Raised beds need minimum 6 inches deep to 12 inches and it is not hard for developing a raised bed because there are so many ready made things in the market. Take redwood or black wood that comes nearly around 20 years. We will soon post our DIY raised beds here so anyone interested in buying can contact to me.
You can build a raised bed for cheap and that gives you a lot of space to plant many vegetables. These raised beds are most useful in gardens and terrace gardening too.
Spinach Mint and some other leafy vegetables are excellent for using raised beds.
Raised beds are suitable only on the earth than on the terrace because the wood at the bottom will get damaged if we used on terrace. Raised beds doesn't have bottom wood of holding soil so the are not suitable. please check the image how a raised bed can be made.