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What can I do to stop rot in a wooden garden shed?
Wood is the predominant material used in garden sheds. It is a fantastic material being easy to use, natural looking, relatively light and relatively cheap. However, its biggest disadvantage is the potential for it to rot away when exposed to the weather. But there are many things that can be done to lessen the spread and impact of this natural process.
Let’s look at what rot in timber is
There are two types of rot in timber: Dry rot and Wet rot
Dry Rot
Dry rot is wood decay caused by a certain species of fungi that digest parts of the wood which give the wood its strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resulted in a darkly coloured deteriorated and cracked condition.
Dry rot is generally a problem in old buildings where the fungus can spread, normally undetected under floorboards, etc.
It rarely is a primary cause of concern in a garden shed.
Wet rot
The spores that cause wet rot are unavoidable, they are in the air. They will only take hold when there is moisture content in the timber of over 30% and cannot survive when the moisture content is lower than this. As they are living fungi they also need oxygen to live.
Wet rot is not a continual process, so if wood gets wet in the winter and rot was to set in, as long as the area dries out in the summer the rot will stop. The sooner the area can dry out after inclement weather the better.
A good example of this would be a fence post, this will always rot at the point where it enters the ground. The ground gets the same rainfall as the top of the post but because it stays wet for longer the rotting process goes on for longer. Further into the ground the post will be less rotten as there is far less oxygen.
What is the solution to wet rot?
Damp is most likely to come from the ground rather from the sky as the ground is damp for most of the year, the sky is only damp when it’s raining.
In addition to this obvious thought, water travels from things that are wet to things that are porous and dry unless there is a barrier. Timber in its natural form is porous, so given the opportunity damp finds a dry wooden shed something it is always trying to access much like a dry house.
Clearly if there is a leak in the waterproof layers around the shed, like the roof, this needs to be fixed urgently. The source of damp is continually in the air and in the ground. As it cannot be stopped, it needs to be controlled as best we can. Assuming that all precautions have been taken in the design, the control is a good method of drying out an area once it has got wet.
The answer to this is air flow:
- Keep damp away from the outside of the building using guttering. A downpipe taking the water down to the ground mitigates the damage caused to the base of the shed from water falling, hitting the floor and bouncing back up underneath the shed
- Do not allow a build-up of debris around the base, which results in restricted air flow as well as creating a sponge effect against the side
- Do not stack items up against the side of a wooden structure as this restricts air flow
- Give ventilation to the internal area during the winter by opening doors or windows on occasion. The air contains damp so unless you are heating it to remove this the internal area can get damp without necessarily having a leak
The crucial design elements of a shed that can help to mitigate damp
- Guttering, together with air flow this is the best protection
- Install on a sub-base to keep the shed from sinking into the ground and losing the ventilation underneath
- The timber that touches the ground be it concrete, or other subbase should be as resistant to rot as possible and at the very least pressure treated. New types of reconstituted plastic bearers are very good for this purpose
- Install a waterproof membrane between the ground and the shed floor
- Ensure that panel joints are sealed in some way, not airtight just watertight
- Weatherproof membrane in the walls and roof
- External cladding which is pressure treated or painted to deter water ingress
- Good quality roof finish
- Glazing sealed into frames with clear silicon