Expert: Roger Zona - 8/2/2009

Question
Roger,

We have recently moved into a new Modular home and I am in the process of finishing the upstairs (the upstairs came unfinished).  One of my goals in the home is to incorporate solar for domestic hot water and heat. My original plan was to build a large under ground heat storage tank near the house, but outside; I still plan on doing this; however, I have been thinking about adding an additional, smaller hot water storage tank in the attic space above the stairwell.   The stairwell is almost exactly halfway in between two "marriage walls"  giving contiguous support to the 2x12 ceiling/floor joists between the first floor and the second floor and the 2x10 attic joists (the attic is essentially a "third floor").   The 2x12 span between the "marriage walls" is 13 feet (again, with the two walls of the stairwell squarely in the middle).  The 2x10 "attic joists" span 16 feet, but, again, are supported in the center by the stairwell.  

My plan is to put down 3/4 Advantech to the top of the 2x10s, then run 2x8s perpendicular to the joists, then deck the 2x8's with another layer of 3/4 Advantech.  My question is, how much weight is safe?  I would like ~1000 gallons of storage, but this would be ~150#'s/SF of static load from the water alone.  The building materials could easily push the total static load to 160#'s/SF.  The tank I am considering building is 6ftx6ftx4ft inside dimensions, with 7.5ftx7.5ftx5.5ft outside dimensions.  I can easily tie the floor joists to the roof rafters either as an integral component of the tank, or just outside of the tank, but I am unsure if this would help.  

Is 160#/SF asking too much of my existing structure?  Obviously making the tank larger and lower would decrease the theoretical load/SF, but intuitively I have a difficult time believing that spreading the 8500# load out more is going to fundamentally alter the way the forces are translated to the foundation.

If 1000 gallons is too much weight, what would you consider a safe and reasonable volume?  

(I have given a lot of consideration to redundant leak protection, baring catastrophic tank failure, I am not concerned with leaks.  The main tank will be fiberglass lined and gel-coated.  The tank will have a 12in "skirt" with a pond liner and two 3in drains.  The tank will NOT be pressurized, but rather will employ heat exchangers.)

Thank you in advance for taking the time to read through my question.  If you need any further details, please post them and I will respond.

Hunter

Answer
Hello Hunter,

Your question comes up quite often - how much weight will my floor system support.

I wished I had a better answer.  

Most wood frame structures are designed and engineered to carry a "live (or static) load" of 30 to 40 lbs/s.f.  Far less than the 160 you propose.  

I remember, as a young boy, our home (in Michigan) had a water tank in the attic.  It was for the collection of rainwater before city water was available.  I never saw the tank in use but it was at one time.

In order to increase the live load capacity you must have bigger joists and they must be installed closer together.  In other words, the 2x10's at 16' span and probably 2' on center, would have to be enlarged to a 3" or 4"x16" member and 12" o.c.  Additionally it would be advisable to have a center support and perhaps a lintel and columns in the wall for added support.  Yes, if you made the tank considerably larger in contact area (but not more gallons of water, you could use the existing framing.  I'm guessing the tank would be much too large.

I suggest you contact your building department and ask about the code requirement for your project.  Also you might want to contact a structural engineer to advise you on what you need.

I hope this has been useful.

Roger