Redwood mystery

[Jamie Buxton]

Last summer I built a big patio table. Over the past year, it has changed in a way which surprises me, and which I can't explain.

The table is made from recycled old-growth redwood. It is 12 feet long. The top is 2 1/2" thick. It is supported by two trestles which are about twenty inches from the ends. That is, the main unsupported span is about eight feet. Despite that length, the table is rock solid. I can jump on it, and there's no flex. I "finished" the table with several coats of deck oil.

patiotablepicsmall.jpg


When the table went into service last summer, the top was flat. It stood on the patio out in the weather over the winter. ("Winter", here, means about 15 inches of rain spread over four months, and a few days with morning frost.) Today, the table top is not flat. It sags about an inch in the middle. It is still rock solid.

I'm surprised at the sag. I don't have a lot of experience building outside furniture, but I've seen wood tables at park campsites that are fairly flat despite decades of exposure to the weather. So what has happened to my table? And how do I fix it?

[Jason Mikits]

Hard to say for sure, but it's probably not sagged from structural weight, but more likely it's warped from differential drying. It wets and dries from the sun more on top and the fibers tend to curl up. It might semi self correct if you flip it upside down on some horses to let nature "have at" the bottom side. Wide flatsawn boards tend to cup and bow more noticeably. Set a board on your driveway and often you'll see it take a banana shape. Oil does a very poor job at sealing the surface to control the moisture gradient. Park tables generally aren't 12' so the bowing isn't as noticeable, and often use pressure treated wood which is usually soaking wet to start with. I have seen plenty of park tables with the same bow. That's my random guess from 3000 miles away.

Very nice looking table BTW! I like the "beefiness"!

[Bradley Gray]

I agree with Jason. Flip it over and hose the bottom every day for a few and see if it fixes itself.

[Ted Reischl]

Gravity. A powerful thing that works 24/7, never stops and always wins. That, and what Mr. Mikits said.

Here is one for you: A garden arbor. The span is 18 feet. The person who built it used 2X12 SYP on edge. Yes, it has a sag in the middle, he thought that a 2X12 would not eventually sag. Gravity wins every single time.

[Lee Schierer]

I "finished" the table with several coats of deck oil.

patiotablepicsmall.jpg


So what has happened to my table? And how do I fix it?

Did you apply the finish to the underside of the table? If not, then the underside absorbed more moisture than the top side did and it expanded. The cure would be to put the table in the sun for a day or two with the bottom side exposed to the sun. Once the table flattens, apply the same amount of finish to the under side. When you turn it upside down be sure to allow air to get under it.

[scott vroom]

If the other solutions mentioned don't work, you could use corbel-style bracing similar to that shown in this random web pic.

[Frank Drackman]

I agree that it is probably caused by unequal moisture in the wood.

I really replied to say that I love the table. It has been on my list for a while but always slips down with more urgent projects.

[Patrick McCarthy]

Jamie, what about doing a center cleat (with only 2 elongated holes) with a single leg; essentially a T formation. Nice table, sir.

[Jacob Reverb]

Did you apply the finish to the underside of the table? If not, then the underside absorbed more moisture than the top side did and it expanded. The cure would be to put the table in the sun for a day or two with the bottom side exposed to the sun. Once the table flattens, apply the same amount of finish to the under side. When you turn it upside down be sure to allow air to get under it.

That would be my first guess, too...

We used to have a big pile of redwood, had to be 3" thick, beautiful stuff, I think it was from a water tower torn down in Camden, NJ -- we never used it for anything, though.

[Jamie Buxton]

Jamie, what about doing a center cleat (with only 2 elongated holes) with a single leg; essentially a T formation. Nice table, sir.

Right now, I’ve added a construction-lumber middle trestle. It has leveling feet so I can jack up the middle of the table. When I jack it up to the point that the real trestles just barely touch the ground, I’ve taken maybe 3/16” out of the 1” sag.