Raised Flower Bed

We recently moved house, and after 6 months solid work indoors we are now looking at getting the rear garden sorted, I am not what you would say a knowledgeable gardener, but both of us like a nicely kept garden, neither of us is interested in growing vegetables at all. We plan to make it easy maintenance. The lawn is quite big and we are going to lose a third of it to gravel, with a paved seating area. She would like a raised flower bed, something about 6ft square, going to put in lots of grasses, plus we are going to have some more smaller planters dotted around varying from about 2ft square to 3ft x 1ft. I am quite capable and will make these myself. I am going to use tantalised timber, either rough cut or the smoother CLS stuff.
For the large bed would 2" thickness be ok, I had thought of 6"x 2", and making it 12" in height, with 4" x 4" posts in each corner and at 3ft intervals to avoid the structure bending out. For the smaller ones should I stick to 6" x 2" or use 6" x 1". I read a lot about lining them with black plastic sheeting, some say yes others no. I would like them to last for a few years, especially the larger one, as I don't want to have to rebuild it in say 3 years time.
Lastly I will be removing a lot of turf where the gravel is going, is it ok to put a layer of this upside down in the large bed, I will dig the ground under it to loosen it up a bit first, then top it all off with top soil, which I am going to have to buy in bulk bags, soil here is very hard clay, lawn gets waterlogged after prolonged heavy rain periods, throughout last winter it was like this for weeks on and off. Its now awful with all the hot weather we have been having, the garden gets almost constant sunshine :-( .Hopefully it will come back again, as long as its green we will be happy, neither of us are that fussy on pristine lawns.

Oh and there is no time scale to the garden, we are thinking along the lines of 9 to 12 months to sort it.
Sorry my first post is a long ask. Have no fear ill be back again for plant and shrubs selection as things progress. All advice appreciated

 

Welcome to the forum @Denwyn. Timber will rot in time, as I have found out - even tanalised timber. The wooden raised beds here that seem to have survived best are made of the old original railway sleepers - they are oozing with a tarry substance. I built some of my own raised beds with broken paving slabs which cost nothing as most of them came from next door when they took up all their paving to make way for something posher
Any time you remove turf, it is best to pile it up somewhere where it won`t be in the way - upside down, as you mentioned, and just leave it. It will then rot away and make lovely soil to add to your planting.
We would love to see photos of your plot if you could upload some. It can be quite helpful to pick a few brains from time to time (even if they`re sheep`s) as me old mum used to say

 

Welcome Denwyn
You can store the turf upside down somewhere or, do as I do, and trench it in in the autumn face down about 30 to 40 cm deep.

 

Oops sorry for replying via private msg, my fault entirely.
Im afraid Railway sleepers not an option,to difficult to get hold of,arkward to cut and expensive. We have about 80 paving flags spare as soon as builders start new paved area,afraid there being skipped. I did think of using them,but there only approx 17" square and by the time I sink them in ground to steady them it's not worth it,plus there old grubby concrete ones and her indoors won't have it at all.
If I was to use tanalised timber 6 x 2 and put in enough 4 X 4 supports and line with black polythene would I get about 8 years out of it,I would be ok with that sort of lifespan . I I had the budget I'd do brick build but the work on the house itself has severally restricted budget now. Thanks