Are old wooden railway sleepers good to use as a sub base for a summer house or will they eventually rot and make the floor unstable
I would suggest not. Unless you are lucky, they're not even across their face so you wouldn't have a level surface. Also, they're a pain to manoeuvre at around 95kg each and not easy to cut to size. Concrete slabs are often given away free on Gumtree, FB and the like and will be infinitely better.
We used this type of stuff under our summerhouse. We are on clay, which always expands and contracts with the weather, so there is some movement but we are generally happy with them.
We have used railway sleepers under our greenhouse as a base. 9 years on and there hasn't been much movement, although I suspect they have rotted away, so I would avoid them.
Re the original poster's question about using railway sleepers; I would use them with caution. Ours are oak and pressure treated, and they have rotted considerably in 8 years, but they were still a better option than brick or concrete (which would have sunk into our clay ground and cracked).
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We have used railway sleepers under our greenhouse as a base. 9 years on and there hasn't been much movement, although I suspect they have rotted away, so I would avoid them.
Re the original poster's question about using railway sleepers; I would use them with caution. Ours are oak and pressure treated, and they have rotted considerably in 8 years, but they were still a better option than brick or concrete (which would have sunk into our clay ground and cracked).