Best Joinery for Boxed Mantel

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Forum topic by kocgolf posted 06-23-2013 05:51 PM 1583 views 0 times favorited 5 replies Add to Favorites Watch
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373 posts in 2418 days

06-23-2013 05:51 PM

I have a pic here of what I am planning to do with a boxed mantel for a friend. This is a project I haven’t had experience in, so I’m shooting from the hip here. The Yellow top and bottom are maple plywood, the lighter fronting piece is hard maple, and the brown at the back is a 2×6 which is lag bolted to a brick facing. I am thinking I will rabbit, then glue and screw as indicated by black lines, with the top screws C being countersunk and plugged. The joint at A will be covered by a decorative hard maple facing strip. There will also be brackets under the mantel to support it against the brick.

I’m just a little unsure of the joint strength I will need for mantel as I haven’t worked on one before. My friend says she is not planning on putting anything too heavy on it. This seems adequate, and is the best solution I could come up with, but I do not have years of experience and maybe I am missing something obvious. I also don’t want to rabbit out too much of the plywood on the top and risk any kind of break at the top surface.

5 replies so far

View Loren's profile


10477 posts in 3887 days

#1 posted 06-23-2013 06:41 PM

If there are brackets underneath you should be alright… you
will have to make sure they sit square though and on brick
there can be some variation.

You may want to allow for scribing on the top.

I don’t have an issue with the joinery, but you do need
to plan to screw it tight to the wall and this can be tricky
to figure out.

View kocgolf's profile


373 posts in 2418 days

#2 posted 06-23-2013 06:47 PM

I was also planning at least a couple interior cross pieces to keep it square which could also be lag bolted to the 2×6.

View realcowtown_eric's profile


617 posts in 2176 days

#3 posted 07-04-2013 06:17 AM

If I’m understanding this, you are gonna use three materials. 2×56 spruce, maple ply and solid maple.

Yer 2×6 spruce aint gonna have square edges, nor is it likely 5.5”, nor is it likely straight. factor that into yer equation. Fstening to the wall ain’t really a problem, Tapcons to masonry, good Hilti toggle bolts, or screws into studs.

Using solid maple for the front, maybe 5’ long, and maybe 6-7 inches wide is about 3-5 bm, maybe 15 bucks in rough material. So why use ply for the top bottom. A sheet of maple ply is gonna run you 50-80 bucks and yer only gonna use a 1/4 of it max. Why not do the whole thing in solid.

Save on fussy finishing and jointery doncha think??

You don’t need no brackets unless it’s for appearances, if you use sufficient anchors for the 2×6. Ain’t nobody gonna set a gadzillion pounds on this.

But what you MUST do is get the info on the fireplace and ascertain the clearances allowed by the manufacturer. You CANNOT just put a mantle of whatever projection wherever you want. The manufacturer specifies the allowable projection of a mantle at different heights above the firebox, Yes grasshopper, even for gas fireplaces.

Moreover, local codes may require more clearance, so you gotta double check that. And while yer checking , see if you require flame retardant finishes? The joinery and material become secondary considerations when it comes to compliance with builiding/fire codes and safety issues. BTW, what about the ends? what kinda joinery are you gonna use there?

I know…it seems like such a simple thing. But that’s why we all pay our liability insurance premiums eh?


-- Real_cowtown_eric

View frosty50's profile


46 posts in 2587 days

#4 posted 07-05-2013 08:57 PM

I have built maybe 200 mantles for different people and contractors over the last 30 years. Your basic idea is fine. I recommend that you dado the face of mantle fo accept the maple plywood or lay the top directly on to the face and glue it well, then trim it out with the moulding. As for attaching it to the wall or brick, offset the back panel of the mantel 2” and use a 2×4 cleat on to the wall. The Cleat needs to fit tight into the mantle when you mount it. you may need to take a 2x and cut it to fit. If you do this, cut a belevel at 30 or 45 degrees to allow you to scribe the mantle top to the wall. After trimming the scribe to the wall, you can use glue and nails or finish screws and attach it. I did this for my mantles, and it presently hold s a 60” flat screen tv with no problem. With brackets installed, it should hold a couple hundred pounds of weight with no problem.

-- frosty

View kocgolf's profile


373 posts in 2418 days

#5 posted 07-05-2013 10:05 PM

First, thanks to the recent very thought out responses. I am building a replacement for an older mantel with almost identical dimensions, so it should be fine for code. The reason for the plywood top and bottom is that I am not a “full shop” wood working and don’t have an ability to process rough cut wood, so with a width of over 9 inches for the top, and only 7.5 inch hardwood available to me pre-finished, I have to use plywood. Which is ok because I have other project to use bits of the leftover. I could also use it for the interior braces instead of pine, but seemed a waste of better material. The ends do not need any joinery as they will be open and abutting walls. The fireplace is a corner unit, so each end of the L shape with be on a wall.

Thanks also for the idea to use a 2×4 cleat system, something I hadn’t though of. I will have to double check on the mounting board up though, as I seem to remember she didn’t countersink the lag bolts that hold it to the wall. I may not be able to flush a 2×4 against it. It’s a great way to do it if it works.

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