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How may joists to straddle for 90cm tank?


3 Aug 2012

Firstly I would just like to apologies not only for my pathetic MS paint skills 😀 But for taking up your time with this post.

I have been out of the hobby for around 10 years and my last tank was a 60L now after getting our own home and my own man cave I wanted to get a tank and start again. I was in between a 60cm and 90cm tank and settled on getting a 60cm but at the weekend I placed an order for a Waterbox 3620 (186L 90cm tank) as it was half price. It cost less money than the 60cm tank but the weight is about double in litres of capacity alone.

Our bungalow is a 2017 new build and I have been doing research about tank weights and joist long before the purchase. Having filled up my room I only really have one space suitable for the tank and that is along a load bearing exterior wall. Tank would currently straddle across just two joists (used a metal detecting wand to pinpoint the joists) I plan on getting a marine ply 18mm board to help as the tank has 6 adjustable feet for pressure points. Cabinet also seems of heavy duty aluminium construction and whilst hardscape shall be limited (lava rocks and not much of it) I am concerned about the weight (I am sure you get asked this every week)

If it was you would you get the board to match the cabinet size (outlined in blue box in the pic - joists are the red lines) so just 2 joists or would you go for a bigger board and have 3/4 joists and just have a plywood board sit below the cabinet? I can make 3 or 4 work I am jsut not sure how many is best practice for such a tank.

I tend to over think things and once a seed is sown it grows 🙂 But its just a concern of how best to approach things. I know its "only 300kg" or whatever all in and to some here that is a nano tank but any comments would be most welcome. I really appreciate the time taken to read my waffle 🙂

I look forward to seeing you all around on the journey ahead!

Best Wishes,

Murray 🙂


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I am no structural engineer but I would think keeping the marine ply board to the size of the cabinet would be perfectly adequate. No need to go bigger. I presume there is some kind of underlay beneath the wood/laminate flooring and then regular floorboards (or similar) running perpendicular to the joists, all of which will be doing a good job of arching the load towards the extra two nearby joists. Extending the marine ply beyond the cabinet is not going to have much practical benefit, not worth it when you consider the potential trip hazard and potential for not looking very nice.