Different garden buildings manufacturers use different types of materials for their shed, summerhouses, garden workshop or log cabin floors. Whilst some are suitable for outdoor use there are many which should not be used as they are not designed to stand up to the wet British climate and are likely to fail quite soon after delivery.
To understand the type of material used we need to know how they are made:-
Chipboard floors:- These are made from wood chips, as its name suggest. These are stuck together and compressed to make a solid material. Absolutely ideal for kitchen cabinets or anything used indoors but outdoors are susceptible to damp. Just a small amount of damp can make the boards fail. As soon as water gets into the shed floor it will be absorbed into the chips making the board swell and then disintegrate. There is nothing you can do to repair this and the floor would need replacing, or at the very least, a floor section.
OSB Floors:- This type of shed floor is made from boards which are made from wood shavings. These are glued and the compressed into solid boards. They have a special waterproof paint applied to the edge of the boards and provided the edges are not cut then this can provide for a cheap shed or summerhouse floor. However, in my experience, this is not possible very often as the boards have to be cut to suit the size of the shed. By cutting the edges of the board water can get in which will make the board swell and will make the boards disintegrate.
Plywood floor:- This consists of thin layers of timber being bonded together and this give the floor its strength. Provided an exterior plywood is used, which uses a waterproof glue, then this can be suitable but is relatively expensive.
T&G boards:- T&G timber boards are made from solid timber. After the tree is chopped down the trunk goes into the sawmill and sliced into boards. These then go into another saw mill where they go through a planing machine. This machines planes both side of the timber and applies a tongue to one edge of the board and a groove on the other edge so these timber boards will slot in together to make a b long lasting garden shed or summerhouse floor. If these boards do get any damp on them then they could expand fractionally but will dry out quite easily. Sheds, garden workshops and summerhouses with floors being made from solid timber do not disintegrate or fall apart.
NO Floor:- If you have a garden building with no floor then you will need, ideally, a concrete base for the shed or summerhouse to go onto. This base should be the exact size of what the floor would have been and you would need to check this with the factory as this size can vary.
You may find that a 10x8 shed would be 9ft 11in x 8ft 1in or possibly 10ft x 8ft exactly so it is vital the base is made to the correct size. By doing this your new building will sit on the edge of the concrete and would normally have the bottom plank of wood overlapping the base. This helps to ensure that rain and wind won't blow in.
If you make the concrete larger then the building can be made flush (the bottom plank of wood on your new shed or summerhouse will not have an overhang on it) so your building will sit straight onto the concrete. In this instance it is highly recommended that you seal around the edge of your new building to stop any rain water getting into the building.
Sheet Materials:- If sheet material is offered as an option on your new garden shed or summerhouse, ALWAYS, ask whether it is made from chipboard, OSB, wafer board, Sterling board or any man made boarding. You will find that some garden shed manufacturers describe these inferior materials, certainly for outdoor use where they may get wet, as sheet material as it does not sound so bad. If they have confidence in the product they would describe the flooring using its proper name. Also you will often find these floors are not covered under their guarantee, always ask and get it in writing, just to be sure.
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.