As garden sheds and summerhouses floors are very important to the life of your new building you need to be very careful in your choice of manufacturers. Besides the type of materials used, many of which are NOT suitable for outdoor use, you need to consider various other things to ensure your new garden building will stand the test of time.
An important question to ask is - how thick is the floor. The very minimum size you should accept is 16mm (12mm finished thickness). This size will create a strong floor if made from proper timber, however, beware of garden shed companies offering shed floors only 10mm thick (this is 3/8 's of an inch) which is smaller than a standard pencil. Just take a pencil and see how easy it breaks. This will prove to you how foolhardy it is to use boards this thin.
The floor boards themselves sit onto floor joists and again you need to check the number of floor joists and the size of the floor timbers used. Some floor joists are only 28 x 28mm (1" x 1") which is crazy and far too small. Ideally aim for 47 x 47mm (2" x 2") floor joists which are very solid and long lasting. Also ensure that the spacing on the shed floor is no more than about 16". Some cheap sheds have batons every 2ft so the floor will bow, or break, under your weight. You will find that the gaps in between the floor joists will allow air to circulate under the shed floor and help to keep it dry.
To ensure the very best floor they should be delivered fully assembled but may come in 2 or 3 sections (or more if the shed or summerhouse is very large). If you have to build the floor yourself, which many of the cheap sheds manufacturers make you do, then this is another area where you may run into problems.
Also ensure that the floor on your new shed or summerhouse is covered under the factory guarantee. You will find that the decent shed companies which use proper timber have no problem with this. The cheap shed makers who use chipboard, OSB, wafer or Sterling board often will not cover the floor as they know when water gets into these boards they will fail. ALWAYS double check and always be wary if they only talk about sheet materials.
You do need to ensure that what ever type of floor you have that you have a solid base for the floor to sit onto. Ideally this should be concrete, paving slabs, timber decking or a purpose built timber sub frame which is a superb foundation for your new shed.
I'm often asked what size to make the concrete base and I always say it should be the same size as the external size of your new shed. Also, ideally, you should make the top of the base 'proud' of the ground by 1 - 2 inches. If you do this then no water will run under the shed and you will never get any rotting from damp. I'm always amazed when so called experts say to make the concrete base larger than the shed or summerhouse. All this will do is to allow rain water to fall onto the protruding concrete and run under your building making the floor wet. This will reduce the life of the shed floor and, the fact remains, that most sheds and summerhouses fail because of the floor rotting.
My advice is based on 39 years in the garden sheds business and you can get more information about shed and summer house floors on this page and this page.