Plaster walls, first-story framing raise fear of collapse
Bill & Kevin Burnett
Q: I am thinking of finishing off my attic and I am curious if interior remodels can overstress a house frame.
I assume it would be lighter to use paneling, but what about drywall? How much do those 4-by-8 sheets weigh? What about the weight of rolled insulation on the ceiling? I have a plaster interior and am not sure how well it can take weight stresses. How do you know if you must reinforce sections of the frame?
Our house is built above a partially finished garage, so I guess you would call it a soft first story. I keep thinking that this house could use some better framing in the garage. Is a roof frame able to take on the extra weight? Adding an interior wall, putting down ceramic tiles in a kitchen, etc. — these things must really put a load on the frame.
A: Depending on the scope of the work, interior remodeling can indeed, in your words, “overstress a house frame.”
You’ve thrown out a laundry list of “what ifs.” We’ll try to address each one in turn by offering some general guidelines, but the devil is in the details. To flesh out these details, we recommend consulting your local building department or a structural engineer before undertaking major structural renovations.
To begin, adding an interior wall or ceramic tile to a kitchen counter will not affect the structural integrity of the house. However, tiling a kitchen floor may require beefing up the floor joists to take out some of the flex in the floor.
Determining whether the framing can handle a particular use depends on the load the use will put on the structure. Design loads are split into three categories: dead load, live load and concentrated load. Design loads are measured in pounds per square foot (psf).
A dead load is the weight of an immovable load on the structure, such as the weight of composition shingles transferred to roof rafters.
Live loads consist of the weight of all moving and variable loads that might act on the structure — people moving across a floor, for example.
A concentrated load might be the wheels of an automobile on a garage floor.
You wrote us a while back about installing an attic ladder with an eye toward utilizing the attic as extra storage space. It seems that now the scope of the work has increased to making the attic usable living space. With this in mind, we make some observations, working from the top down.
Roofs in older homes are usually framed with 2-by-4 rafters. Since you have room in the attic to stand, the pitch of the roof is fairly steep. To determine whether the rafters will handle the extra weight of the drywall, you’ll need to know the number of layers and type of roofing, the length of each rafter and the spacing between them. With this information, you can determine whether the rafters can handle the dead load of the drywall.
A horseback guess is that a piece of drywall weighs about 60 pounds, or a dead load of about two psf. If you determine that the rafters are undersize, it’s a fairly straightforward job to sister another rafter from the ridge to the top plate, doubling the thickness and increasing the strength of the rafter to allow for the installation of drywall.
Rolled insulation is not heavy enough to affect the plaster ceilings. However, if you are planning on laying a plywood or oriented strand board (OSB) subfloor on the ceiling joists and walking up there, the added live load could cause the plaster below to crack.
The existing ceiling joists are probably 2-by-4’s — not hefty enough for floor joists.
Depending on the span between bearing walls (the exterior walls and an intermediate wall or two transferring loads to the foundation), floor joist size can range from 2-by-8’s and up. The solution is to sister new joists to the existing ones, making sure that the new material rests on bearing walls, to strengthen the existing joists.
The attic renovation should not affect the overall structural integrity of the house. But given that you live in earthquake country and the house is built over a garage (the soft first story), the prudent course is to explore the possibility of a seismic retrofit.
A structural engineer can help you here. In addition, there are firms that specialize in seismic retrofitting. You may be asked to pay a fee for the estimate, but if you get a detailed plan and the estimate fee is credited back if the company does the work, it’s well worth the cost.
Copyright 2008 Bill and Kevin Burnett