Need new water tank (drinking water) lid as concrete one has crumbled away, plywood safe?

Forums › Home Workshop DIY › Need new water tank (drinking water) lid as concrete one has crumbled away, plywood safe?

TeaLeaf

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#310516 27-Oct-2023 12:23

I got a rental that the in ground water tank concrete lid just falls through as the edges have worn away and one corner has snapped off.

Is any plywood safe to use? I was just going to make a template of the hole with one piece by stenciling the shape, so it fits snuggly in the concrete lid hole and then screwing that to another piece that is inches wide all round so it doesnt fall in, and just put a bsic handle on top.

I just dont know if plywood is toxic? If so any other ideas for an easy buiild? I did consider another concrete but its going to be painful to fit into the hole perfectly again. I rang a tank guy and he said that is why they dont rebuild tank lids......

mdf

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#3152670 27-Oct-2023 13:00

Standard plywood rated for external use (H3.2) is treated with pretty nasty stuff, I wouldn't want to be drinking anything that came into contact with it. You can get "untreated" marine grade plywood. It's quite expensive though and you would want to check that "untreated" means drinking safe, not just "we don't use arsenic" (which is what H3.2 includes... and we use in playgrounds).

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TeaLeaf

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#3152718 27-Oct-2023 13:04

mdf:

Standard plywood rated for external use (H3.2) is treated with pretty nasty stuff, I wouldn't want to be drinking anything that came into contact with it. You can get "untreated" marine grade plywood. It's quite expensive though and you would want to check that "untreated" means drinking safe, not just "we don't use arsenic" (which is what H3.2 includes... and we use in playgrounds).

Cheers all the things I was worrying about.

any ideas on another material can use?

mdf

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#3152725 27-Oct-2023 13:12

How big is the hole? Plastic and steel are options, but both would require a bit of fabricobbling starting from scratch. You might get lucky with an exact fit, but otherwise would probably be best to buy something off the shelf and adapting the tank to the cover. Assumptions on my part, but I would guess most straightforward would be a cover that is slightly too big then making a collar to fit around the mouth of the tank.

TeaLeaf

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#3152731 27-Oct-2023 13:47

mdf:

How big is the hole?

about 3ft by 2ft off the top of my head. I did consider making a mould and pouring some diy cement, but unsure if that is safe either. another idea was just to lay some kind of rubbery plastic over the hole and some how creating some clips in the tank to anchor the cover too....

Gurezaemon

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#3152734 27-Oct-2023 13:54

Maybe cut a plywood cap and then coat it in something safer? Roof paint is supposed to be safe, as lots of people get water off their roof.

Alternatively, what about a couple of layers of fibreglass or something?

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pih

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#3152738 27-Oct-2023 14:11

Gurezaemon:

Maybe cut a plywood cap and then coat it in something safer? Roof paint is supposed to be safe, as lots of people get water off their roof.

Roof paint is safe after 2-3 heavy rain falls, so if using roof paint you'd want to coat it several times and weather it before use, my feeling is that even marine ply may delaminate over time if the edges are exposed to the weather.

With a hole that size you'd want to make sure that it was strong enough to walk on and not likely to blow off in strong wind. Another thought is something like roofing polycarbonate sheet (>=6mm solid), or ACM (Aluminium Composite Material) expensive for a whole sheet but you might be able to find an offcut cheap. Ensure the UV treated side is up (for polycarbonate). It will need to be weighed down or strapped on somehow, but it should hold the weight of an average adult, depending on hole size.

TeaLeaf

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#3152783 27-Oct-2023 16:09

yeah u do not want any sun light getting in your tank.

Yes the fit is important, none of them are built safe enough to walk on. which is odd as kids plat on the damn things. Not sure about the polycarb versions. actually I wonder if the poly carb guys could make me one.

Bung

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#3152862 27-Oct-2023 18:59

TeaLeaf: Yes the fit is important, none of them are built safe enough to walk on. which is odd as kids plat on the damn things. Not sure about the polycarb versions. actually I wonder if the poly carb guys could make me one.

Do you really mean that you have buried water tank with a lid that can't be walked on or is it only partially buried?

TeaLeaf

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#3153147 28-Oct-2023 17:30

Bung:

Do you really mean that you have buried water tank with a lid that can't be walked on or is it only partially buried?

Partial of course :-). House was built new during that change in regulations. You have to make effort to climb onto it. Currently its sitting flush by itself. but we also have some thick reo (thicker though) rods as well, it does sit in there by itself without them and you could put weight on it, we are just wanting a new one before it gets to a state that it doesnt.... its not something you can readily replace ie ring someone up to buy one.

gzt

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#3153184 28-Oct-2023 20:24

Is it flat? The giant heavy steel plates used in roadworks come to mind with some additional welding or simple bolts to locate it securely in the hole. Expensive.

larknz

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#3153189 28-Oct-2023 21:21

Who made the tank? They may have something suitable.

andrew75

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#3153233 29-Oct-2023 08:32

I had exactly the same problem. I tried and failed to find a suitable sheet of polyethylene.

In the end I used H3.2 plywood, reinforced with a couple of 45x45 batons. I laminated a sheet of aluminum from Mitre 10 to the underside (water facing side), wrapped around at the top and sides (but open at the bottom). This lies over the hole in the roof of the tank. The roof is sloping so no water exposed to the treated wood will be exposed to the tank water. Its held up fine over the past 6 years now.

TeaLeaf

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#3154983 2-Nov-2023 15:38

larknz:

Who made the tank? They may have something suitable.

No, oddly not, was my first port of call.

gzt: Is it flat? The giant heavy steel plates used in roadworks come to mind with some additional welding or simple bolts to locate it securely in the hole. Expensive.

no sadly not, and they ar held up by the angled cut into the concrete.

I have thought about using the old one to make a mold but slightly bigger, then just grind it where needed so it fits in the hole.

TeaLeaf

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#3154986 2-Nov-2023 15:43

andrew75:

I had exactly the same problem. I tried and failed to find a suitable sheet of polyethylene.

In the end I used H3.2 plywood, reinforced with a couple of 45x45 batons. I laminated a sheet of aluminum from Mitre 10 to the underside (water facing side), wrapped around at the top and sides (but open at the bottom). This lies over the hole in the roof of the tank. The roof is sloping so no water exposed to the treated wood will be exposed to the tank water. Its held up fine over the past 6 years now.

That is full one :-). Is untreated ply poisonus at all? I know in time it may delaminate, but would about non toxic paint?

mattwnz

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#3155020 2-Nov-2023 16:33

You really need a replacement concrete lid. They do seem to fail over time and crumble. Our concrete water tank has a plastic lid, but it clips on. However our concrete septic tank has a concrete one. There are a few companies that make concrete septic tanks such as this one https://www.decoprecasters.co.nz/precast-concrete-septic and these companies should be able to supply a new lid. You may also be able to find some paver stones large enough.