The next step is to check for headroom and over all floor space. Most building codes call for 7½ ft. of headroom over 50% of the “usable” floor space, which is defined as any space with a ceiling height of at least 5 ft. Remember that these minimums apply to the finished space—after the flooring and ceiling surfaces are installed. Other things can affect headroom, as well, such as reinforcing the floor frame, and increasing rafter depth for strength or insulation.
You may also find various supports in your attic that are there to strengthen your roof but may limit your space. Collar ties are horizontal boards that join two rafters together in the upper third of the rafter span. They prevent rafter uplift in high winds. Often collar ties can be moved up a few inches but can't be removed. Rafter ties join rafters in the lower third of their span to prevent spreading. In most attics, the ceiling or floor joists serve as rafter ties. Purlins are horizontal boards that run at right angles to the rafters and are supported by struts. These systems shorten the rafter span, allowing the use of smaller lumber for the rafters. You may be allowed to substitute kneewalls for purlins and struts. If you’ll need to have any support system altered or moved, consult an architect or engineer.
You can add to your floor space and headroom by adding protruding windows called dormers. In addition to space, dormers add light and ventilation to your attic.
The rafters themselves also need careful examination. Inspect them for signs of stress or damage, such as cracks, sagging, and insect infestation. Look for dark areas indicating roof leaks. If you find leaks or you know your roofing is past its useful life, have it repaired or replaced before you start the finishing process. and even if the rafters appear healthy they may be too small to support the added weight of finish materials. Small rafters can also be a problem if they don’t provide enough room for adequate insulation.
At this point, it’s a good idea to have a profession al check the structural parts of your attic, including the rafters and everything from the floor down. In some cases, finishing an attic is like adding a story to your home, which means that the structure must have adequate support for the new space. Attic floors are often built as ceiling frames for the level below and aren't intended to support living space. Floors can he strengthened with additional joists, known as sister joists or with new joists installed between the existing ones.
Support for the attic floor is provided by the load-bearing walls below and , ultimately, by the foundation. If these elements can’t support the finished attic, they’ll need to be reinforced. This may be as simple as strengthening the walls with plywood panels or as complicated as adding support posts and beams or reinforcing the foundation.
In addition to these structural matters, there are a few general code requirements you should keep in mind as you inspect your attic. If you plan to add a bedroom, it will need at least one exit to the outside. This can be a door leading to an outside stairwell or an egress window. Most codes also have minimum requirements for ventilation and natural light, which means you may have to add windows or skylights.
One of the largest expenses of finishing an attic is in providing access: You’ll need a permanent stairway at least 36” wide, with room for a 36” landing at the top and bottom. This is an important planning issue because adding a stairway affects the layout and traffic patterns of the attic as well as the floor below.
Finally take an inventory of existing mechanicals in your attic. While plumbing and wiring runs can be moved relatively easily other features, such as chimneys, must be incorporated into your plans. This is a good time to have your chimney inspected by a fire official and to obtain the building code specifications for framing around chimneys.