Stacking cabinets: too much weight?

[Ken Cobbing]

Hello all,

I'm continuing to build cabinets and shelving for my first time. I've designed a wall of storage cabinets as pictured below. Each unit is 3' wide x 2' high and 2' deep with a shelf in the middle. The cabinets are frameless and I intend to install inset doors with euro hinges (the sketch does not show the middle shelves being recessed to accommodate the inset doors). I'm using 3/4" baltic birch and pocket screw joinery.

What are your various thoughts on stacking these units 4 high as I have shown in my sketch? I'm wondering if the weight will become too great once loaded - or even before. I intend to attach the cabinets to the wall, although I haven't decided the best way to do that yet (structural screws through the backs and into the studs? French cleat?). Anyway, the floor bears all the weight rather than the wall.

The reason that I designed this as eight separate cabinets rather than, say, two eight-foot-tall cabinets is so that I could accomplish the construction by myself. I also thought (perhaps incorrectly) that stacking numerous smaller cabinets will be more sturdy compared to larger frameless cabinets.

All feedback is appreciated! Thanks for reading. And sorry for another very basic question!

Utility Room.jpg

[Jamie Buxton]

You're okay. All the weight goes down the walls to the floor.

Your floor might not be flat and horizontal. You might look into leg levelers or the like. The leveling scheme needs to support not only the outside walls but also the middle wall.

[Ken Cobbing]

Thanks, Jamie.

My main worry is that I don't understand how much weight the bottom frameless cabinet can bear loaded on top of it. I think each cabinet will weigh about 125 lbs when complete and empty. So that's nearly 400 lbs sitting on top of each of the bottom cabinets. When loaded the bottom cabinet might have three times that or more - 1200 lbs+. Can it really bear all that weight?

By the way, I think what you referred to as the "middle wall" is actually not a wall per se, but where the 2 bottom cabinets butt together. I'm not sure if that was clear from the drawing.

[Adam Herman]

a couple things:

depending on the weight on the shelves, a 24in span of 3/4 ply will sag a pretty good amount over time. you may find you need something to stiffen the shelves.
are the shelves going to be dado-ed into the boxes or adjustable?

the double thick center section will be your strong point. I would only worry about the strength if the shelves on the bottom units are not fastened to the outside verticals. the ply could bow enough that the shelf would fall because of the increase in width. What are you planning for a back? 1/4 ply?

[Malcolm McLeod]

Thanks, Jamie.

My main worry is that I don't understand how much weight the bottom frameless cabinet can bear loaded on top of it. I think each cabinet will weigh about 125 lbs when complete and empty. So that's nearly 400 lbs sitting on top of each of the bottom cabinets. When loaded the bottom cabinet might have three times that or more - 1200 lbs+. Can it really bear all that weight?

By the way, I think what you referred to as the "middle wall" is actually not a wall per se, but where the 2 bottom cabinets butt together. I'm not sure if that was clear from the drawing.

The sides of each box are essentially a column. And as drawn, the greatest unsupported 'height' of any given column is 12". So a 2' deep by 12" high column should easily support the expected 600lb load on it (1200lb / 2 sides).

I'd daddo/rabbet all the shelves/tops into the sides, then screw from the outside (into shelf edges). And if you're planning to store gold ingots, I might be cautious and use the same 3/4" material for the box backs , also screwed into edges.

Also securely screw the top of one box to the bottom of the one above (add adhesive if you don't plan to move?). Use enough that top and bottom become one piece essentially. Result is a 1-1/2" thick 'shelf' (bet it won't sag).

That leaves only the 2' span of front edges of middle 3/4" shelves that might sag - - from all that gold.

[Thomas Bank]

A question - what is the floor this is bearing on? I see a roller cabinet next to it, so possibly this is a concrete garage/workshop floor? Hopefully this isn’t a 2x framed floor in your house with 1200# bearing on it!

[Ken Cobbing]

Hi Adam,

Thanks for your thoughts.

The shelves will be fixed with pocket screws to the sides and back. The back is inset and uses the same 3/4" baltic birch used for the rest of the cabinets.

Regarding sagging shelves: First off, unfortunately I know knowing nothing on this subject from personal woodworking experience - only what I've read. So I used the sagulator to estimate how much weight the single shelf inside each cabinet will hold. I found that being 24" deep and 36" wide, each shelf can hold 150 lbs/ft and still meet the target sag (0.02 in/foot). That is far more than enough weight for my needs. I think I'm correct that the bottom of each shelf will hold much more weight than that since it is effectively 2 x 0.75" thick (the bottom of each cabinet sitting on the top of the one below it). Feel free to correct me if I'm missing something on this subject!

I don't understand why the double-thick center section will be a strong point in terms of bearing the load of each cabinet on top. I visualize each bottom cabinet to independently bear the weight of the 3vcabinets set on its top. I understand that if something were to be placed across the top of the double-thick center section that this would be a strong point, but I do not see that being the case as currently designed.

Thanks again for your help

[Ken Cobbing]

A question - what is the floor this is bearing on? I see a roller cabinet next to it, so possibly this is a concrete garage/workshop floor? Hopefully this isn’t a 2x framed floor in your house with 1200# bearing on it!

Good question! Something I should have specified in the original post. This is a utility room on a concrete slab. I just laid new solid vinyl tile on the floor with a weight rating of 800 PSI. I also applied a vapor retarder coating on the bare concrete floor.

[Adam Herman]

i was assuming that they would all be screwed together. even stacked together the center will be better able to resist bending stress. a dado joint will be stronger than just pocket screws for the shelves, I would not use pocket screws in a pure shear application unless there was no other way to do it.

