Can I Cover A Summer House With Plastic To Stop Rot?

Associate
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Plan is to build an insulated summer house out of timber with 18mm plywood sheets for the exterior.

I just pulled a shed out of the same location which was rotten around the back.

The summer house will be on a concrete base (not done yet), probably coming up 100mm above ground.

I wanna cover every part of the summer house with plastic to prevent wood exposure to elements.

Is it wise to buy some massive sheet plastic panels and screw them onto the 18mm plywood, then seal any joints with mastic?

And if the front of the summer house is covered with composite wood-effect panels, can I seal the joints between those as well?

I'm asking because this would essentially lock any air out so the wood won't be able to breath. Insides of summer house will be plasterboard + skim coat.

Thanks.
 
Soldato
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Was the old shed hard up against a wall or hedge etc. Because with proper treatment a shed can last 50 years as long as it has air flow and periodic treatment.

At first glance I would worry about trapped moisture rotting the wood at the plastic boundary’s due to condensation. Also the plastic will UV degrade and let water in eventually.
 
Soldato
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You're depending a lot on a piece of plastic that can crack, degrade, be punctured etc. Can't you just paint the shed? Paint can be repaired, covered, stripped, and also inspected annually.

I wouldn't be comfortable with screw/nail holes long term. Sooner or later they'll let something in.
 
Soldato
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Bad idea. Aside from issues with keeping the wood wet or humid, sitting inside a plastic cage for a summerhouse will be unbearably hot in summer and freezing in winter.
 
Man of Honour
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With a decent wood preserver and periodically reapplied it should last decades - back in the day we used to use Creosote but I think that is now banned. My family/relatives have/had sheds and summer houses, etc. well over 50 years old still in good shape with proper maintenance.
 
Soldato
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As others have said, don't use plastic! Use a breathable membrane, with a gap between the membrane and outer cladding. I wouldn't seal the joints where the cladding clips together as they are usually designed to expand and contract with changing temps/humidity, plus they are designed so that water can't get in anyway.
 
Soldato
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Posts 6 and 7 have hit the nail on the head- You can get creosote online - the old real stuff - Guttering is a must and piped away from base of summerhouse - another important point is your base should be slightly smaller than summer house and a few inches above ground level - any rain that runs off the sides should hit the dirt not drop on concrete base to bounce back up to the bottom of the sides - If house is bigger than the base then water won't run under the base - another point is put a layer of gravel all round house to stop water bounce back.
Next door put a big summerhouse up but as soon as it was up he went on to another project -He didn't put gutters up - he sold and now it's rented by a old couple who are treating it as if they own it - he is doing all the bits the original owner didn't do and he told me there is damp marks on rear right corner due to rain splash back - he is going to fit some gutters on it.

I read your post again and I built a garage and used ply for the one long side that was against a hawthorn hedge - The ply started to separate after a few years and it would have been better to have used T&G like the other three sides - This was at least 30 yrs ago and each time I have driven past my old house that garage is still solid as a rock. - I don't know if the new owners did anything but I would have covered ply with T&G planks. One more thing - do not used cheap thin felt -buy good fit once. --Never underestimate water.
 
Soldato
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I don't know if creosote is a good idea for a summer house as it stinks. OP said he was using composite cladding, which should last many years without maintenance.
 
Soldato
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I don't know if creosote is a good idea for a summer house as it stinks. OP said he was using composite cladding, which should last many years without maintenance.

God I love the smell of creosote - reminds me of summer as a kid and walking past my neighbours fence on my way to play with my mates in the fields. Good times.
 
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