The life span of your garden building is subject to good maintenance. 
Timber is a natural product and is subject to climate changes and can expand in cold weather and contract in hot weather, this is a common occurance in most timber buildings.

Even kiln dried timber can shrink in high temperatures and can dry out further which can result in warping, splitting, knot holes falling out etc.

It is always a good idea to treat your shed with a preservative that allows the wood to move naturally rather than a varnish or something that does not allow any moisture to penetrate. Always read the label to see what level of protection the preservative offers and what it contains.

Some are water based which protect the shed against mould or rot usually up to 12 months, the shed can still absorb moisture but it is not a repellant against heavy rain fall in most cases.

A spirit based preservative usually offers a better protection against water seapage and also allows the building to breath as it is absorbed into the wood.

Also most good water repellants allow a natural movement of timber and offer a good protection against water seapage during heavy rainfalls and storms. 

Wax based treatments are also available.

Where your shed is fixed at the seams may pose a risk of water penetration, as water can find its way into the smallest of spaces,
it may be worth while to use a silicone product to seal these areas against this, Also around the windows would benefit from being sealed with either putty or silicone sealant or similar product.

A putty or wood filler can also be used to fill any knot holes that appear due to the drying out of the timber in hotter weather. 

The roofing felt on your building has a limited life, and can be more affected by deteriation if the building is under trees etc. 
If you notice dampness on your roof it is important to find the source of the problem before the timber is affected, this may only be a raised felt tack or staple that needs replacing, or it may be a rip or damage to the felt, which may require a refelt. Please ask the manufacturer for the average life span of there roofing felt.

Usually a good green mineral felt is ideal and can be found at most DIY stores or a builders merchant.

These are just a few tips on how to look after a wooden garden building after you have purchased it, and is only an opinion based on our own experiences within the trade, For information on individual products please contact the manufacturer for detailed advise etc. 

This is not a service we offer but tips on what the customer can do to help prolong the life of your garden building.