Size Of A Room In An Attic Truss
Question:
Does SBCA have any guidelines regarding the size of a room in an attic truss? I have used a formula, length of room in inches divided by the depth of the bottom chord equaling less than 22. I was curious if any extensive studies or published articles giving more exact guidelines are available.
Answer:
The guidance you mentioned of bottom chord depth no greater than l/22, where L is the floor span, is not unusual. l/24 is often used as a maximum ratio for floor span/depth, but that is not codified in any way that we know of. Some designers use more conservative ratios, such as the l/22 you mentioned or even l/20. The reason for these types of limits is always to avoid problems with floor vibration. Vibration is unpredictable and that unpredictability probably explains why the floor span/depth ratio is not codified). We do not know of any other guidance that is used to limit the size of a room in an attic truss, other than structural calculations or code provisions that limit room height. For specific design or pricing, contact one of our members in your area (see our Members Section for truss manufacturer listings).