Specification for wooden shelving (thickness vs length)

[Mosaic]

This question arises because I was never taught woodwork at school. I want to hang shelves to fill the alcoves either side of the chimney breast in my living room. These are about 1.5m wide. I have lots of books and don't want the shelves to sag in the middle. Can anyone advise me about what thickness of timber I would need to achieve this?
Thanks in advance.

[Francis Asis]

Your best bet would be to fit battons round the wall and use 18mm contiboard with a front batton for extra support.
So says mr francis the chippy.

[HowardKennitby]

1.5m seems very wide for shelves to hold up books. Paper is heavy.
I have done some 0.75m wide using an adjustable shelving system which has 2 verticals screwed to the wall with clip on brackets for the shelves. The shelves were 16mm Contiboard and this has worked fine.

[Mosaic]

Thanks folks!

[The Builder]

Moses ........... 1.5m is too much for anything less than 40 or 50mm thick timber. Even then, it would probably sag eventually.

18mm is fine........... one suggestion would be to fit a couple of "dividers" to each shelf. Starting from the floor, each one is fixed vertically above the one below. All the load is taken down through each divider to the floor.
Cut each one a couple of inches narrower than the shelf so that it's partially hidden by the books each side of it. 10 or 12mm thick would do it.

[HowardKennitby]

What The Builder suggests is fine and will work. The only drawback I can see with it is that the height of each shelf is fixed.
Personally I would split the alcove in half vertically and have adjustable shelving in each half.

[The Builder]

........... even more sensible, Howard :o)

[mrs_overall]

Oooh, I love it when you go all technical Monsieur le Bodger.

Found any porn stashes this week? ;-)

[The Builder]

............. Monsieur Le Constructeur .......... if you don't mind Mrs Overflow ;o)

No stashes.......... but I did have to get into another airing cupboard ;o)

[Mosaic]

Thanks to you all again! I'm liking the internal divider idea.