We are going to make modular kitchen and wardrobes for our home using pure hard wood like teak wood. The depths of the shelf vary from 1 feet to 1 and half feet. For shelves, can we use planks of width 1 feet or we need to join planks of 6 inch? In case if we need to join...what kind of joint is the best for wardrobes and kitchens...? Thanks.
For shelves, can we use planks of width 1 feet or we need to join planks of 6 inch?
There's no easy answer to that, because as with most things wood-related the wood itself if the key factor. This is each piece, not the wood species in general.
With the way that most wood is cut even subsequent boards from the same stack, sawn from the same tree in sequence, are all slightly different. And some will be much more likely to be stable than others (generally the ones closer to the centre*).
So the answer is really yes, IF the wood is suitable.
In case if we need to join...what kind of joint is the best for wardrobes and kitchens...?
It has long been standard practice to use butt joints to make up wider boards or panels from narrower stock. Edge-to-edge joints with no further reinforcement are stronger than the wood around them, if done right.
See this for some further details, Not-so-obvious disadvantages of butt joints
*Because the closer to vertical the grain at the ends of the boards is the more stable the wood tends to be, as long as the heart is not in the centre of the board (if present this should generally be cut out and discarded). This is one reason for quarter-sawn wood usually being sold at a premium, even in woods where there isn't a specific figure showing on the face of the board that results from the wood being cut this way.
The idea of using multiple planks is that you would make sure the grain runs in opposite directions on each plank that you join to minimize any kind of cupping which can happen with wider pieces.
With flat sawn boards, the cut means the board will have a change of direction at the middle of the board, and this will make cupping more likely than a quarter sawn board.
With quarter sawn, the log is first cut in quarters (hence the name) then the boards are cub more or less with the grain running perpendicular to the face of the board, and the grain runs in the same direction in the board.
With this example, if you were to use the flat sawn boards, you'd cut them down the middle, then flip one of them over, and then rejoin them using a simple butt joint.