Needing some advice and/or help - Inching forward on the fireplace mantle

[MarkDarby]

All,
I am making slow progress on the mantle. the mantle is going to be a box -- should be simple. Length is 9'4", height is 6", and depth is 11". The front and rear pieces will join to the side pieces with half-blind dovetails. This work is complete. The issue is the front piece has developed a 1/4" upward in the middle (see the attached sketch). I will have three doors that open on the top and, with the crook, won't sit squarely on the front.
I was thinking of trimming the front piece -- but that will leave a gap at the bottom. The other alternative: I can force the front back into "true". I plan to have two "styles (??)" that connect the front to the back at the top and bottom -- to provide stability and keep the front from bowing out. I could make the styles solid from top to bottom, force the front back into true, and depend on glue to keep it true over time. I plan to use two dominos at the top and bottom of each style.
I would be grateful for your thoughts.

Thanks in advance,
Mark

 

[Wilsoncb]

Do you have a picture of what you have built at this point.

 

[Mike Davis]

Sliding dovetail vertical dividers will hold much better than glue and dominoes.

 

[MarkDarby]

Here is the carcass...
- The back side, which you see in the photos, will sit against the wall and on top of the stone fireplace facade.
- The cross bars that you see is where I was going to use dominos -- but will likely replace those for top-to-bottom dividers that I connect with sliding dovetails.
- The top will be three doors.

Thanks again to everyone.

Mark

 

[Wilsoncb]

Oh boy, I can see the issue better now. I was hung up on doors that hinge up on the top. I was thinking they were on the front, but now it makes sense. 9’-4” is a long span for 3/4 x 6 and hanging 11” off the wall is a big ask. I didn’t completely understand how you were going to mount this, but it’s probably going to have to also support the shape of the box unless you beef up the box some more. I think the two front to back cross braces should be full (6”) high to help transfer the load and support the front. I would also consider making more smaller doors so you can have more cross bracing.

Before you do anything you can test it to simulate how it’s mounted before you actually commit to anything.

Take the following for what it’s worth as ultimately you will have the best perspective and “feel“ for what you have.

Concerning the bow, one option would be to reinforce that (9’-4”) span with another board on the inside (or even aluminum plate). Either is going to add unwanted weight, and it might be necessary to do the same at the rear depending on how it’s mounted.

The good news is it’s bowed up in the center, so hopefully you can support the ends first and let the center come down with gravity? If that doesn’t happen another option might be to mount that assembly on a sled board and run it through the table saw to trim off the bow in the middle. Not sure about this as you will still need to “help” support this span with whatever you do to mount the assembly.

Just spit-balling ideas…Good luck.

 

[Mike Davis]

Use a full length brace in the back as a French cleat. It will stiffen the top and provide full support.

 

[MarkDarby]

Was going to use six lag bolts through the inside of the back of the mantle into the wall. I was going to put one lag bolt at each end and two on either side of the cross-bars. I'll up the # of cross-bars to four -- do you think that would be sufficient? That would bring the # of lag bolts to eight.

 

[MarkDarby]

For the curvature in the front, I think I'll force it back into shape as I glue the cross-bars.

 

[Mike Davis]

Lag bolts are usually rated around 20,000 pounds per square inch shear strength. So 1/2 inch is a quarter minus a bit of that rating and 1/4 inch is 1/16th. Pull out strength depends on the supporting material, how it is attached, and depth of bolt into the wall.
For my usual over engineered safety factor I figure about 200 pounds per bolt.