When looking for additional areas to create more living space in the home, the attic space—the empty territory between the ceiling of the upper story and the roofline of the house —is a logical place to consider. If you're not able to build a full-fledged room addition, the attic (along with the basement) is one of the few unused spaces that has potential for meeting your needs.
However, there are several considerations that must be met for an attic to be suitable for converting to living space, and some involve the structure of the floor. Unless the floor is suitable for carrying the additional weight of an active living space, the attic can't be converted.
Framing Structure: Rafters and Joists vs. Trusses
The way your home was framed during construction will have a major impact on the potential for converting your empty attic into additional living space. The reality is that many modern homes are not suitable for attic conversions, simply because of the way in which they were framed.
Traditional Joist and Rafter Framing
In older homes (before the 1960s), the top ceiling platform was usually framed with a standard platform made from parallel joists spanning across outer load-bearing walls. The roof structure rests on the outer portion of the platform and is supported by the load-bearing walls, while the floor of the attic and the ceiling of the upper story are the same platform. If the joists used to frame this platform are large enough, then there is a good chance that they are sufficiently sturdy for the attic to be finished off as living space that can support flooring, furnishings, and people.
If your home already has a walk-up attic that is used for utility storage space, then it may have the 2 x 10 or 2 x 12 joists necessary to support the added load of a finished attic. But if the ceilings are framed with 2 x 6s or 2 x 8s, this will not provide the strength necessary. If access to your attic is through a hatchway or drop-down ladder, the joists may be too small to support the added weight of finishing the attic.
All is not lost if this is the case, however. It is possible to reinforce the attic floor by installing additional "sister" joists alongside the existing 2 x 6 or 2 x 8 joists. This can increase the load-bearing capacity of the floor and make it possible to finish the attic. Consult a builder to determine if the existing attic floor structure is suitable, or to learn what additional structural work might be required.
Truss Roof Structure
The issue becomes more clear-cut in a home where the roof structure is formed with trusses. These prefabricated units are commonly found in newer construction (1960s and later). Trusses are factory-built roof framing units consisting of metal connection plates that join the framing lumber pieces together—angled roof rafters, ceiling joists, and reinforcement struts—into a web-like unit truss that can be quickly added atop the framed outer walls by a construction crew. Once all trusses are installed, they provide the framework for attaching the ceiling drywall as well as the roof sheathing and roofing materials. The structure of trusses allows the outer wall to carry all the weight, which means the ceiling members need to be only strong enough to carry drywall and insulation in the ceiling.
If you have a home where the attic space is framed with roof trusses, it generally cannot be converted to a walkup addition—the nature of the framing makes it impossible. Not only will the floor be insufficiently sturdy to support the weight, but the presence of the truss webbing will make it impossible to create a wide, accessible space in the attic. The best you can hope for is perhaps to lay some thin plywood platforms down over the bottom truss members and use them to store lightweight items.
If you do have an existing walk-up attic with traditional floor framing, the principle concern when converting the space into a living area will be weight. Even if you have 2 x 10 or 2 x 12 joists spaced 16 inches on-center (OC), it is a good idea to limit the amount of weight the attic floor will bear. If the joists are spaced wider (24 inches on-center is quite common), the weight-bearing capacity is even less. It is usually a bad idea to place large, heavy bathtubs, pianos, or other heavy items in a converted attic space unless a skilled builder reinforces the floor. And natural stone, ceramic tile, and other heavy flooring materials should be used with caution, and only after consulting a structural engineer or building contractor well-versed in live load calculations.
There are many insulation and ventilation issues that come into play when converting an attic to living space, but the floor itself is sometimes overlooked. Attic floors are often heavily insulated, and if the existing insulation extends well above the floor joists, laying down floor sheathing over the insulation will compress it. Insulation derives its resistance to thermal transfer (its R-value) through the presence of dead air space, so compressing the insulation will actually reduce its R-value. In colder climates, this may not be a serious problem, since the heat rising up into the attic from below may be welcome. But in warmer climates or in the summertime, a well-insulated floor, combined with good ventilation, will keep the attic cooler and lower cooling costs.
The ideal solution is to make sure the joist cavities below the attic floor sheathing are fully filled with bat fiberglass or blown-in cellulose or styrofoam insulation. Not only will this minimize heat loss or gain in the attic, but it will create a good sound-deadening layer to muffle the sound of footsteps to the level below.
The proper flooring materials can also improve the thermal- and sound-insulating properties of the floor. Carpeting with a good underlayment pad, or laminate flooring with a dense foam or cork underlayment, are good choices.
If more insulation is wanted, it is also possible raise up the attic subfloor by laying sleeper strips across the floor joists before installing a subfloor, thereby increasing the vertical space that can be filled with insulation.
Informal Storage Options
Where a full walk-up attic is not practical, there are some easy options that will help you create more storage space in the attic.
Partial Attic Flooring
If you are just looking for extra storage space, consider the minimalist approach of installing flooring in just part of the attic. For example, if your attic is accessed through a hatchway or dropdown ladder, you can install a few thin plywood panels across the joists or attic trusses within reach of the hatchway. This can serve as a useful overhead "closet" without adding too much to the weight or detracting from the thermal seal of home.
Attic Deck Panels
There are several do-it-yourself attic flooring solutions available from retail vendors. These lightweight plastic squares are designed to be attached directly onto the joists in the space and distribute weight placed atop them in an even and safe manner. They are also built with open grate slots that allow for ventilation of the insulation. These products are generally rated for a load of about 250 pounds, so use caution when storing heavy items or walking around in the attic.