I have an open attic with 2x8 ceiling joists, 16" on center. I would like to convert this space to low-traffic living space, eventually to house a model railroad layout. No bedroom, no bathroom, no real seating, just one large open area with the layout against the walls. Since my roof is stick-built (just the ridgepole and stringers, no trusses), I don't have to worry about roof support, do I? Are the single 8x2 joists sufficient subfloor to support the occasional weight, or should I sister them with another 2x8 each? I'm planning on putting in 2x6's for the wall joists prior to insulating it and then drywalling it.

I'm also planning a renovation of the house with an addition, which will expand the attic space and may change the requirements for attic flooring. For thoroughness, here is a floorplan of the space below the attic area I want to finish prior to the renovations: pre-renovation
The span between the outer wall (left-most wall) and the rightmost kitchen wall is ~24 feet. The converted space would stop there, and would only be as wide as the garage for the area above the garage. Ceiling joists are currently strung left to right in these floorplans.

Post-renovation, here is the floorplan: post-renovation
The master bathroom is conditioned space, the workshop will most likely be slab, unconditioned space. The space above the new master bathroom would not be converted to living space, the space above the workshop would.

  • What is the span? Are there walls below that will take some of the load? – Ed Beal Sep 11 '16 at 15:36
  • The maximum span would be about 31 feet - the longest span unsupported by walls is 20 feet, and most of the space would only be a 14 foot span. – John Sep 11 '16 at 21:21

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