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Shelf Structure

Posted by snookums2 (My Page) on
Fri, Nov 2, 12 at 15:30

Hi all,

I have a question on 3/4" x 12" pine shelving. Span ranges from 6'-7', 3', and very long, maybe 12' shelves. How important is it to have a strip of wood (about an inch) placed across the front of the shelf. Also does it need to be permanently screwed down to the support strips running under them and along the sides? What about brackets? Some will be storing heavy items, other lightweight, but certainly items would change over time.

Thanks!


Follow-Up Postings:

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RE: Shelf Structure

pine sags a lot. All boards do, but pine especially.

Here's the link for The Sagulator, a calculator for wood sag.

http://www.woodbin.com/calcs/sagulator.htm

You'll need some sort of intermittent shelf brackets simply to hold them up, and also to prevent sag.

The edging strip can do a lot to prevent sag on the shelf--or you can run a metal angle brace along under the shelf as well (and a wider-than-the-shelf edging bracket can camouflage it).

And yes, the edging strip should be glued and nailed (I don't think you need to screw it--I *think* nailing is enough, but wood glue would be needed too) at somewhat frequent intervals.

Here is a link that might be useful: The Sagulator


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RE: Shelf Structure

Thanks Tally. Would poplar be stronger? I'm accustomed to seeing simple shelves with bullnose edges. I did have some very deep pine shelving, about 4 feet wide, no strip or fastening down, that warped over the years.

Does the shelf itself need to be screwed down into the support strips around the perimeter to avoid warpage?


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RE: Shelf Structure

Rule of thumb: do not go over 2' for books (weight) and the wood along the bottom of the shelf provides (grain direction different) added strength to the shelf.

If you must go wide, then add center supports in the center of the shelving.


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RE: Shelf Structure

"wood along the bottom of the shelf provides (grain direction different) added strength to the shelf. "
The grain direction is actually the same, the wdth of the shelf from side ti side.

It is the extra thickness (top of shelf to bottom of strip) that adds the stregth.

Poplar, while a hardwood, is not especially strong.

Ash, oak, or any other number of hardwoods make fer batter shelves.

Using 5/4 lumber (nets out 1 inch thick) or even 6/4 (nets out to 1.25 inch) is stronger if you do not want to put in the ledger strip at the front of the shelf.


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RE: Shelf Structure

Thanks all.

I will look for the thicker wood and check out harder woods. I usually see closet shelves that are not thick when looking at photographs. Not sure what that's about.

Do you think screwing the shelf into the support strips will also keep it from warping?


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RE: Shelf Structure

" I usually see closet shelves that are not thick when looking at photographs. Not sure what that's about. "

Books are not usually placed on closet shelves (or anything all that heavy beyond a single box or two).

Blankets and linens are not all that heavy.

Books are heavy, and put a real premium on wood strength.

Even metal shelving does not have all that much strength until an edge is turned down.


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RE: Shelf Structure

I'm doing pantry shelves too. There are some clothes closets that I can fit more shelves in for book overflow, as well as some other stuff (some heavy, some not so heavy). For the pantry, I'll probably do flexible bracket shelving even though I would prefer a built in.

I think I'll do the strip across the front but put some trim across to dress it up.

Thing about screwing it all down is then they are not flexible if I need to change height or depth down the road. The holes will be filled and painted over so dismantling would be difficult knowing where the screws are later.


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RE: Shelf Structure

If you have a 3-sided ledger, like the pantry shelves at the link, it goes a long way towards preventing sag because the center is supported.

Adding a 1x2 strip across the front, nailed and glued, turns the shelf into a tiny box beam and resists sagging even more.

Here is a link that might be useful: look at the ledger under the shelf


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RE: Shelf Structure

Yes, that's the style. I love those simple shelves, as shown. I might do the face strip but face it with trim.


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RE: Shelf Structure

Plywood is probably the best bet for shelves, especially if you're willing to add a facing strip--Because then it doesn't matter what the wood *looks* like, because the facing strip will cover it. And plywood is always stronger than wood (because the grains in the plys alternate directions).

Also, plywood comes 1" thick, which is another way to keep a long shelf from sagging.

I don't think screwing the shelves into the support strips will make much of a difference; the sag will come between, and the wood will stretch. So it won't really matter.

