It's an unclaimed space, full of architectural potential. But remodeling an attic can be tricky because its floor joists may not be up to modern rigidity standards, those appealing rafter ceilings may have low-slung collar ties that block all that apparent headroom, and the roof rafters may not have sufficient depth to accommodate the necessary insulation.
Stiffening Spongy Floors
Existing attic floor joists may not be stiff enough to meet current codes for a bedroom. If you have enough headroom to double-up attic floor joists with 2x10s or 2x12s, you're lucky — most of the time, old attics have 2x6 or 2x8 floor joists at best, and the joists typically have a notch cut out at the ends to accommodate the sloping rafter ceilings, which makes it impossible to insert deeper members. An obvious idea is to double up the existing joists by sistering (or scabbing) another 2x6 or 2x8 alongside the existing joists, but that won't solve the problem. Floor stiffness is a matter of joist depth, not width — a double 2x8 sandwich will not resemble the stiffness of a single 2x10.
Attics are tempting spaces to reclaim and remodel, but often the floors were not built to handle the loads typical of living areas. Remodeling an attic may entail beefing up the floors as well as widening stairway access and lifting the ceiling ties to obtain more headroom.
The solution to stiffening up a floor requires a combination of adding midbay joists, laminating the subfloor and possibly adding girders. Although sistering joists won't add much stiffness, reducing the joist spacing by adding new joists between the existing ones will. If you consult a joist/rafter span table (available at most lumberyards and in every building code book), you will find the maximum distance that your existing attic joists can span to meet current code as bedroom floors. (Note that bedroom floors have lower stiffness requirements than living room floors: 30 pounds per square foot (psf) for bedrooms vs. 40 psf for other living areas.) You may find that adding another joist between each pair will create enough stiffness to comply with codes: For example, 2x6s spaced 12 in. OC are equivalent to 2x8 joists spaced 16 in. OC. Even if the added joists alone are not sufficient, they may be part of a two- or three-pronged solution.
You can further stiffen the floor by adding a thick layer of subfloor, such as 3/4-in. tongue-and-groove plywood (which is lighter and stiffer than oriented-strand board) glued and screwed to the joists. I know one builder who laminates two layers of subfloor perpendicular to each other to strengthen attic floors without adding new joists. However, a structurally sound floor can still feel bouncy, and floor-joist codes are based on minimum prescribed stiffness, not just holding up the weight of people, furniture and walls.
The final weapon in the floor-stiffening arsenal is to add a beam, but not necessarily under the floor joists because that may interfere with the ceiling below. Instead, you can add a beam (or girder) on top of the joists (illustration below). This solution works best when it coincides with a wall location that can hide the beam — otherwise you'll trip over it. Here's how it works: Set an adequate-size beam — probably a 4x10 — near the midspan of the floor, where you may be building a wall anyway. Fasten the beam to the joists below using framing straps or oversize joist hangers. The beam will distribute the load across all of the joists to enhance stiffness without compromising headroom.
Hanging from a beam: Although a beam usually works from underneath a load, in certain circumstances you can place the beam above and use joist hangers to attach the joists to it. This works best in attics where new walls can act as bridge girders to hold up the structure without having to lower the ceiling below. Often, spreading the load across the rafters is all you need to stiffen an otherwise structurally sufficient floor system. Illustration from Affordable Remodel: How to get Custom Results on Any Budget (The Taunton Press 2007). Illustrator: Robert LaPointe.
Sometimes you'll have to transfer this load down to a bearing wall below or to the foundation by means of posts or vertical supports. An experienced carpenter with a code book in hand or a creative structural engineer can usually resolve even the surliest floor-reinforcement situations, so don't worry too much about the solution, just know that solutions exist.
Remedies for Headroom Headaches
Collar ties keep the weight of the roof from spreading the exterior walls of your home. Floor joists running parallel to the roof rafters serve the same purpose, so you may be able to remove existing low-slung collar ties entirely. But you should consult with a building official or structural engineer to make sure this solution is adequate for handling snow loads, wind and other roof stressors. If you're going to install a flat portion at the apex of your attic ceiling (as seen in the photo), you can replace the existing collar ties with short ceiling joists attached to your existing rafters using plywood gussets firmly glued and screwed to both sides of the joists and rafters, essentially using your new attic ceiling as a replacement for the old collar ties you removed.
Raise the Ceiling: Lifting the ceiling can make a dramatic change in the appearance of a room. If your attic isn't quit big enough to turn into living space, consider raising a portion of the ceiling below. Illustration from Affordable Remodel: How to get Custom Results on Any Budget (The Taunton Press 2007). Illustrator: Robert LaPointe.
Handyman Club Life Member, Fernando Pagés Ruiz, is a homebuilder and remodeler with 30 years' experience and the author of Affordable Remodel: How to Get Custom Results on Any Budget (The Taunton Press, 2007).