Instinct or Sagulator?

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Forum topic by spunwood posted 10-01-2010 05:19 AM 2501 views 0 times favorited 6 replies Add to Favorites Watch
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1202 posts in 3040 days

10-01-2010 05:19 AM

Topic tags/keywords: question pine weight sturdy sag failure seat bench design

A few weeks ago, a friend asked/was convinced to let us build a bench/bookcase for her house. She really wanted something sturdy, and I originally planned to build it out of 2×4’s. But lowe’s has wide boards that are 3/4” thick. I really wanted to build it in this style:

I love this style, although I believe his top is 1” thick.

My design has a 4.5’ x 17” top supported by 4 legs just like in his picture which are each about 17” apart and fit into feet which he does not have in his design. The bench will be high (23”) with two middle shelves and a full bottom shelf. Our friend’s highest priority was book space.

I will have a 2 support beams running accross the back horizontally.

My question is this:

If sagulator the sag factor is okay, should I listen to it? I am still worried about building a bench for more than one person to sit on with only 3/4” thick wood.

Am I underestimating the strength of the pine, the joinery, and supports?

Love to hear your thoughts.



-- I came, I was conquered, I was born again. ἵνα ὦσιν ἓν

6 replies so far

View Bobmedic's profile


381 posts in 3006 days

#1 posted 10-01-2010 06:17 AM

I would use some supports under the seat between the aprons you could pocket screw them in there.

View TopamaxSurvivor's profile


18394 posts in 3880 days

#2 posted 10-01-2010 06:23 AM

OR, if you want to stick to traditional jointery, you could tennon them through the ends :-)

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View spunwood's profile


1202 posts in 3040 days

#3 posted 10-01-2010 01:05 PM

Thanks fella’s, that is really helpful.

I’ll go for it.

-- I came, I was conquered, I was born again. ἵνα ὦσιν ἓν

View rhett's profile


742 posts in 3871 days

#4 posted 10-01-2010 02:14 PM

If you’re building something for someone else, its always smarter to overbuild it. Once out of your shop, if it fails, even by blatant missuse, it looks like its your craftsmanship that failed.

It stands to reason that using a 3/4” top is atleast 25% weaker than a 1” top. I agree with Robert in adding cross members from apron to apron.

-- Doubt kills more dreams than failure.

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Peter Oxley

1426 posts in 4079 days

#5 posted 10-01-2010 02:47 PM

If you run cross members front to back (which is what I think is meant by apron to apron), your cross member will be running across the grain of the seat. I believe that softer woods expand and contract even more than harder woods, so you may cause some splitting and/or buckling by attaching cross-grain pieces.

If you are trying to prevent sag between the legs (along the grain), you won’t gain anything by putting supports between the aprons (across the grain) – they would just travel down with the seat when it sags. You need more support along the grain. You could make extra aprons and cut notches in the tops of the legs to receive them, or you could just run stringers (that are the same depth as the aprons) between the legs.

-- -- --

View Gregn's profile


1642 posts in 3188 days

#6 posted 10-01-2010 08:34 PM

Having worked with mostly Southern Yellow Pine 3/4” will work fine with the design of the bench. I built something simular to the same design and it supported 3 people with very minimal sag.

-- I don't make mistakes, I have great learning lessons, Greg

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