Number of Dominos needed to support shelf?

[woodworker432]

Hello,

I am building a book shelf, the width is 1100mm and 340mm deep. I am going to use 25mm MDF for the shelves and sides and 18mm MDF for the back.

I don't want to see any visible supports on the shelves if possible, so just need to fix to the 25mm side and the 18mm back, having done some research it looks like dominos can be used for this
( i have a Domino XL)
I would like some advice on how many dominos to put along the 1100mm length please, also if the dominos and some glue alone will be enough to prevent the shelves from sagging when books are put on them.

I don't want to build it then find it wasn't substantial enough and have to re-make it.

Any advice would be appreciated!

 

[MorrisWoodman12]

Somewhere on this site is the Sagalator. It estimates the sag you'll have in a shelf given the materials you using. Sorry I don't know exactly where it is.
I would think it will be better to rebate/dado the shelves into the sides rather than relying upon dominoes but that's just MHO.
Martin

 

[woodworker432]

Thanks for your reply,
unfortunately, i don't have any means of routing the dado/rebate...

Thanks mentioning the Sagulator, i have tried this and it seems ok, however - i am not sure how the method of fixing effects the result.
I.e the calculation shows acceptable but is this using a better method of fixing than the method i will be using?

does anyone have any experience with fixing shelves with just dominos?

 

[Yojevol]

Hello,

I am building a book shelf, the width is 1100mm and 340mm deep. I am going to use 25mm MDF for the shelves and sides and 18mm MDF for the back.

I don't want to see any visible supports on the shelves if possible, so just need to fix to the 25mm side and the 18mm back, having done some research it looks like dominos can be used for this
( i have a Domino XL)
I would like some advice on how many dominos to put along the 1100mm length please, also if the dominos and some glue alone will be enough to prevent the shelves from sagging when books are put on them.

I don't want to build it then find it wasn't substantial enough and have to re-make it.

Any advice would be appreciated!

Depending on your shelf loading, I would expect some sag on the front edge of 25mm MDF over that length. The sag may take some time to become noticeable. I would suggest 6 dominos along the back edge, more to keep it straight than for strength. The shelves themselves could be loose, just slid into place on the dominos. This would allow them to be turned over at a later date if sag becomes excessive. To prevent sag altogether you could put in central supports made of the 25mm material.
Brian

 

[Tris]

https://woodbin.com/calcs/sagulator/

This is the one Martin is referring to

 

[woodworker432]

Depending on your shelf loading, I would expect some sag on the front edge of 25mm MDF over that length. The sag may take some time to become noticeable. I would suggest 6 dominos along the back edge, more to keep it straight than for strength. The shelves themselves could be loose, just slid into place on the dominos. This would allow them to be turned over at a later date if sag becomes excessive. To prevent sag altogether you could put in central supports made of the 25mm material.
Brian

Thank you, I will try six.

great idea to leave it loose, I will go with this and adjust at a later date if it becomes and issue.

 

[woodworker432]

Thank you all for your help.

 

[Spectric]

I think the MDF will need more support than just domino's / dowels because I have found that MDF can split open, the domino / dowel can act like a log splitter. The rebate gives better support.

 

[recipio]

Just for fun I used the Sagulator to calculate the sag using a 30 kg load and you will get a 17 mm sag in the middle. I'm afraid MDF has little structural strength and you would would need to glue on something like 40 x 40 mm wood battens under the front and back edges. The 18 mm back would also make it very heavy. A 6mm panel set into a rebate would be more than enough to stabilize the structure. With a rigid carcass the shelves do not need to be attached to the verticals although I tend to use pocket screws as they are virtually invisible using the plastic covers, I have posted before about using cheap flush doors for shelves - just glue in battens into the cut edges and this is now my preferred way to make bookshelves.

