What would you suggest is the most suitable timber to use when shuttering a concrete base slab for a garage.
I have read an article on the web which was recommending 150 x 47 timbers which seems a bit over the top to me ?

does seem a bit OTT to be too. If you put stakes in behind the shuttering at appx 2 ft invervals you shoud be able to get away with something far thinner ( I'd use 2x2 for stakes), I've used 18mm OSB in the past cut into strips to suit slab thickness - shoud be able to get all from one 8 by for sheet (circa £15) , if you overlap the corners and fit together with a piece of 3x2 behind this should do the trick, obviously you will need to join pieces on runs over 8ft by a smaller piece behind screwed thro. Benifit I found using sheet material you can accomodate for any gradient in the gound easier.

Good luck