Pocket Joinery in a humid environment

[Bill Stock]

I'm building another aquarium stand for a 90 gallon tank. The previous stand was built with 2x4's using half lap joints and a plywood skin. I wanted to build the new stand (open top) using face frame construction with 1x4 oak for the rails and 1x3 oak for the stiles. I was originally considering using pocket joinery to fasten it all together, but I've been reading about pocket screws working loose in moist environments. Does this story hold water or is it over blown?

[Mike]

I think that you'd be fine as long as you use glue on the joints in addition to the pocket hole screws. Make sure that you use a glue made for damp applications, such as Gorrilla Glue or Titebond III.

Apart from the joinery, are 1x3's and 1x4's strong enough to hold the weight of a 90 tank? This will depend largely on your construction details.

[RayV]

750 lbs just in water, I wouldn't trust that to a frame that is just screwed together butt joints.

[Pat Barber]

Pocket hole joinery is meant to "hold until the glue dries" not actually create a joint. It is quite strong without glue but only in non-loadbearing situations, which is rare.

It should have been called pocket hole clamping...

Do a little read> Bill Stock wrote:

[Larry Blanchard]

I built one for a 30 gallon tank using 3/4" plywood. No problems yet.

And oak is a lot stronger than plywood. Wood in compression is remarkably strong. If this is the standard 90 gallon tank (48x18) I'd use two vertical dividers and tie all together with a fixed horizontal shelf.

Don't forget to rest the tank on a piece of rigid insulation foam. Makes up for any little irregularities in your construction.

For those still wondering about strength, visit your local pet store and look inside some of the particle board stands they sell :-).

[Bill Stock]

Yes I plan on 16" supports, so even 1x4 is probably overkill based on the span calculator I checked.

Thanks Larry, but this one won't have a top, just an edge frame. The tank has an overflow (bottom plumbing) so it will just be edge supported.

[CW]

Below is a quote directly from the site you referenced.

How strong is a pocket hole joint compared to a mortise and tenon joint? This is a question we get asked quite often. A independent lab completed testing a few years back that showed that a pocket hole joint failed at 707 pounds when subjected to a shear load while a mortise and tenon joint failed at 453 pounds (approximately 35% stronger). Pocket hole joints are tremendously strong for a couple of reasons. 1. The use of a mechanical fastener (screw) is significantly stronger than the material around it (wood), and 2. The amount of direct clamping force placed on the joint by driving the screw combined with today's glue technology makes for a sensationally strong bond.

[Bill Stock]

Yes, but this would appear to be at a point in time and does not address the strength of the joint over time. Although they do address the "weather" issue in another point.

[Quote] I am afraid to use pocket holes because I am worried about wood expanding and contracting with changes in the weather. What do you recommend?

There are a couple of easy ways to compensate for this type of wood movement. Let's assume an example of attaching a rail or apron to a solid tabletop. First, set your depth collar on the drill bit so you barely drill through the edge of the rail or apron. This will create a larger pilot hole for the screw shank and will create the effect of a "floating top". Secondly, drive the screw into the top until tightened, then back off 1/4 of a turn.

[C & S]

Snipped

"Sheer load"

Of course this is dependant upon the OP's design, but I suspect that the mode of failure that would be of greatest concern would be racking force resulting in screw withdrawal.

-Steve

[bf]

My big tank stand was made with pocket screws and glue. The high end stands at the big pet store are built similiarly.. in fact, they go so far as to pockethole the plywood sides together on the carcass.

Anyhow, my stand has worked fine for approximately 5 years.

The scary thing I've seen is people using particle board/MDF for big tank stands (even commerical ones)

[bf]

Good point on racking, but I'm sure the OP will put a plywood back on his stand. That will hold the case square.

[Larry Blanchard]

That makes no difference. All tanks are supported solely by the edges,=

regardless of whether or not the top of the stand is solid. The foam st= ill evens out any little irregularities in the stand that might put a heavi= er load on the tank frame at a specific point and cause it to rack. You c= ould cut the center out of the foam for the plumbing.

I will admit that this is an issue on which there is a lot of argument.= Some say it's overkill. But I say it's cheap insurance.

--=20 It's turtles, all the way down