How to reinforce a firepit wooden table top

[Dumb_Polack]

So, the other weekend I caught an episode of Ask TOH in where Tommy and Kevin built a wooden cover for a firepit.  I've been looking for a cloth cover for our firepit and can't find one so I thought I'd make one out of the scrap white cedar I have laying around from my fire pit chair build.  I glued all the pieces up and cut out the circle with a trammel attached to my router.  The firepit is 35" in diameter and my cover is 36 1/4" in diameter.  I'd like to add a circular ring 1/2 thick attached to the underside of the wooden top.  This will do 2 things:

1) Stiffen up the top 
2) Allow water to drip down vs getting sucked into the fire pit via capillary action.

(the cover is only about 1/2 thick so I will also be adding cleats that'll run perpendicular to the strips.  Obviously these cleats will need to taper toward the outside so they don't raise the cover above the firepit.  You can see the wide lip on the fire pit.  These will add some rigidity too).

So, I have some left over mahogany (you can see some of it in one of the pictures) from the deck build of 15+ yrs ago so I think I'll use that.  My questions are these:

A) For a 36 1/4" ring and the mahogany is 3/4" thick, how many segments and what are the angles at the end of each segment?   (I don't recall covering this in 8th grade Geometry).  I realize it'll be a LOT of segments given I'm starting with wood that's only 3/4" thick and it'll need to come down to 1/2", but I've got the time to do this.  I think I'll also add splines to the segments to strengthen them.

B) Once I glue up my "circle" what's the best way to bring them down to 1/2" thick?  I suppose I could try to create a jig with the router and trammel, but are there other ways to do this?  I figure the circle will be 1 - 1 1/2" tall so I don't know that my router could reach that deep.  I guess I could flip the "circle" over and get the other side that way.

C) I do have some thicker cherry scraps laying around (probably 1 1/2" thick.  Should I use those?  I'm a little hesitant to go that route as I'm not too keen on having cherry exposed to the elements).

Your thoughts?  Thanks!

[Woodshop]

if I understand you ,your trying to trap that iron top with a wood top and left 1/8 inch all the way around for expansion /contraction . IMHO 
bad idea. but I could be wrong.

[Dumb_Polack]

if I understand you ,your trying to trap that iron top with a wood top and left 1/8 inch all the way around for expansion /contraction . IMHO 
bad idea. but I could be wrong.

You think the iron will expand/contract?  Or the top?

[jteneyck]

If you put a glued together ring around or near the perimeter it will break from seasonal expansion/contraction of the top.  If your purpose for the ring is mostly to keep water from wicking under then I would cut a small dado, maybe 1/8 wide and 1/8" deep on the underside about 1/4" from the perimeter and forget the ring.  That will cause the water to fall off instead of migrating further underneath.  

Any cleats you add will raise the top above the metal ring.  If your top were thicker, you could let the cleats into the underside, but I don't see that being an option with the top only being 1/2" thick unless maybe you use aluminum bar stock.  

John

[Dumb_Polack]

If you put a glued together ring around or near the perimeter it will break from seasonal expansion/contraction of the top.  If your purpose for the ring is mostly to keep water from wicking under then I would cut a small dado, maybe 1/8 wide and 1/8" deep on the underside about 1/4" from the perimeter and forget the ring.  That will cause the water to fall off instead of migrating further underneath.  

Any cleats you add will raise the top above the metal ring.  If your top were thicker, you could let the cleats into the underside, but I don't see that being an option with the top only being 1/2" thick unless maybe you use aluminum bar stock.  

John

Well, I plan on having the cleats be inside of the rings o they shouldn't raise the top.

And I'm worried about the seasonal expansion issue.


Need to think on this a bit more

[brianwelch]

If you put a glued together ring around or near the perimeter it will break from seasonal expansion/contraction of the top.  If your purpose for the ring is mostly to keep water from wicking under then I would cut a small dado, maybe 1/8 wide and 1/8" deep on the underside about 1/4" from the perimeter and forget the ring.  That will cause the water to fall off instead of migrating further underneath.  

Any cleats you add will raise the top above the metal ring.  If your top were thicker, you could let the cleats into the underside, but I don't see that being an option with the top only being 1/2" thick unless maybe you use aluminum bar stock.  

John

Similar to what John said, I would probably go with a shallow V groove rather than full square dado, but the premise is the same, water rarely flows uphill (wind driven being an occasional example)

[R Clark]

I made a cover for our fire pit a number of years ago; used the same basic construction you're talking about.  I made it from 4/4 cypress, left it full thickness.  I edge glued the boards and used cleats underneath across the boards to reinforce.

Only lasted two seasons.

It cupped wildly.  I didn't have a cracking problem, and the wood itself was still solid with no rot.  But it wold not lay flat, and the method I used to secure it to the fire ring was ripped apart by the cupping.

In hindsight, I might have reduced or eliminated cupping by ripping the boards lengthwise, edge jointing, and then gluing them back together.  Perhaps using boards only 3-4 inches wide would have been better.

[jteneyck]

I made a cover for our fire pit a number of years ago; used the same basic construction you're talking about.  I made it from 4/4 cypress, left it full thickness.  I edge glued the boards and used cleats underneath across the boards to reinforce.

Only lasted two seasons.

It cupped wildly.  I didn't have a cracking problem, and the wood itself was still solid with no rot.  But it wold not lay flat, and the method I used to secure it to the fire ring was ripped apart by the cupping.

In hindsight, I might have reduced or eliminated cupping by ripping the boards lengthwise, edge jointing, and then gluing them back together.  Perhaps using boards only 3-4 inches wide would have been better.

Gluing the boards into a panel is a bad idea for outdoor projects.  It would have a lot less tendency to bow if the individual boards are just nailed or screwed to the cleats underneath.  That way, each one can expand/contract as it wants w/o affecting its neighbors.  If you are trying to shed water then use tongue and groove construction with a healthy gap.  

John

[R Clark]

Gluing the boards into a panel is a bad idea for outdoor projects.  It would have a lot less tendency to bow if the individual boards are just nailed or screwed to the cleats underneath.  That way, each one can expand/contract as it wants w/o affecting its neighbors.  If you are trying to shed water then use tongue and groove construction with a healthy gap.  

John

You make a lot of sense, John.

If I try again, it may be with something synthetic, perhaps Trex.

[jteneyck]

You make a lot of sense, John.

If I try again, it may be with something synthetic, perhaps Trex.

That likely would yield far better results.  Less maintenance, too.  

John