I built a box out of 0.3" (8mm) plywood sheets framed by 3" (75mm) by 2"(50mm) staffs. The box is about 3' (1000mm) each side and 2' (600mm) deep, has a floor but is open at the top. The box should act as a water reservoir, therefore needs to be waterproof (at least on the inside).

Additional materials

In order to waterproof the box I bought sheets of HDPE of 0.04" (1mm) thickness. The sheets are somewhat "semi-rigid", not as flabby as foil, but also not rigid enough to support itself, hence the plywood structure.

My Ideas

I planned to nail the HDPE sheets to the plywood sheets and weld the adjoining edges together. Additionally I wanted to seal the nails in a similar way.


Today I did a few test weldings (I could only get my hands on 0.15" (4mm) welding cord) and was less than satisfied. Probably because of the thickness ratio of cord and sheets the heat distribution was off, the sheets getting too much heat while the cord barely melt.


What other ways are there to waterproof my box (preferably using most of my setup)?


  • I know there are easier ways to gather water, I deliberately want to do it this way.
  • I welded HDPE sheets before, but always with a thickness of more than 0.3" (8mm).
  • 1
    Seems like you're asking a lot from your plywood and framing to hold that much water. Water is heavy, and you're going to have up to 600kg of it.
    – JPhi1618
    Sep 6 '17 at 20:45
  • @JPhi1618 That's why I framed and cross-braced the plywood sheets with 3" by 2" staffs. Additionally I did a calculation concerning water load as well as a simulation using CAD software. All loads are well within strength boundaries.
    – pat3d3r
    Sep 7 '17 at 8:54

When dealing with water, look at marine

People make boats out of plywood, just make one inside out. They have several techniques, including involving fiberglass, but one of the most prominent is epoxy systems that give you a lot of options. One such is West System epoxy.

Keep in mind water is heavy (1 tonne per cubic meter, in fact) so your cross bracing needs to be tip top.

  • Coincidentally I already used epoxy for another water container. That is actually the reason I wanted to try it a different way this time.
    – pat3d3r
    Sep 7 '17 at 9:41

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