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Dado joint in white oak for the outdoors-help

pizza's picture

Hi.

I’m making a wooden cross out of white oak for my cousin’s grave marker. This cross will be outside. The white oak is not quarter sawn, the pieces are basically half rift sawn with the rest being flatsawn. The horizontal cross piece is 15 “ long by 3” wide by 1 1/8” finished thickness. The vertical piece is 36” long  by 3” wide” by 1 1/8” thick (with one foot of the bottom being buried into the ground, essentially making the cross 24” tall by 15” wide). The bottom tip of the cross will have a point on it so we can hammer it into the ground.

I’m connecting the two pieces by cutting a wide 3” wide by 9/16” deep dado in each piece where they intersect and then using Titebond III waterproof glue and 2 stainless screws.

I will be finishing and sealing the wood with a finish of some sort (haven’t decided yet).

My questions are these:

  • Should I make the dado slightly wider than 3” to allow for expansion of the wood? I don’t want a tight fit to cause it to crack if in case the wood swells. If I do make it slightly wider, by how much do you think?
  • I know screws in oak will tend to cause black marks when they react with the tannins in the oak. Should I leave the screws out? Will using stainless screws not cause black marks as opposed to non-stainless screws?

Thanks to all in advance.

RalphBarker's picture

White Oak, outdoors (post #170825, reply #1 of 5)

White oak is one of the better woods for outdoor use. Still, it can't be expected to last "forever" - especially the portion in direct contact with the earth.

I'd make the half-lap joint to fit, but only use one (stainless) screw from the back side, centered on both pieces. That will allow for movement, away from the screw location.

pizza's picture

WHITE OAK OUTDOORS (post #170825, reply #2 of 5)

Thanks, will do the one screw, good point.

The cross is temporary but the gravestone makers have been taking their time getting the stone marker made so this cross will serve in its place till then. I still want the cross to be well made and able to withstand the elements so I chose the white oak for it's ability to withstand water (has benn used in shipbuilding) as well as it's strength and beauty (especially over pressure treated, yuk)

Thanks

PSeverin's picture

drainage (post #170825, reply #3 of 5)

in detailing similar joints on a house or fence, getting the water to drain out is important.   I would suggest putting a slope on the lower edge so that water drains.   You might even consider putting a slope of a few degrees on the arms, and top.  It is one of my complaints of most fence or guard  rail designs that they create flat surfaces where water stands and lifts any finish you put on them.

I would also suggest that one foot is probably not deep enough to hold the cross vertical for long.  Any of these concerns and responses are only relevant if the cross will be in place for some length of time.

Peter

pizza's picture

White oak drainage (post #170825, reply #4 of 5)

Thanks for the reply, I will consider your points. The post will actually go 16 inches into the ground, 2 ft actual cross exposure and 16" ground penetration for a total vertical part of 3 ft 4 inches. So for temporary purposes till the stone marker arrives I hope it will stay upright.

hammer1's picture

Back in the 70s I built a (post #170825, reply #5 of 5)

Back in the 70s I built a truck ladder rack in red oak. Although some of the joints were dovetail shaped, they were half laps. A center strut was just a square half lap. I used a marine polyurethane for the finish and that didn't last too long. I'm guessing I used the rack for 15 yrs. then it didn't fit the stake pockets on newer trucks, the poly was peeling so It got relegated to the scrap pile out back. Sat outside for several years until I cut it up and reclaimed some of the lumber. Back in the day, there wasn't any Titebond II or III. The common glue for exterior work was Resourcinol. I took a picture of a portion of the rack when I cut it up. All the joints stayed together just as I had made them, I couldn't pound some of them apart. Over it's life, I loaded that rack beyond good sense.

I think your concerns about the joint aren't anything to worry about as long as you have a good fit. The issue would be the point stuck in the ground. It should last several years but I'd soak it in a preservative just to add some life. A couple stainless screws wouldn't hurt. The hook in the picture was chromed but the screws were ordinary steel, couldn't get those out, no tanin stains on the red oak.

Beat it to fit / Paint it to match

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