Friday, January 19, 2007
Trusses are popular roof choices
By Ann Robinson and Annie Vernon
It is not uncommon for remodels to call for changes to a roof. You may be adding a new addition, which will require a roof for the addition and perhaps changes to your existing roof. Or you may be adding a second story, which will most definitely call for a new roof.
Fortunately, the process of building a roof has gotten more efficient in recent years with what has become the most common of roof structure: the roof truss.
It wasn't all that long ago that roofs were routinely "stick framed." A roof went up "stick" by stick" on a job site, and individual pieces of wood were braced in the attic. It is a labor-intensive process,, but this method also left most of the attic open and usable for storage or future living space.
Today the vast majority of roof framing is done with trusses. Trusses are still made of sticks or 2-by-4 lumber, but they are assembled before they reach a job site. They are engineered by the manufacturer and assembled in controlled conditions in a plant. Then they are transported by truck to the job.
For normal residential construction, most trusses can be lifted onto an 8-foot wall and tilted up into position without special equipment, but very large trusses or those going onto a second story are placed by a crane.
Trusses have become a popular choice for a number or reasons. First, they allow for a more open and flexible floor plan because they can span a greater distance than single pieces of wood. They are also easier and faster to install and can be handled by less skilled labor than is needed to hand-frame a roof.
Laborers can often install trusses in a single day, and sheathing can be added in another day or two so the construction job can be dried in quickly — a benefit in any season.
Trusses are also efficient in their use of lumber. While 2-by-4 wood may look a little spindly, when individual pieces are combined in the right configuration, they are amazingly strong. A truss is a series of triangles, which, if you remember your geometry, is a shape difficult to distort under a load. But thanks to computers and engineering software, trusses don't have to be just simple triangle shapes. All sorts of different roof shapes can be engineered to be built with trusses, including hip, mansard, gambrel, and saltbox shapes.
Because of they way they are constructed, trusses do not allow the use of the attic space. One option is an attic truss, which is specifically designed to create a small space in the middle of the truss large enough for a small room or storage area.
Another truss option is a scissor truss. Picture a pair of scissors in the most extreme open position. These trusses have sloped bottom chords to allow for some vault in the ceiling but are appropriate only for very specific applications, since the slope is too low for anything but a small room or a contemporary style home.
Of course, there are still very complicated roofs that do not lend themselves to using trusses. So the days of stick framing are not over, but trusses are definitely here to stay. As always, we welcome your home architect design questions at firstname.lastname@example.org.