Many attics, old and new alike, are not framed with storage or living space in mind – they are simply a dead space between the top of the ceiling and the bottom of the roof. When looking at the ceiling joists to determine their weight-bearing capacity for storage, there are three things to take into consideration – the size of the joists, the center-to-center distance between them, and the distance between the walls that support them.
Once you've determined these three things—all of which can be ascertained with a quick look in the attic—discuss the situation with your local lumber yard. They have lumber tables know as span charts that will allow them to help you determine how much weight the attic can support, and what, if anything, you may need to do to reinforce things.
If you have a newer home and your attic framing has been done with engineered, manufactured trusses, storage becomes more difficult. Engineered trusses are very strong, but their configuration of intermediate members between the ceiling and roof make accessible storage somewhat impractical. Remember that you cannot cut trusses, so if you would like to try and devise some storage up there, your best bet is to discuss the situation with an experienced remodeling contractor or with a local truss manufacturer.
If the attic will safely allow for storage, then access to it is the next consideration. Most attics are accessed via a simple hatch in the ceiling, which is practical only for very limited storage that you won't need to access very often. A better solution is to replace the attic hatch with a pull-down attic ladder. These come in both wood and aluminum varieties, in sizes to fit just about any standard application. They are inexpensive and easy for the do-it-yourselfer to install, and are readily available through most lumber yards and home centers.
Next, you need a floor to place your stored items on. Plywood is an ideal material for this use – use ¾-inch thick tongue and groove sheets, and install them using adhesive and either screws or nails. The drawback to plywood, however, is that it's often difficult to get the sheets into the attic. If plywood is impractical, 1x6 tongue and groove boards may prove to be an easier solution. In either case, you'll find it easier to measure and precut the sheets or boards on the ground before taking them into the attic.
Remember also that you cannot cut any of the intermediate supports or braces that help support the roof structure, so you'll need to work around them. You will also need to be careful with any electrical wires that are laid across the tops of the joists – you'll either need to reroute them into the joist cavities, or furr up the joists so that the wires are safely below the storage area floor. Whatever you do, make sure that the wires are not subject to damage when installing the attic floor, or from inadvertently stepping on them when moving around in the attic space.
Lighting is the next thing that makes for a safe and easily accessible attic – in fact, most codes require that if the attic is accessible by stairs or by a ladder, it must have a light that can be turned on and off at the attic's entry. Depending on the amount of headroom you have, simple porcelain single-bulb fixtures may be easy to install and provide enough light. If you cannot maintain an adequate amount of clearance between the bulb and any combustible material, then you'll need to use a fluorescent fixture instead – they produce considerably less heat.
Remember that attic conversions that require structural framing or electrical wiring will require a building permit, and your local building department can help you with understanding the applicable codes. If you have any questions whatsoever about the structural integrity of your attic or how to work with the necessary structural, electrical, or other changes involved, do not proceed with any work – consult with your building department, an experienced remodeling contractor, or even a structural engineer before going any further.
