In your experience, what material is better resistant to sagging from
the weight of heavy books, 3/4" plywood or 3/4" solid lumber? In the
case of solid lumber I suppose it could depend on a number of factors,
not the least of which is the species of wood, but let's just assume a
variety of the most popular species (oak, walnut, mahogany, maple,
cherry, etc.) and good straight kiln-dried lumber that is showing no
outward signs of stress or a tendency to warp. Would that resist
sagging better than a good quality plywood faced with the same species
"Our beer goes through thousands of quality Czechs every day."
(From a Shiner Bock billboard I saw in Austin some years ago)
Solid wood is much better. But if there is going to be a heavy load,
anything will sag over time. When I build bookcases, I always put in
dividers. This not only allows you to catagorize and separate books, but it
makes the unit much stonger. No sag.
If you are making movable bookshelves, you may need to reinforce the board
to eliminate sag. A simple 1 X 2 fastened to one or more edges will usually
do the trick.
...with four foot shelves, I had to put two: front and back.
While I was building them, I did different weight tests on different
configurations (loaded with books and left for a few days)
and that was the only one that resulted in no (negligible) sag.
I've been back, a couple years later, and still no sag under load.
"Playing is not something I do at night, it's my function in life"
Ply or solid wood is about equally sag resistant. The book shelves I
built have a 1.5" wide pine lip on the front and the back of the 3/4"
ply shelf. The lip hides the plies and adds a lot of strength. Even
with that, try to avoid a span of more then 30". Books are heavy.
Ignore 'sagulator'; it omits age and moisture, both of which can
the bookshelf situation.
Flakeboard, plywood, and oak are the worst. That's because the glue
(or in the case of oak, the sap) is important in these materials, and
it will absorb moisture and stretch, or expel moisture and shrink, in
ways that strength-of-materials calculators ignore (because the
load tests are too quick to see the effect).
Pine, with paint, is a very good shelf material: lightweight, stiff,
If you want a fast-drying finish, some woods without lots of sappy
knots will only need a wipe-on shellac. My last project used knotty
pine, and came out well, BUT the plan was dark-stain and polyurethane
varnish: the finishing steps took a week (sand, wood-conditioner, dry
time, stain, dry time, touchup stain irregularities, dry time,
clear polyurethane, dry time). Next time, a different plan.
Most of the shelf surface is invisible in use: a decorative strip on
or (better for stiffness) a front skirt board that adds some depth,
an appealing design choice.
I used red oak, with a 1"x2" front lip at 90 degrees (like angle iron)
and after 11 years of holding hundreds of computer books, there
~might~ be a 1/16th inch of sag in the middle. Lip was attached using
biscuits and glue, shelf was attached into dado on upright with glue
and screw, and some plugs to hide the screw heads along the side. I
made 2 cases, 36" wide 8" deep, 8ft tall. ymmv.
Just to amplify on this, that front lip 2x1 inch on a 1x7 inch flat
is about half the stiffness of the whole assembly.
If pine is half the cost and half the stiffness of oak, we can
the cost and stiffness of a shelf design.
Oak 2" lip / oak 7" shelf: cost 9, stiffness 15
Oak 2" lip / pine7" shelf: cost 5.5, stiffness 11.5
Oak 2.25" lip/pine 7" shelf: cost 5.75, stiffness 15
Oak makes a pretty face, pine holds its share of the load; I'll take
third design and call it good.
Plywood shelves sag more than solid hardwood of the same dimensions. Even
without a preciase numerical answer, a few moments of reflection will make
it clear that this is inevitable, given the softwood cores and internal
voids and patches of typical plywood. With that said, I'm reasonably
satisfied with 3/4" plywood shelves for spans shorter than 30", given normal
sized books packed end to end on the shelf. Substantially heavier loads,
such as a set of encyclopedias, beg for a different solution.
The primary issue is what are you trying to do? Looks, ease of build,
cost of materials, and so on are the key factors. Oak is certainly
stronger than plywood, and looks a lot better. Look for quarter sawn
more than flat sawn lumber. Also, when you buy 3/4" Hardwood at a mill
or lumber yard, you generally get more than 3/4, often 7/8". Fixed
shelves are stronger than floating, Face frames are really strong if
shelves are attached on all four sides and sag is then almost a non-issue.
Right now I'm looking at some cheap ass book shelves my wife bought
about 15 years ago. They are cheap veneered particle board. The shelves
are floating 3' long that I attached to the back, no face frame and
loaded with computer books. The sag is about 1/16". If they had a
facing, they wouldn't have sagged at all. If they were oak, I don't
think they would have sagged either. Personally, I like bookshelves with
attached sides and a face frame. Floating shelves are about never
adjusted. Once you decide where to place the shelves, they pretty much
never move again, so might as well make them permanent and design
something strong and nice looking.
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