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Sizing floor joists for a 2nd floor addition.

Question: 

Hi George--

we have a question about a new addition to our home. We want to add a living area upstairs; it appears that the home originally had only one floor but someone converted the attic (at the highest point, with normal high ceilings) into a couple of small rooms.

We want to add two dormers the full length/width of the house to increase the living area (with full steps going up). At present the floor-ceiling is composed of 2x4's. In order to make it an up-to-code living space, what are the minimum requirements for the floor joists? Any help would be appreciated.

Thank you. (If you can recommend any book we can read on this subject, that would be great).

Kim and Jose

Answer: 

Hello Kim and Jose,

O if things were just that simple! The size of floor joists depends upon the span (length) between load points, floor loads, joist spacing and a few other assorted factors. Its quite obvious that the joists will be larger than 2x4s or 2x6s but may not have to be 2x10s or larger. But none of that is going to help you with your calculations. Let me suggest the following steps:

(1) This is a major project and one that requires design work, some engineering, and permits. And all of that must be done before any physical work begins. That may sound complicated but it doesn?t have to be ? but you can?t do such a job without these basic steps! If you try to by-pass these basis steps then I predict that you will be sorry that you did so. Take a look at: http://www.soundhome.com/topics/topic_building.shtml.

(2) Part of the design work may help you decide if full length dormers fit your style of home. In some home styles, such dormers don?t look very good to me. And you will want the benefit of the designer?s help to help you decide about the look of house you like.

(3) The actual decision about the joist sizes will be one part of the designers and/or engineer?s job. They might also be able to tell you if using a special type of joist, for example a wood ?I? beam may help you solve some construction issues and/or reduce costs.

Let me just get back to the first point. There are many ways to save money in a project like this but skipping the design stage of the process is not one of them.

As for any books, let me suggest that you look at my bookstore page and avoid the ones that tell you about loads and spans. Look for the books about designs and planing.

George