 

[woodworker432]

would a 20mm thick oak edging prevent this movement?

i would rather not put batons below shelf

 

[recipio]

would a 20mm thick oak edging prevent this movement?

i would rather not put batons below shelf

No, I'm afraid you will never get a totally non deflecting shelf of 20 mm over a 1100 span. ( I'm amused to see all those TV experts with groaning bookshelves behind their laptops ). The trouble with sheet goods is that they all need to be edged with something - preferably solid wood. You could glue on 40 x 40 mm battens directly on the the front and back edges preferably using some biscuits if you have a jointer.

 

[Ttrees]

Welding would be my preference, not too costly all in for forty quid.
Pick up some hefty drills for the job in there too, to fit whatever bar you might choose.

Invisible and trustworthy, for a big ol heavy pile of books.

 

[Yojevol]

Here are my efforts in this field.
Note the use of uprights to give short shelf spans which gives the possibility of varying individual shelf positions
In the middle pic the LH unit span is over a meter and I was a bit concerned about sagging (not my design) so I included a central support on the wall. I don't know whether this would have stood the test of time as the whole unit has now been demolished.
The first pic is made from a melamine faced honeycomb sandwich material lipped with ash. The 2nd is out of 25mm MDF and the 3rd is 20mm solid ash with ladder strips to support the shelves.
Brian

 

[woodworker432]

View attachment 167040View attachment 167041View attachment 167042

Here are my efforts in this field.
Note the use of uprights to give short shelf spans which gives the possibility of varying individual shelf positions
In the middle pic the LH unit span is over a meter and I was a bit concerned about sagging (not my design) so I included a central support on the wall. I don't know whether this would have stood the test of time as the whole unit has now been demolished.
The first pic is made from a melamine faced honeycomb sandwich material lipped with ash. The 2nd is out of 25mm MDF and the 3rd is 20mm solid ash with ladder strips to support the shelves.
Brian

Many thanks for sharing.

It looks like I should Rethink my design to include vertical supports.

 

[recipio]

Many thanks for sharing.

It looks like I should Rethink my design to include vertical supports.

I think units with symmetry look better than random shelves. For thinner material up to 25 mm I tend to make them as 400mm ( 16 " ) cubes x 400 mm deep. The verticals stop the books from falling over. If I have to span anything wider than that I use the flush door method assuming they will be painted. If they are to be varnished I use a matching veneered MDF and front/ back lipping. I'd use veneered plywood if I could find it.

 

[paulrbarnard]

Many thanks for sharing.

It looks like I should Rethink my design to include vertical supports.

Don’t be too hasty. Using dominoes along the back will provide a lot of support which means no sag at the back. The only sag possible is on the front edge. As it is a book shelf the width is not so great the effective span is quite a bit less. Also if you are glueing the dominoes on the ends there will be tension also preventing sag. The sag calculations are for unrestrained spans.

That said a nice hardwood edge along the front would look good and add a lot of strength.

 

[Wattsjoinerybarn]

Domino the back every 150mm
Domino the front every 150mm to a piece of hardood that can be sanded flush and decorated, or slightly chamfered to create a nice v join detail. You could also have it lipping down from the shelf to hide a light detail.
No need to rebate the sides, dominos will be more than good enough for this.

Without the hardwood lip there will be some sag

 

[Doug71]

MDF is good for some things but isn't great for shelves. I get paranoid about shelves sagging so I normally end up making them out of solid wood.

The other problem with MDF is the weight. You talk about making the sides and shelves out of 25mm thick, I don't know how tall the unit is but at 1100mm wide and 340mm deep you will get through quite a lot of material, 25mm MDF weighs over 50kg a sheet plus the 18mm for the back is about 35kg a sheet so it could end up a weighty unit!

 

[Spectric]

As it is a book shelf

Real books are heavy, you see so many saggy bookshelves but the idea of a box shelf would give much more rigidity whilst using thinner material.

 

[johnnyb]

I reckon getting all those dominoes in the right place will be a pain. I'm frankly amazed you have a domino xl but no means to make some housing joints! How about ply or solid? To a man with a hammer everything looks like nail. With more appropriate materials a few screws would be as good maybe better and much quicker/cheaper. If your painting just fill holes.