How to attach a wooden shelf to a concrete wall

Hello - I am from India and i am writing regarding a query i have on attaching a wooden shelf. My grandfather used to own a good quality wood (unsure if its teak etc..) table, over the years, the legs began to deteriorate and i did not have the heart to throw it away. so i sawed off the legs and planed the base to get a regular wooden box shelf.
It is about
2 feet in length 1 foot in width 0.5 foot in depth.
weight should be about 5 Kilograms.
I am planning of attaching this to my patio, here we have mostly concrete wall construction. My current method is to get 2 'L' shaped metal brackets and attach it to the wall, placing the brackets beneath the wooden shelf.
The only problem with this method i have seen earlier is that the whole unit would tend to irritatingly "sag/ tilt" after it is attached. I am unsure if the spacing between the brackets is too close or too far apart.
If there is a better method , i'd appreciate your ideas and suggestions on this.
best regards, cc
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I did the same in my basement (concrete block wall). Used the 'L' brackets, drilled and installed the plastic screw plugs and used 'dry wall' screws. Hasn't sagged in over 20 years. I used the heavy duty brackets with the extra brace running diagonal between the legs.
Harry K
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wrote:

TapCon screws or your local equivilent
http://www.buildextapcon.com /
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The brackets are bending under the weight of the shelf, right?
Use better-quality brackets. They make metal brackets with diagonal braces, and there are also wood brackets.
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wrote:

Attach a (2x4") ledger board to the concrete wall using concrete taps or screws. Make the length of the ledger board slightly shorter than the shelf. The shelf will rest on the ledger and you can fasten it using deck screws. You may still need a diagonal brace for support. Oil or stain the wood every 3 years to prevent rot.
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If you can drill holes in the concrete then you can use lead anchors or drive pieces of wood into the holes and fasten one edge of your box directly to the wall with screws (machine screws for the lead anchors or wood screws for the wood plugs. Wood can be used to make a brace running from the outside edge of the box/shelf to a point lower on the wall. Fasten using the same method. Some glue on the pieces off wood driven into the drilled holes won't hurt.
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