sheathing and isn’t strong enough to be used as a floor. The building code allows for using OSB in place of plywood for subflooring.

There may be some risks in using OSB, which you should consider. Due to how it is manufactured, it is much more susceptible to the effects of moisture, specifically absorbing moisture. So if you have moisture problems or rainwater leaking into your attic, you should not use OSB, as it won’t last and could be dangerous if it gets wet enough.

Whether you use softwood plywood or OSB, the sheets should be lain perpendicular to the joists, either the original joists or the 2″ x 4″ joists that constitute your riser. Always make sure to cut the sheets to length so that both ends of each sheet land on joists and aren’t hanging off in the air somewhere. The materials we are using, especially the OSB, need that support.

Few attics have access that makes it possible to carry a 4’x8 sheet of plywood up the stairs or ladder, but if you rip it the long way, making the sheet into two ′ wide by 8′ long strips, it is easy to carry up.

There is also a product called “loft panel flooring,” made specifically for flooring attic spaces. This is a particle board plywood, already cut into two-foot panels, for ease in getting the material into your attic. The panels are made with tongue-in-groove long sides, making it easy to join them together.

Like the riser, the plywood decking should be screwed down rather than nailed for the same reason. While nailing would be safe for the floor you are laying, it could cause problems for the drywall and texture on the ceiling of the room below.

Another option is plastic decking panels, a new addition for attic storage. These look like grating, allowing for ventilation of the space underneath the floor. They are engineered to hold up to 250 pounds per panel and come in 16 and 24-inch widths to fit your existing joists. These panels interlock and are attached to your existing joists with 2″ screws.

The real advantage of attic decking is the ease of installation. It is designed as tiles, built to go right onto the existing joists. Their lightweight and size make them easier to carry up into the attic. But they have one drawback; they don’t allow adding additional insulation above the height of the joists.

What about trussed roof attics?

Generally speaking, it is impossible to put flooring into a trussed roof attic and use the attic space for storage. Not only is it difficult to negotiate a safe path through the attic, but the truss’s bottom stringer is only a 2″ x 4″ and not a 2″ x 8″. That’s not as strong.

Okay, so why is a 2″ x 4″ used for the joist rather than a 2″ x 8″? The answer is in the design of the truss. All trusses are a series of triangles, with the occasional pentagon thrown in. That increases the strength greatly, as any weight on the joist will pull down on the webbing, transferring some of the weight to the rafters. While that will flex the rafters slightly, it will mostly push the ends of them tighter together, reinforcing the strength of the truss.

There are limits to this, so don’t try parking an armored personnel carrier in your attic, but if you floor it over, you can walk on it and store things up there.

Now that we’ve got the weight issue out of the way, the next big issue is insulation. If you try to put the plywood flooring directly on the joists, it will crush the insulation down, and you’ll end up with the equivalent of R-15, regardless of what you started with.

To solve this problem, add another course of 2″ x 4″ or even 2″ x 6″ stringers, either on the existing joists or attached to the sides of the truss. To hold the new 2″ x 4″ s in place, cut pieces of plywood that will span the two pieces and use them to screw the two together. This will give the space not only to prevent crushing your existing insulation but also add more on top of it, increasing the R-value of your roof.

With the structure in place, you’ll need to cut your floor decking to match the spacing between the trusses, usually 24″. In addition, you’ll need to notch the pieces of decking to go around the truss webbing. Then lay them in place and screw them down. It’s not ideal, but it can be walked on and used for storage.

In addition, shelves can easily be added between the rafters, spanning the supporting members from the roof rafters to the webbing. Add 1/2″ plywood or OSB to make a shelf.

Additional storage space in the attic

While flooring over an attic will provide considerable storage space, you can consider that just the beginning of getting the most out of your attic. Every nook and cranny in your attic can be used for storage if you can find a way to get to it.

Insets for sculptured ceilings are a common area that goes to waste in most attics, even after the flooring is added. But if you plan it out right, you can install your flooring right up to the edge of those spaces and then build a long, deep shelf over the inset area. I did this over several of the insets in the attic of my former home, creating good storage areas for Christmas decorations, suitcases, and other lightweight items.

Another area that can be treated the same is over ductwork. The existing ducts will often limit the area you can floor over. But that area can still be used for storage if you build a light platform over the ducts. Don’t just lay things on the ducts, as they are weak. Even though they may not crush immediately, they probably will over time, reducing the airflow through your home’s HVAC system and increasing your heating and cooling costs.

Adding shelves around the room’s perimeter is a lot of work, but organizing your things in the attic can make it much more manageable. Two shelves, made of 1″ x 4″ lumber, with plywood decking and leaving some open space below, work out to be ideal for storing most household goods.

Then you can do specialty storage for items that don’t fit neatly into boxes. Fishing rods can be hung on hooks and mounted to the ceiling joists.

Other long, thin items can be stored in the eaves, behind the shelving, where nothing else will fit easily. Backpacks and other camping gear can be hung on the studs of the end walls. It’s just a matter of looking at the problem creatively and seeing what will fit into your available space.

Regardless of what you store or how you store it, be sure it is not sitting directly on the insulation, as the weight will crush down the insulation, reducing its insulating value. Left long enough, it will stay crushed, even after removing the item.

Using attic space as living space

Some people turn their attics into additional living space rather than storage space. This is a much different prospect and requires considerably more work. Since the insulation is in the attic floor, you will need to add insulation to the rafters to make the attic space into usable living space. That will then have to be covered with drywall or paneling and finished.

Building code will also require that you have an actual staircase into the attic room rather than a drop-down ladder. To meet building code requirements, you must also add heat registers and electrical outlets. That shouldn’t be hard, as you will probably already have existing ductwork and wiring that you can tap into to add the additional registers.

When adding in those registers, remember that whatever branch of your ductwork you add them to will probably need to be augmented, in its overall cross-section, to prove the required additional airflow.

While I don’t want to get into detail about this, the idea is that the cross-section of the duct should equal the area of all the registers attached to it combined. So, your best bet is to find a full-size duct with only a few registers. That will allow you to add additional registers without having to do a major redesign to the system.