[Thomas Bank]

Good question! Something I should have specified in the original post. This is a utility room on a concrete slab. I just laid new solid vinyl tile on the floor with a weight rating of 800 PSI. I also applied a vapor retarder coating on the bare concrete floor.

An engineering office I worked in put a bunch of flat files in on the second floor to hold sets of paper engineering drawings. A few days later one of the engineers noticed a big crack in the ceiling below the files and that the ceiling was bowing… Ummm… He went over to the structural department and asked a couple questions about floor loading and how much paper weighed. They quickly determined that the files were about three times the allowable floor load…

One would think that engineers would be more aware of such things.

[Ken Cobbing]

The sides of each box are essentially a column. And as drawn, the greatest unsupported 'height' of any given column is 12". So a 2' deep by 12" high column should easily support the expected 600lb load on it (1200lb / 2 sides).

I'd daddo/rabbet all the shelves/tops into the sides, then screw from the outside (into shelf edges). And if you're planning to store gold ingots, I might be cautious and use the same 3/4" material for the box backs , also screwed into edges.

Also securely screw the top of one box to the bottom of the one above (add adhesive if you don't plan to move?). Use enough that top and bottom become one piece essentially. Result is a 1-1/2" thick 'shelf' (bet it won't sag).

That leaves only the 2' span of front edges of middle 3/4" shelves that might sag - - from all that gold.

Hi Malcolm,

You've really addressed the heart of the issue for me by explaining the structure in terms of a series of 12" supported columns. When you say that a 2' deep by 12" high column will easily support 600 lbs, do you say so from experience or from theory? I've searched the web to find a reference for this type of information (basically a vertical "sagulator"), but have so far come up empty handed. This is my major concern with this project.

I'd prefer to use the joinery techniques you mentioned (daddo, rabbet), but am planning instead to use pocket screws. This is because of my limited experience and workshop area/tools, as well as limited time. On the one "test" box that I've constructed so far, the top and bottom sit in-between (not on top of) the sides and are pocket-screwed into them. The 3/4" back is inset and is then screwed into the top/bottom/sides. The middle shelf is also screwed into the back and sides.

[Malcolm McLeod]

I speak from 'assumed experience' - - I've built similar shelving, but never actually weighed everything that came to be piled on it. I've overhauled 2 or 6 relative's houses that had 3/4 plywood (improperly) installed in a load-bearing fashion. I'm sure sizes were slightly different than yours, but the loads were nothing short of staggering and there was never any sign of 'column buckling'.

Dado/rabbet recommendation is my being cautious. I'd still recommend it, but understand time/material/tooling constraints.

Load it till it creaks, then put the rest in SWMBO's closet! When revolt is impending, offer to strengthen shelves. Promo flyer for router or tablesaw in-hand.

Edit: Adam has a good point about screws and pure shear. If unavoidable, use a lot of them - - ~4-6" OC??

[Robert Engel]

Quick answer: A lot. The structural strength of the material is what counts. In this case, the vertical weight bearing strength of plywood is extremely high, as long as the material is prevented from bowing, which is what a back and the shelves will do.

I have frameless cabs made of melamine supporting granite coutertops standard procedure, no problems.

BTW I usually don't go over 30" on storage cabs with 3/4 ply. I recommend a hardwood bullnose on the shelves to keep sagging to a minimum.

[Ken Cobbing]

I speak from 'assumed experience' - - I've built similar shelving, but never actually weighed everything that came to be piled on it. I've overhauled 2 or 6 relative's houses that had 3/4 plywood (improperly) installed in a load-bearing fashion. I'm sure sizes were slightly different than yours, but the loads were nothing short of staggering and there was never any sign of 'column buckling'.

Dado/rabbet recommendation is my being cautious. I'd still recommend it, but understand time/material/tooling constraints.

Load it till it creaks, then put the rest in SWMBO's closet! When revolt is impending, offer to strengthen shelves. Promo flyer for router or tablesaw in-hand.

Edit: Adam has a good point about screws and pure shear. If unavoidable, use a lot of them - - ~4-6" OC??

Good to know! Thanks for shairng your real-world experience.

LOL @ SWMBO. Never heard of that acronym before. This is the impetus for the project

On the test cabinet I spaced screws 6" or less apart and 2" from edges.

[Ken Cobbing]

Quick answer: A lot. The structural strength of the material is what counts. In this case, the vertical weight bearing strength of plywood is extremely high, as long as the material is prevented from bowing, which is what a back and the shelves will do.

I have frameless cabs made of melamine supporting granite coutertops standard procedure, no problems.

BTW I usually don't go over 30" on storage cabs with 3/4 ply. I recommend a hardwood bullnose on the shelves to keep sagging to a minimum.

Good to know, Robert. I did try searching for base cabinet load bearing capacities and found that granite and concrete countertops weigh about 18 lbs/sq ft. For my cabinets, which have 6 sq ft tops, that's about 110 lbs - heavy to be sure, but only about 1/4 the weight of the 3 cabinets sitting on top.

Thanks for your feedback on the cabinet length. I do agree that 36" is quite long. But the dimensions worked out best for my room size using 5 x 5 sheets of Baltic Birch, and the sagulator suggests that the middle shelf can withstand a load of 150 lbs/ft (450 lbs total over 3 feet) without noticeable sag. That seems like a LOT to me, and is far more than I'll be loading the shelves. But maybe I've used the sagulator incorrectly? The sagulator also does not account for the shelf being fixed to the back, which I imagine increases the capacity substantially?