By deep liter do you mean the US system where they go onto deep bedding and then a thin layer is added daily, to encourage them to scratch the bedding around regularly? I overwinter my breeding birds in concrete pens under cover and use a fairly thin layer of shavings, cleaning it out completely and rebedding as soon as it's about one third droppings. The pens are well ventilated but this helps keep the atmosphere dry and I hope keeps the likelihood of coccidiosis, etc., down.