Span Tables

By Tom Tynan

Not knowing what size lumber to use when building any type of structure, whether it is a house, a deck or perhaps a second floor patio, can create a great deal of confusion with the homeowner. Since detailed information is not readily available, many people play it by ear and choose whatever they feel is best or choose the size lumber based on the best deal a store is offering. Unfortunately, what we see happening later is a myriad of problems such as second floors beginning to sag, sheetrock bending or ceiling joists in a garage flexing under pressure. These types of problems are serious and very expensive to remedy. Many times the homeowner attributes these types of problems to foundation failure when in reality it is a basic framing problem. This is the reason that I constantly try to lead people to use what is known as a “SPAN TABLE”.

There are a number of different span tables available; however, some are very involved and confusing. They involve the use of formulas, dealing with bending moments, live and dead loads, and other things of this nature. This is the reason most individuals involved in the building industry have always relied on the “user friendly” Span Tables published by the FHA. I have printed the FHA “Span Tables” in this issue of the magazine so you will have these requirements readily available when you undertake your next project. As you can see, these tables address floor joists, ceiling joists and of course the roof rafters.

The floor joists are the most critical because they take the greatest load, both a live and dead load. The dead load is designed into the span tables, but the live load can be done different ways. Basically for homes, you design with either a 40 or 50 pound live load weight. The difference would be if you wanted to be real scientific and use 40 pound for sleeping areas, bathrooms, and areas prone to a lower number of people. Yet, the living areas you would go to a 50 pound because it will have higher stress due to the number of people. If you decide that you just want to create a stiff structure, 50 pound would be your choice. Most homes are designed with a 50 pound throughout the entire house because it works well, is easier, as they can use the same span table, and it eliminates future problems with sagging (particularly with the use of waterbeds, bookcases, heavy antiques, etc.)

Correct sizing of ceiling joists is important and needs to be based on what type of ceiling you plan to hang. For years plaster water was used, now we use sheetrock, and depending on whether it is on 16 or 24 inch centers you have the option of either - or 5/8 inch sheetrock. This type of ceiling can be heavy, and if not sized properly, a few years down the road problems like sagging can occur. Think about it, if you combine both and you have the floor above and the sheetrock below, it is critical that the design has allowed for this heavier load.

As you will also notice on the rafters, there is a 3 in 12 pitch dealing with 3 in 12 or less, and over 3 in 12. This is very self-explanatory if you follow the rules. Just a quick tip; anytime you use rafters, you are going to have a ridge going down the middle of the structure. The ridge needs to be at least one size larger because the rafters must fit flush at the top of the ridge. Always make sure that you get a full face fitting, and that usually means at least one size larger on typical roofs. If you are dealing with a very steep roof, two sizes larger on your ridge are recommended.