The garden shed normally comes already treated, mostly with a water based treatment or a spirit based treatment which is far better. It may even come fully pressure treated, also known as tanalised treatment. Or you may find the cheap garden sheds have a treatment with is just like coloured water and about as much use as that and in these cases you need to be very careful.
As a rule the normal water based treatment needs to be re-coated within 4 weeks of being delivered. If it's treated with a spirit based treatment then ideally you should ensure that you re-treat your building before the first winter after assembly. The reason for this is that first treatment acts as a primer and undercoat and the coat you put on is like a top coat.
After putting this coat of treatment on you wouldn't normally have to re-treat for 2-3 years except in very exposed areas. However, bear in mind when you buy a shed offering a 10 year guarantee you will find, if you can find the small print, that you have to treat your new building every year to ensure it's covered. Clearly this means their treatment is totally useless and it's only the fact that you are treating it which gives the shed a long life. This applies to all garden sheds in spite of any warranty offered.
The pressure treated buildings don't need any extra treatment against rotting for 15 years normally and, although, it's a little more expensive can be worth considering. The only thing I would say is - where the shed or building faces into the weather and during driving rain - you may get damp patches on the inside of the shed. This won't hurt the building and will dry out very quickly however if you put Thompson Water Seal treatment on this will provide a waterproof coat to the building.
If you are thinking of buying a pressure treated shed then ensure you buy one which has all the timber pressure treated including the floor, framing, walls and roof. You will find many shed companies offering tanalised or pressure treated shed but it's only the walls which are treated. This is OK to a point but you can't beat ALL of the shed being pressure treated. 1st Choice range of Platinum and garden sheds are all fully treated including the cladding, frame and floor.
After the initial treating you need to keep an eye on the building to ensure no problems crop up. You will find that the Autumn is a good time to be doing this before the onset of winter. Take a look at your garden shed or garden building and ensure the treatment is in good shape and that it is not splitting, cracking peeling or going grey. If so try to re-treat while the timber is dry and this will keep the worse of the winter weather out and your shed nice and dry.
You could possibly choose from a variety of colours if you want to brighten up your garden during the winter. You will find that the Protek range of colour treatments are perfect for this and are available for all 1st Choice garden sheds and buildings.
Besides the treatment take a look at the felt and ensure it's laying flat and that all the nails are in place. If any nails have moved then remove then and replace but ensure you put a dob of mastic in the old hole to stop any damp getting it. Ensure any cracked panes of glass are mended and the beading is in place. On the door ensure that the catch lines up so is easy to close and apply a drop of oil to the hinges and the lock so that it does not become to hard to operate.
By doing this now this will ensure that during the colder months, when you do not want to go out into the garden, that your shed will be protected and that will protect all your valuables inside. By doing this you will get a nice warm glowing feeling.