Expert: Bruce E. Johnson - 5/2/2011

Question
QUESTION: I want to build a 15x18 room under my house. I have a few concerns that I would like an experts opinion on my plans.

flooring: currently it is gravel with 6mil vapor barrier. Lowes carries a handiblock that allows you to lay joists into the handiblock and it raises the floor about 6 inches off the ground. So i had planned on using those, and the joists would be 2x6 pressure treated. I would also add another 6mil vapor barrier before setting the handiblock. The subfloor would be 23/23x4x8 tongue groove OSB. self adhesive tile would cover the OSB.

walls: currently 2 of the walls I would be building would be against block walls that have blown insulation on them. I had planned to use standard 2x4 studs untreated, r19 insulation and vapor barrier on the back. The interior of the wall would have mold resistant drywall.

ceiling: drop ceiling so I would have access to gas, electric and water lines.

My main concerns are rot and mold. I want to make sure to keep that out as best I can, but my budget can't afford to have that area poured with a slab and then build the room. I'm open to other foundation methods, or insight into mine, as it is also a concern to not build out a room and then have a major settling issue.

ANSWER: hi Matthew, it sounds like you have a decent plan in mind but it seems to me that pouring a concrete slab would be cheaper than building a floor system unless you are able to build the floor system yourself and have to subcontract out the concrete work which may cost a little more.  A concrete slab with a good vapor  barrier underneath outperforms a floor system in this instance as far as keeping moisture under control.  With a floor system you have to keep air circulating to avoid mildew problems, hence the foundation vents  you may have around your existing foundation.  Most crawl space floors are required to be 18" clear of the ground or 24" to the bottom of the decking if a building permit is needed for this project.  I can't say what your existing soil conditions are and whether or not your support system for the floor joists will settle over time.  If the area in question stays relatively dry throughout the year and the soil is fairly stable (not mucky or all clay) then you shouldn't have a problem.  An average soil bearing capacity should be around 1500 pounds a square foot..if those foundation blocks are 12"x12" then each one should theoretically be able to support 1500 pounds if your soil is compacted properly.  Don't forget to spray the area for termites and use pressure treated wood for the floor joists and post and beams..sincerely bruce e johnson

---------- FOLLOW-UP ----------

QUESTION: Thanks for the prompt response. the concrete work would cost a little over $2000 since I would have to subcontract that all out. my entire project budgeted is right at $2500. If I removed my flooring costs and added in the concrete it would be just under 4000 to do this room. the flooring currently is clay/dirt with about 3 inches of gravel on top and then a 10 mil vapor barrier.

Answer
Hi again Matthew, well, like I said before, most standard building codes require crawl space floor systems to have at least 18" of clearance to the bottom of the joists.  But good luck with your project and should you proceed with your plan as stated make sure you allow for some screened vents around the perimeter to allow the floor system to breathe.  These vents can be found at Lowes as well.  Sincerely Bruce Johnson