building a wooden bench overhanging a very damp concrete frog pond?
dear friends —
in anticipation of a wedding, i’m about to tackle a long-delayed project of rebuilding a bench all around a concrete frog pond i built some years ago. the pond is a roughly rectangular (all dimensions are inexact — one of the problems) 6 by 8 foot structure topped with 2 by 6 pressure treated beams bolted into the concrete. last time we built a bench on top of that, screwing it into the pressure-treated stuff, it was made out of wood (painted oak, if memory serves?) that turned to rot in no time, because it is such a damp environment. also, because the dimensions are inexact, the diagonals cut at 45 degrees didn’t butt up exactly, and they pinched and scraped people and looked ugly.
to address this, six years or so ago, when i thought i was about to enter a period of spare time (ha!) i bought four very expensive and no-doubt sustainability-incorrect tropical boards from a fancy millwork. (mahogany, if memory serves?) the argument i was given was that, whatever they are, they are as water resistant as you can get without being ugly. they are 11 1/2 inches true by 1 and 1/2 inches true by twelve feet. they have been lying out on the porch, out of the wet, but exposed to the air, all this time, with steel bands and braces to make sure they don’t warp or cup.
i’m planning on buying three dozen 2 3/4 inch brass screws, and fastening one of them every one foot, in a zig zag pattern to try to defeat cupping. i’m also planning on installing the slightly-longer-than 8 footers, cutting the diagonals according to whatever i find the angles actually to be from the inside corner of the pond to the outside corner. and then trying to exactly match up the angles to make the cuts in the slightly-longer-than 6 footers.
two questions:
the fancy millwork said at the time that before i install these boards, i should treat them with linseed oil. my wife says somebody told her that for outdoor wooden furniture, something has been found to be better than linseed oil. what might it be? and does it have any toxicity at all? remember this is a delicate ecostructure full of tadpoles and baby fish and water plants.
meanwhile, do you see anything you’d recommend i do different in the above plan?
cheers, and thank you —
joel garreau
broad run, va.
Replies
attempting to add a photo of said pond
as a p.s.
j.g.
joel
If there's concern about moisture and warping/cupping you might be well advised to rip down the boards you have into more of a slat type "bench". With cross members below it to raise it off the surface of the treated wood (is that what's shown in the photo?) it would stand half a chance of being able to dry out and not be in constant contact with the moisture transferring treated stuff below.
Coatings or protectants for this, I will not comment on.
However, brass screws I would not use. They are too soft and will be a pain in the rear to work with. Predrilling almost the perfect hole while screwing by hand so you don't strip the heads out isn't a good time.
More suitable would be a treated wood resistant screw, perhaps SS, but if you check fastenmaster or GRK you should find suitable screws to work with in that environment.
just a wag
No real insight to offer, but wondered why not check with some of the marine supply places, e.g., West Marine in Alexandria. See what people are using for decking for docks, etc. and see if some of the newer composites are suitable?
You should not attach your boards directly to the treated board, but should provide some sort of spacer to maintain at least 1/2" air space.
Use stainless screws, not brass.
An open/slat sort of structure would indeed be your best choice. Not only will warping be minimized, the wood will dry a lot quicker giving many more days of dry sitting time. As to finish... I'd guess that toxicity really isn't an issue since you have those pressure treated planks already bolted to the concrete where they can leach their innerds directly into the pond. In any case, I'd probably leave the bench unfinished. Anything you apply will have to be periodically re-done thus adding more toxins (however slight) to the environment.