You can also use a trim strip that's wider than the board. That can increase the strength a lot. And it can be an artistic choice--you can use a trim piece that's 2" wide, and the shelves will then look as though they were made of a nice, thick 2" slab of wood. Just align the trim w/ the top of the shelf and let the rest extend below the shelf. (another advantage: if you use a shelf support, a lot of it will be hidden by the trim piece, so it won't be very obvious)

http://www.familyhandyman.com/DIY-Projects/Woodworking/Woodworking-Tips/how-to-install-edging-for-plywood/View-All

If anything, you might be better off *not* fastening them, and then you can flip the shelf over if it does sag. (can't do this if the trim piece is wider, of course)

(another name for that "ledger" under the shelf is "cleat"; and those shelves are often nailed into the cleat. But not always, especially if the cleat is wide enough )

If you want long, long shelves that don't have much support in the middle, you can create support for them w/ a "ladder" system--I just saw this, so I don't know how sensible it is. See the link below.

Here is a link that might be useful: look at the


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RE: Shelf Structure

<< You can also use a trim strip that's wider than the board. That can increase the strength a lot. >>

So are you saying a face strip can be the same width as the shelf and somehow still work to stop sagging and warping? Would that be due to using the grain running in a different direction?

What about bullnosing a strip that's the same width as the shelf. Would that weaken it's purpose? I've always liked a simple painted, 'thin' shelf with a bullnosed edge.

I'm thinking screwing the shelf down into the support strips would function the same as a strip at the front to keep the boards from warping. I had some pine shelving in a pantry, I believe multiple 12" boards for deep shelves as I recall. But they had became all warped.

I like the idea of using a stronger plywood. Will check that out.

Thanks for all the input!


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RE: Shelf Structure

"And plywood is always stronger than wood"

Not always.

It depends on how the plywood is used.

if you support the shelf at each end only, plywood is significantly weaker than solid wood.

"So are you saying a face strip can be the same width as the shelf and somehow still work to stop sagging and warping? Would that be due to using the grain running in a different direction? "

Thr grain is rubb\\nning the sae direction as she shelf, the width.
It is the additiona thickness te strip ads that stiffens the shelf.

You can use 3/8 in thick plywood for pantry shelves, if you use a 1 inch tall by 1/2 inch thick hardwood strip at the front and back of a 4 ft shelf running the width.
If the shelf was pine the strip would still be adding a lot of strength, ad the grain is all running the 4 foot direction.

Just fastening that small strip of stronger wood to the long dimension of the shelf greatly increases its weight bearing without sag.

One of the problems with the 'sagulator' is that it does not account for ledger strips.

They make the math a lot more complicated (especially if they are a different wood (strength) than the shelf material).

If all else fails, make up a sample shelf, support it at each end on some scraps of wood, then load it up as it will be in use.

If you measure the height in the middle carefully (bottom of strip or shelf to floor) you can find the sag for the load you have applied.

If you use just some 1x wood scraps to support each end on a flat floor, you will quickly find another scrap will not just slide under the middle of the shelf.

It has sagged.

Support on three sides helps a lot as long as the depth of the shelf (front to back distance) is smaller than the width (side to side).


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RE: Shelf Structure

I think I am understanding this.

Thank you.


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RE: Shelf Structure

Here's another way to avoid sag without adding much in the way of thickness or needing center support:
Attach a metal L-angle or C-channel strip along the front edge. Use the slightly wider trim piece to hide it.

The bent metal will stop a LOT of sag! Even if they're only 1/2" wide.


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RE: Shelf Structure

I'm not sure what those pieces are. It doesn't come up clearly on a search. An architect once told me that you could put a steel strip underneath in order to create wider shelving.


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RE: Shelf Structure

"An architect once told me that you could put a steel strip underneath in order to create wider shelving."

Only if you can make sure the steel is adequately fastened.
You need very tight screw shank to screw sized holes (nearly zero clearance).


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RE: Shelf Structure

I think he said the strip gets recessed into the wood with a groove (forget what that's called). Not sure how that would be fastened.


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RE: Shelf Structure

I reinforced the shelves on an IKEA bookcase by sliding aluminum C-channel over the back of the shelves. I bought channel that the shelves barely fit into, and cut the channel a couple of inches shorter than the shelves.

File off the rough edges, slide the channel on and put the shelves back. They are holding a ton of books with no sagging.


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RE: Shelf Structure

Now I see what the c-channel is about! Thanks.

That's a good idea.


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RE: Shelf Structure

You need a VERY tght fit.

Putting metal between piees of wood in frmaing makes what is called a 'flitch' beam.

You could put the metal in a shallow groove on the long edge of the shelf, but you must make sure the metal cannot bow away from the shelf under loading.

the back of the bookcase might be ade3qte to hold the metal in place, and if it is C channel, the metal can move a little without losing contact with the wood (depend on the exact shape of the metal, no taper allowed in the C channel).

It will tend to 'pop out' of the slot without sufficient fastening.


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