# Structural use of back of cabinet



## nosuchhounds (22 Nov 2022)

Im looking to make some drawer units for my shop and wondered if the back of a unit adds any structural integrity, especially if rebated or encased?

The units will.be made of 18mm ply and was looking at an overall height of around 700mm, width of around 500mm


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## Spectric (22 Nov 2022)

The back panel if done correctly will prevent the cabinet from lozenging, it adds a form of triangulation.


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## recipio (22 Nov 2022)

You don't want the carcass to rack which will jam the drawers so yes, a 6 mm back will prevent that. You could also use 12 mm ply unless you are trying to use up the 18 mm stock. Finding the _optimum _solution is the fun bit in furniture design.


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## Jacob (22 Nov 2022)

nosuchhounds said:


> Im looking to make some drawer units for my shop and wondered if the back of a unit adds any structural integrity, especially if rebated or encased?
> .....


Yes it will, but not in a good way. 
Basically the back needs to be loose. If structural, any movement will be greater at the front instead, and drawers and doors could stick. Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.


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## nosuchhounds (22 Nov 2022)

Jacob said:


> Yes it will, but not in a good way.
> Basically the back needs to be loose. If structural, any movement will be greater at the front instead, and drawers and doors could stick. Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.


So captured back with a bit of play in the groove? Deeper groove than the size of the panel?


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## eribaMotters (22 Nov 2022)

I would not worry about having the back loose as you are dealing with manufactured board and not solid.
At present I'm making a small bookcase unit out of 18mm MR MDF. It will be painted and the 9 mm back will be glued and pinned on before hand.

Colin


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## Jacob (22 Nov 2022)

nosuchhounds said:


> So captured back with a bit of play in the groove? Deeper groove than the size of the panel?


Sounds good!


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## rogxwhit (22 Nov 2022)

Jacob said:


> Nearly all trad furniture has very loosely attached backs, thin ply, panels, T&G boards etc just nailed and not structural.


He's not doing trad, Jacob. Poke your head out of the trench for a minute. He's doing plywood. A trad carcase would quite likely have a construction (dovetailed corners, tenoned rails) that provided anti-racking at the front _and_ back. Here, there's presumably little of that, so anything helps! So yes, fit the back panel tightly, and fix it in. Wood movement isn't a significant issue here. Structural movement is.


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## Jacob (22 Nov 2022)

rogxwhit said:


> He's not doing trad, Jacob. Poke your head out of the trench for a minute. He's doing plywood. A trad carcase would quite likely have a construction (dovetailed corners, tenoned rails) that provided anti-racking at the front _and_ back. Here, there's presumably little of that, so anything helps! So yes, fit the back panel tightly, and fix it in. Wood movement isn't a significant issue here. Structural movement is.


Still applies. Any loads causing movement could show at the unbraced front more so than at the heavily braced back.
It's like a box with one side missing (the cupboard or chest front). The open side will be bendy (e.g. corner to corner) but the opposite side (the back) much less so.


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## rogxwhit (22 Nov 2022)

You're only right as far as you're right. That's no excuse for casting the whole caboodle to the winds. Give the job half a chance - it's got little chance at all otherwise. You want the whole thing to slop about? Come on, man, pull yourself together!


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## Jacob (23 Nov 2022)

rogxwhit said:


> You're only right as far as you're right. That's no excuse for casting the whole caboodle to the winds. Give the job half a chance - it's got little chance at all otherwise. You want the whole thing thing to slop about? Come on, man, pull yourself together!


OK he might get away with it but it just happens that I've recently seen a cabinet very carefully made with expensive timber and a solid back etc but the doors no longer fit well as they did, for the reasons I've given!
Good idea to look at a few cabinets and see how they are made and how they are getting on.


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## Jacob (24 Nov 2022)

Jacob said:


> OK he might get away with it but it just happens that I've recently seen a cabinet very carefully made with expensive timber and a solid back etc but the doors no longer fit well as they did, for the reasons I've given!
> Good idea to look at a few cabinets and see how they are made and how they are getting on.


In fact it's simple logic:
In any structure prone to movement from whatever cause, if you stiffen part of it then there will be more movement in another part. If you strengthen the back then the movement will be at the front........


