How to Put Plywood Flooring in an Attic (Over Insulation)

The average home today doesn’t have enough closet space for the things we own. One way many people like to make up for this is using their home’s attic to provide a little extra storage space, especially if the home doesn’t have a basement that can be used for storage. While attics really aren’t designed to be used for storage, with a little bit of work, many attics can be transformed into a useful 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 by compressing the insulation, reducing your attic insulation value and causing an increase in your home energy cost. Obviously we don’t want to do either of these, so the method we are going to discuss will avoid those problems.

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, if you do it yourself. 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 are able to support your weight, the space between them cannot. That space is just ½” drywall, covered with insulation and mounted to be the ceiling below. Accidentally stepping into the space between the joists will pretty much guarantee that your foot is going to go through the drywall, adding a ceiling repair to your project.

Can you put that stuff above your ceiling?

Before doing anything, you have to 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 is constructed. If the supporting structure is constructed out of trusses, it’s much harder to floor it. 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.

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 the 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, so that they can 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 all the way to the eaves. Sculptured ceilings, with inset areas, located in the floor below may also limit the area that is able to be floored, as it is impractical to floor over those insets, since they are above the floor height in the attic area.

Even so, those areas can still be used for storage, if platforms or shelves are built above them. Any time you are going to put shelves around the floored area, such as creating a perimeter for the floored-over area, be sure to make the flooring extend 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 be sure to 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 valued for homes in the Southern part of the country and an to 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 up this 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 actually 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 that 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 going to be putting in a floor, you won’t be able to insulate your entire attic to R-49; about the best you’re going to 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 that 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.

If you are doing this with blown-in insulation, 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. In order 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, in order 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, as well as 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 the installation of these new joists easier, while spreading the weight across several joists. The riser should be built on 16″ centers and the ends capped with a single cap later. Some people put in a floor without this step and end up crushing their insulation, but that’s not a good idea.

When building and installing the riser, don’t use nails, 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 in place, install a layer of unfaced R-13 fiberglass insulation. This layer will help deal with the crushing of the existing insulation, as well as adding additional insulation to your attic, helping reduce your heating bills. This is your only opportunity to do this, as having a floor in place will eliminate any future opportunity to add insulation in this part of the attic.

It may be that you end up with too much insulation under your flooring. This is not really a problem, other than it means that you are paying for insulation that isn’t doing you any good. 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.

Flooring the attic

An attic can be floored with 1/2″ CDX plywood, if it is being used only for storage. However, the same can’t be said if you are planning on using 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 the use of OSB in place of plywood for subflooring.

There may be some risk in using OSB, which you should consider. Due to the way 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 actually be dangerous if it got wet enough.

Regardless of 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 2′ 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, so that it is 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 are 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, made to fit your existing joists. These panels interlock and are attached to your existing joists with 2” screws.

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

What about trussed roof attics?

Generally speaking, it is not considered possible 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, there is also the problem that the bottom stringer of the truss is only a 2”x 4” and not a 2”x 8”. Obviously, 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 that is put on the joist is going to pull down on the webbing, transferring some of the weight to the rafters. While that will flex the rafters slightly, mostly it will 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 out with.

To solve this problem, add another course of 2”x 4” or even 2” x 6” stringers, either on top of the existing joists or attached to the sides of the truss. This will give the space to not only prevent crushing your existing insulation, but also add more on top of it, increasing the R-value of your roof. 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.

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 simply 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 a considerable amount of storage space, you can really 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 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. Often, the existing ducts will limit the area that 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 themselves, as they are not very strong. 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 perimeter of the room is a lot of work, but can make it much easier to organize your things in the attic. Two shelves, made of 1”x 4” lumber, with plywood decking and leaving some open space below, work out to be just about 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, mounted into 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 the space you have available.

Regardless of what you store or how you store it, be sure that 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 down, even after the item is removed.

Using attic space as living space

Some people turn their attics into additional living space, rather than into storage space. This is a much different prospect and requires considerably more work. Since the insulation is in the floor of the attic, you will need to add insulation to the rafters as well, so as to make the attic space into usable living space. That will then have to be covered over 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. You will also have to add additional heat registers and electrical outlets to meet building code requirements. 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, keep in mind that whatever branch of your ductwork you are adding them to will probably need to be augmented, in its overall cross-section, to be able to prove the additional airflow needed.

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 duct which is full-size, yet only has a couple of registers. That will allow you to add additional registers, without having to do major redesign to the system.

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