A leaning water heater is not a tourist attraction, but can be a safety concern.
When a water heater leaks and the water penetrates the wooden platform, the wood weakens, especially if this condition goes on for a long period of time. The weight of a 50 gallon water heater; approximately 150 lbs. for the tank and 420 lbs. for the water, equals about 570 lbs. With all this weight, the platform may sag or the legs of the water heater sinks into the wood or drywall, and the water heater is no longer plumb, it leans.
Basically the two main causes for water heaters to lean, is that the platform had water damage and is no longer as strong as it was, or a larger water heater was put on a platform built for a smaller water heater.
Add a piece of plywood on top of existing platform?
If the basic platform structure appears sound and only the plywood sheeting has sagged, then laying a stronger (thicker piece) of new plywood over the old may provide the needed support for the water heater. If the platform is badly damaged, then rebuilding the platform is best.
If slightly leaning
When the water heater is just a little out of plumb, many owners will just monitor the water heater and if the situation gets worse, then they fix it. While checking the water heater, remember to look at the water connections. If the connections are hard-piped, then leaning water heater may put undue stress on the water pipes. If the water heater supply lines / connections are flexible tubing, then there is probably less stress.
Stains, Mold and Mildew
Since moisture damage to the water heater platform is one of the primary reasons for a leaning water heater, it just makes good sense to check for evidence of mold and mildew in the area. Moisture stains and dark discolorations on the wall or platform may be a possible warning sign. Another warning sign may be a musty or moldy odor. If the wall area has possible mold evidence, then if possible, check on the other side of the wall for similar evidence and if the water heater is on the second floor, then check the ceiling area under it.
Should mold or mildew be suspected and it’s in the wall, then it may also have penetrated any insulation in the wall. Homeowners often correct and repair small mold and mildew areas themselves, but at times will hire a professional to investigate and correct any problems. (Read more on mold and mildew)
Is there a pan under the water heater?
Having a properly plumbed pan under the water heater generally reduces damage to a water heater platform. If you are replacing the water heater or rebuilding the platform, remember to put a pan under the water heater. There only a few dollars.