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## rogxwhit (24 Nov 2022)

Jacob said:


> In any structure prone to movement from whatever cause, if you stiffen part of it then there will be more movement in another part.


That's bunkum, Jacob. Your simple logic has yawning gaps in it ... ;-)


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## Jacob (24 Nov 2022)

rogxwhit said:


> That's bunkum, Jacob. Your simple logic has yawning gaps in it ... ;-)


Why?


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## peter-harrison (11 Dec 2022)

i


Jacob said:


> Why?


If I understand your argument right, it's like saying that if one of your legs is broken, you should break the other one to make it easier to walk!


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## Jacob (11 Dec 2022)

peter-harrison said:


> i
> 
> If I understand your argument right, it's like saying that if one of your legs is broken, you should break the other one to make it easier to walk!


er, not really, you haven't understand my argument right. 
It's not an argument anyway it's a simple observation; it's just one of those things which most furniture makers all seem to know and follow. You only have to look behind a few chests of drawers to see.
Have a look at "Techniques of Furniture Making" Joyce. I haven't checked but I guess, if anything, he will say something similar.


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## Hornbeam (11 Dec 2022)

If the back is a plywood or mdf based panel and the grain of the solid timber is running along the edges of the panel ( which would be normal for a set of shelves or a cupboard)then it is better to rebate or groove and glue all round. And yes it will significantly add to the strength of the cabinet. If it is a solid timber panel or a slatted/T&G back yoh need to allow for expansion/contraction and should fit without glue and slightly loose


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## Jacob (11 Dec 2022)

Hornbeam said:


> If the back is a plywood or mdf based panel and the grain of the solid timber is running along the edges of the panel ( which would be normal for a set of shelves or a cupboard)then it is better to rebate or groove and glue all round. And yes it will significantly add to the strength of the cabinet.


Will add "strength" yes, but to the back, and shifts any movement to the front. Wouldn't matter for open shelves but with doors or drawers could cause them to stick.
Imagine an open ended tea chest on its side - if you stood on top of it the open end (where doors or drawers might go) would bend and distort, the closed end would stay square.


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## Hornbeam (11 Dec 2022)

Your tea chest analogy is slightly flawed but stay with it. If the base wasnt there or didnt offer any rigidity then the whole thing would collapse. by folding along the corners. The second point is that the chest is quite deep. The hallower the chest, the more the rigidity from the bottom is transferred through to the open end. If the chest was only 50mm deep it would be totally rigid. If it was 5metres deep then you would be correct that the base would offer minimal rigidity to the open end


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## peter-harrison (12 Dec 2022)

Jacob said:


> er, not really, you haven't understand my argument right.
> It's not an argument anyway it's a simple observation; it's just one of those things which most furniture makers all seem to know and follow. You only have to look behind a few chests of drawers to see.
> Have a look at "Techniques of Furniture Making" Joyce. I haven't checked but I guess, if anything, he will say something similar.


I don't think it is a simple observation. I've made lots of furniture out of both solid and manufactured boards. For the solid, I allowed for expansion/ contraction; the ply etc I didn't. I never had a problem with either, ecxept a couple of times when I ignored some basic rules like not sticking a long bit of end grain to a piece of long grain. I have had a look in Joyce. He says, "In all carcass work it should be remembered that while a well-fitted carcass back, grooved, *screwed or glued* in position will help to achieve overall stiffness and squareness, it will not automatically ensure such rigidity"


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## Jacob (12 Dec 2022)

peter-harrison said:


> I don't think it is a simple observation. I've made lots of furniture out of both solid and manufactured boards. For the solid, I allowed for expansion/ contraction; the ply etc I didn't. I never had a problem with either, ecxept a couple of times when I ignored some basic rules like not sticking a long bit of end grain to a piece of long grain. I have had a look in Joyce. He says, "In all carcass work it should be remembered that while a well-fitted carcass back, grooved, *screwed or glued* in position will help to achieve overall stiffness and squareness, it will not automatically ensure such rigidity"


Yes I checked with Joyce.  He presents every alternative! He says thin or loose backs are a feature of cheap furniture. Oh well, cheap furniture is all I know!
But think I could be right in certain circumstances.
I only thought about it at all because someone I know made an immaculate cupboard with sliding doors, which stuck. He said he thought the solid fixed back would have stabilised it, and it had, but at the back not the front.
I checked several other cupboards and they all had loose (ish) backs, both modern and traditional. Presumably cheap furniture!
Trad chests of drawers need structurally tight front, loose runners, and the back loose without being particularly structural


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## Jar944 (12 Dec 2022)

We solidly attach cabinet backs over here. Typically glued and screwed or nailed into a rebate or groove.


