Agreed. It’s one thing to build one for yourself because it’s only your stuff you risk damaging. However, if you are building one for someone else and it fails and damages their house and belongings, then you can bet they will be suing you. In addition, before you sell something like that, you really should have at least built and tested several of them and worked out a design so that you have documentation supporting your build design in the event that you are sued, especially if an injury is involved. Lastly, you should be covered from an insurance standpoint so that if something bad ever happened and one of your tanks damaged someone else’s property or caused injury, your insurance would be responsible for paying out the damages.
As you can see, all of this stuff costs quite a bit of money and is a big reason why professionally made tanks cost so much.
Now, if he buys the materials and you build the tank for him for free out of the kindness of your own heart, then you are not liable unless they can prove you intentionally did something to cause property damage or injury. Same is true if he buys the materials and builds it with your help and guidance, as long as you take no money.
My recommendation, if you want to help him out, then do it on a free basis. It will still help you because you will gain more experience in building plywood tanks. However, do not accept any form of payment. A grey area would be if he offered a case of beer or to buy you dinner for helping him out. Technically, it should count as a form of payment, but it would be really hard to get that pushed through court in the event of a lawsuit.
