Expert: Bruce E. Johnson - 12/12/2008

Question
Hi, I have a victorian terraced house and would like to use our attic either as storage or as a room if possible. I haven't been up there yet as the access is hard to get to. I intend to have a proper recce after Xmas. Can you tell me what size the joists need to be to put down a floor that is safe for walking on, or for using it as storage please? If our joists are not suitable then I will know when I've seen them and will be able to shelve this idea!
Thanks!

Answer
Hi Amy, floor joist spans can be figured using an old "rule of thumb" method.  Generally the floor joist spans from an outside wall to an interior bearing wall or several interior bearing walls.  The distance from one wall to the next is the span.  The rule of thumb is one foot of span per one inch of board thickness, provided the board is 2x material (1 1/2" nominal width.  In other words if your joists are 2x8 then the suggested span would be eight feet.  Now you can fudge on that a bit, especially if your home has been built with "rough" lumber and the wood is actually a full two inches wide as opposed to more modern lumbers that are planed down to nominal thicknesses.  I wouldn't take a 2x8 more than ten feet for a solid floor system.  You didn't mention what the ceiling joist size is so I can't say one way or another if it is acceptable.  One thing you can do if the joists are inadequate is to add a new thicker joist alongside the old one, attach them together and then use the new joist as your floor joist.  I hope this information helps, feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com