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## Terry - Somerset (12 Dec 2022)

Just a non-professional perspective.

Without a fixed back, stability relies on the joints to sides, top, bottom. How do traditional joints (eg: M&T or dovetail) compare with those on a plywood carcase (glue, screw, batten?). 

I can understand how a fixed back means that most deformation occurring will happen at the front. But with thick man made boards I would expect this to be limited. 

Does traditional design contribute to structural integrity - eg: fixed shelves, cross bracing.

In conclusion is it a balance of risk - a loose back allowing transformation into a parallelogram, or distortion at the front causing drawers to stick. Or is it a fuss about nothing - my MDF or chipboard based kitchen cabinets after 14 years of use still haves running drawers and working doors.


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## Jacob (12 Dec 2022)

Terry - Somerset said:


> Just a non-professional perspective.
> 
> Without a fixed back, stability relies on the joints to sides, top, bottom. How do traditional joints (eg: M&T or dovetail) compare with those on a plywood carcase (glue, screw, batten?)
> I can understand how a fixed back means that most deformation occurring will happen at the front. But with thick man made boards I would expect this to be limited.
> ...


A chest like this would have 3/4 to 1" drawer rails 3 to 4" wide stub tenoned into the sides, and either rails or solid top/bottom dovetailed to solid sides. Bomb proof! Back panel just a detail.








Terry - Somerset said:


> In conclusion is it a balance of risk - a loose back allowing transformation into a parallelogram, or distortion at the front causing drawers to stick. Or is it a fuss about nothing - my MDF or chipboard based kitchen cabinets after 14 years of use still haves running drawers and working doors.


Your MDF cabinets would have planted-on doors or drawer fronts rather then set-in, so hiding any movement. Still need something to keep the thing square - so it'd be the back, or being built in.
PS I'd forgotten how extensive and detailed Joyce is - the book is an essential read!


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## doctor Bob (12 Dec 2022)

Jacob said:


> OK he might get away with it but it ...............................................


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## Drifter2406 (13 Dec 2022)

Backs do help the structure especially on cheep furniture, if you have a square box with no back or bracing it will wobble around and the bigger the box the more the wobble. I worked in a fish tank industry before this job and with the amount of weight the cabinets were expected to take without a back they would have collapsed, no fixings other than a groove to sit in all the way round, I work in a kitchen industry now and we staple the backs in, tall units get a brace too.


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## Fatherclive (13 Dec 2022)

Jacob said:


> Will add "strength" yes, but to the back, and shifts any movement to the front. Wouldn't matter for open shelves but with doors or drawers could cause them to stick.
> Imagine an open ended tea chest on its side - if you stood on top of it the open end (where doors or drawers might go) would bend and distort, the closed end would stay square.View attachment 148903



How did you get a picture of the tv stand I have just made??


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## Agent_zed (19 Dec 2022)

If it were me I wouldn't worry too much about expansion with plywood. If you are using draw runners you have a couple of mm play anyway. I'd do 18mm box and a much thinner back fixed in place, probably in a rebate to make it neat and strong. 

What I haven't seen mentioned though is adding bars across at the front. A couple of horizontal 18mm x 50mm bars will prevent any bowing of the side panels if you put heavy things on top of the cab. If you put them flat so the 18mm is the vertical face then your draws can be made to hide it if you want.

I built a speaker cab out of 18mm ply with finger joints on the corners. Its an open back cab so only has 2 pieces (top and bottom) tight fittting and screwed in place, with about 2/3 of the back open. The front is a floating (couple of screws) baffle of 12mm ply holding the speaker. Its rock solid.


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