Determining The Load Capacity Of Trusses
Question:
I am doing research for clients who have flat roof trusses to which they are adding a second story. We are now looking for information on the trusses to determine what their load capability is. What do you need to know to help us determine this?
Answer:
SBCA is a trade association that represents truss manufacturers all over the country. As such, we don’t actually perform truss design and can’t give a definitive answer to your question. However, we recommend that you contact an engineer that has experience in trusses before you continue building a second floor on top of the existing trusses. Roof trusses generally have different loading conditions than floor trusses, and assumption that you can safely add additional loading to the structure is a huge one.
Because the trusses were manufactured so long ago, it’s likely you’ll have trouble finding the truss design drawings. In absence of the original designs, you would have to re-create the design by looking at the parts that make up the truss itself and then analyzing the truss.
First look at the metal plates. In the last few years, plates have been marked to distinguish the plate manufacturer. For example, a 20-gauge TrusWal plate is marked TW-20. Once you know the plate manufacturer and gauge of plate, you then need to measure the plate sizes. The type and size of the plates will determine the capacity of the metal plates. Next you need to look at the lumber. What size, grade and species is it? This will also affect the loading capacity of the truss. Once you have all this information you would need to re-analyze the truss to determine if it is capable of carrying the new loading.
Again, we strongly suggest you contact a professional who can actually sign off on the load capacity after looking at all the factors (see our Members Section for truss manufacturer listings