you can use just about anything for compost - as long as you allow for drainage. Make sure you have some good holes in the base of your builders' bags - make some, don't trust the drainage they currently have. I have two old pedal bins in my small garden that are always full of leaves. Every few months I empty them out and get fab sticky leafmould. The only protection they get is a newspaper over the top. Go for it!
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Some people put an old piece of carpet on top to stop the rain and keep the heat in as heat rises so you don't want to lose it.
you can use just about anything for compost - as long as you allow for drainage. Make sure you have some good holes in the base of your builders' bags - make some, don't trust the drainage they currently have. I have two old pedal bins in my small garden that are always full of leaves. Every few months I empty them out and get fab sticky leafmould. The only protection they get is a newspaper over the top. Go for it!