Kitchen Bench build - do I need to add support underneath?

[Sean Moore]

Im building a pair of fairly straightforward kitchen benches for someone, out of 10/4 ash. The bench will be 72" long. Should I add a long stretcher underneath to help support the bench, or will the thickness of the stock be enough? See the attached rendering of the design.

Thanks in advance!

[Jim Becker]

Given the length, even with the stout thickness, I think I'd likely to some kind of stretcher, even if it was hidden (as best as possible) up under the bench seating surface. The concern would largely be for racking and resulting stress on the joints. It could also be something contrasting and minimal in keeping with the simple bench design, such as a painted steel rod, end to end and centered to keep things solid.

[Jamie Buxton]

Me, I wouldn't bother putting a stretcher under a 10/4 seat. You could park a truck on that seat. But I would be concerned about how you hook the bench ends to the seat. How are you going to do that joinery?

[Sean Moore]

Me, I wouldn't bother putting a stretcher under a 10/4 seat. You could park a truck on that seat. But I would be concerned about how you hook the bench ends to the seat. How are you going to do that joinery?

Jamie I was planning on using a double row of 14mm Dominos. Figured 10 of them per side should suffice?

[Jamie Buxton]

If you use Dominos, pay attention to getting facegrain-to-facegrain glue surfaces. Facegrain glue surfaces are stronger than endgrain glue surfaces.

That 14mm domino has a cross section of 14mm by 30mm or so. The easy way to insert it into your joint would be to orient the 30mm direction parallel to the end of the seat. Unfortunately, this gives you endgrain glue surfaces. The much-stronger way is to insert the domino with the 14mm direction parallel to the end of the seat. That way gives you facegrain glue surfaces.

[Mel Fulks]

If you weigh all who will be “sharing the grub” , get the weight distributed correctly ,and make it a rule that , “ No One Leaves The Table
Without making sure that their “ weight partner” is texted to be ready to “move out” when you text him the exact “move out time”. Then
all will go smoothly and no one will be catapulted into the ceiling.

[Jonathan Jung]

The joint between 2.25-2.5" thick wood is plenty strong on its own. I'd only use a few dominos for alignment. Only exception is if you're not able to make an even mating of it.

No stretcher is needed. Put a piece of 10/4 anything between two buckets 6' apart, stand on them, see what you think.

[Jim Dwight]

It is a deflection question, not a question of whether the bench will break. A simple way to calculate the defection would be to use a sagulator program - meant for shelves. I think they would allow you to input the plank dimensions. If not, it is not a hard calculation and the formulas are available over the internet. How much deflection is too much is a subjective thing but the rules of thumb used for shelves could be used.

I also agree with the simple experiment already suggested. Even one piece of ash - planned or not - supported roughly as the bench will be with a person or two on it will give you and idea of how much deflection there will be. I don't think it will be much.

[Brian Tymchak]

... A simple way to calculate the defection would be to use a sagulator program - meant for shelves. I think they would allow you to input the plank dimensions.

As others mentioned, I also felt there is no need for any additional structure. But, just for fun I entered your dimensions into sagulator, along with a few assumptions. Sagulator says that your bench should support ~350 lbs per foot while maintaining an acceptable sag of 0.08" over the span.

[Prashun Patel]

I built a bench like this. 84".

I put a center stretcher butted up against the bottom of the seat. It has worked like a charm. No sag. You can't see the stretcher except from a very flat angle. I highly recommend you do that. Cheap insurance.

[Thomas McCurnin]

I would not be concerned about sag, and stretchers wouldn't fix that issue anyway.

What I would be concerned about is the horizontal-vertical connection and whether the leg(s) will splay in or out as a result of stress. My fix would be a dovetail joint, or a 45° stretcher connecting a couple inches off the leg bottom and to the bottom of the seat, or both.

[Prashun Patel]

Not true. If u out the stretcher at the top it stops sag. Racking is not a concern on my bench. It works.

[John Kananis]

Without some time off support, gravity always wins over time. Imho