I have a space on top of some fitted cupboards where I would like to use for book shelves.
There is a wall on one side but the other side is open, so I was thinking of building a frame to sit in the space with through dado joints used for the shelves. I don't want to use brackets on the wall as I would like to keep with the style of the house.
As it will be painted I don't mind using MDF or plywood but I wondered what would be the best choice for strength? They shelves will be used for storing lots of heavy technical reference books, so something which will prevent sagging would be best.
If you have a router, you could inset 4 adjustable shelving strips down either side. If not! They do a surface mounted version. Then use in preference 25-30mm MDF for shelves.
Google this link "sdslondon.co.uk/adjustable-bookcase-strips.html"
I think you will have a problem with sagging shelves with the method that you're suggesting
You've said that you'll be storing "lots of heavy books" so any shelf without central supports will sag, the longer the shelf = more sag
MDF, chipboard, ply, all will sag in time
You could either cut some supports from same material as shelves and screw these vertically between top and bottom shelves at approx. 500mm centres or depending on length of shelves, space out equally so looks neat cosmetically
Another method is to fix shelves as you mention, then cut timber battens, 1 per shelf - W -30mm x D - 20mm x length of the shelf. Mark a centre line along length of shelf, drill pilot holes and screw through shelf into edge of batten, ie 20mm side (using timber this way will give more strength) Fix small right angle brackets (or plastic furniture blocks) under ends of battens and into side of storage unit
Just bear in mind that you will lose the depth of the batten when working out your shelf spacing so need to allow extra
how about using a strip or two of steel angle under the shelf as a stiffener? i did this for a shelf i made that bridges two units above my fridge for a 36 bottle wine rack. the shelf is 16mm laminated chipboard so clearly wouldn't take all that weight. worked a treat. conceal the steel with a cleat along the front edge of the shelves.