Help me hang these shelves..

[Bflkgjropign]

I'm hanging 3 sets of 'gallery' shelves. Mine are homemade to my specific needs 1.2m long and 15cm deep by 18mm thick. They'll be reasonably heavy by the time I've filled them with pictures. I'm pondering the best fixing to the wall which is lath/plaster in a 30s house. I could drill multiple long screws through the back of the upright direct into the wall to spread the load (10 maybe?), or I could countersink keyhole flush fittings into the back of the shelf. I want a solid secure fixing. What do you think is best?

I should mention, I don't want to use brackets

Really good wall fixings would help. You can get some that screw in. They've been used in our house and are a bugger to remove, so definitely hold well.

If you can find a supporting strut to drill into, all the better.

lath and plaster

you must screw into the supporting studs.

You can locate them using my special tool.

Thanks Piglet Any idea how many fixings would be best for this length of shelf? And would countersunk keyholes be better than long screws direct through the back?

If you have uprights (I can't see any in your pics) then the top one is the most important due to leverage. Put one screw two inches from the top and one six inches down, then one about half way and one near the bottom just to stop warping and hold them straight and flat.

If you have no uprights and no brackets, they will bend down and fall off.