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Thank you for your question. Tents should be kept dry inside because damp can encourage mould growth and respiratory infections.

There are two possibilities. The first is that the tents are indeed leaking when it rains, and the second is that condensation from breathing, sweating and cooking, is building up in the tents causing the canvas to remain damp. If the tents are leaking then it will be noticeably worse when it rains, with dripping from specific places, and damp patches are likely to be localized around the seams. However, the area of roof directly under the plastic sheet should stay reasonably dry. If condensation is the issue then the damp will be fairly evenly spread across the roof.

If the tents are leaking it is possible to purchase waterproofing spray, but the tents will need to dry out before using it. You will need access to alternative accommodation for the families close by so you can rotate them into it while you treat a small number of tents at a time.

It is more likely that condensation is the problem, and plastic sheet touching the canvas will trap the moisture in the tent. Fixing it away from the canvas may help. This can be done by attaching guy ropes to the four corners of the sheet and at one meter centres, then pegging out the ropes so the plastic sheet is at a shallower angle than the roof of the tent. The plastic sheet may still provide some benefit by trapping warm air just above the canvas, which is the principle of a double fly tent. In general single fly tents should not be used for family shelter.

For details on how to attach ropes to plastic sheet please see: Plastic Sheeting: A guide to the use of plastic sheeting in humanitarian relief, Chapter A.5, Fixing Plastic Sheeting p26. https://policy-practice.oxfam.org.uk/publications/plastic-sheeting-a-guide-to-the-specification-and-use-of-plastic-sheeting-in-hu-126734

To reduce the condensation the tent must be ventilated as much as possible. This is not easy in cold weather because it means letting in cold air, but if the family can leave the doors open several times a day this should help. Reducing the number of people staying in each tent will also contribute, if you have enough land, additional tents and families who are willing to split their members between tents.

A more radical, but common, solution is to locate the tents in a large indoor space such as a hangar or hall, where the air immediately outside the tents will be dryer and warmer. An added benefit to this arrangement is the large covered space outside the tents that families can use without getting cold.