Expert: Bruce E. Johnson - 5/9/2009

Question
We're getting a 120 gallon aquarium with about 35 more gallons of water in the sump below, in the ballpark of 1500lbs.  The footprint is about 2' x 4', so it's getting close to 200 lbs/square foot.  It will be about 4-6" from an exterior wall.

I just took a look under the house, not much space under there and what I can see is on the opposite end of the room from where the tank will be.  It's post and pier construction with the piers in something like a 3' x 4' grid in the part I can see.  The beams on top of the posts are 4x6", then there's 2" thick boards on top of that (2x6 I think) and then hardwood flooring on top of that (and now carpet over that).

How strong is this type of floor, and will the tank be ok or should the floor be reinforced?  I'm not sure yet if a single beam will be running under the tank or if it will be closer to having a beam on each end.  Thanks.

Answer
Hi Brian, your beams where they bear on the wall have a shear of 175-300 psi (depending on the type of wood) which, given a cross section of about 3.5 inches wide and 4 inches of bearing is approximately 14 square inches per each beam end. So your beams should be able to hold the weight where they go into the wall and where they are supported by posts.  Most post and beam floor systems have posts set approximately 4' apart and it seems that yours is no different.  It sound like your beams are 3' apart and your post spacing is 4' based on your email.  Your floor system itself is tongue and groove 2x6 topped with 3/4" hardwood which makes your floor very capable of spreading the weight of your aquarium evenly over the beam supports.  Picture eight 200 pound men standing in the 8 square feet your tank is going to occupy..It won't collapse. As long as the wood in you floor system is not rotted or incapacitated in some way you should be fine. The obvious best location of your tank would be straddling two beams instead of one if possible to distribute the weight over more structure.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com