Our hot water tank (in the downstairs utility) is probably going to weight about 350kg.

 

This will sit on 18mm plywood, a thick anti vibration mat, then 22mm chipboard T&G and finally 45 x 145mm joists at 400 centres.

 

The span of the joists is quite reasonable at 2 meters and the tank is well placed next to wall.

 

Is it irrational to be worried out this weight?  

 

If it ever became an issue would I notice a sag rather than the joists snapping 

That's a good sized tank, but the timber size sounds normal for that span, even if the load is more than normal (1.5kN/m²); personally I wouldn't be worried. And yes, the joists would flex if there were to be a problem - so without a ceiling beneath not such a big issue.

Edited March 11, 20206 yr by Mike