The canvas we use weighs 12-ounces (12oz) per square yard (410 grams per square metre) and is woven with 2-ply yarn in both the warp and weft directions. The hydrostatic head is 40 inches (1000mm) or 10kpa (kilopascal). This means the canvas can withstand a column of water that high before it begins to penetrate. Through the Australian Standards AS 3567-1988, the Department of Defence call for a minimum hydrostatic head of 500mm or 5kpa. Imagine, apart from being uncomfortable, 1000mm of water on top of your swag and you still would not have water leaking through.
The fact that water can still be forced through the fabric means that it is not impervious to air. In other words, most importantly because it still breathes, it will not condensate. It is rot-proof and mildew resistant. An all-cotton canvas is cheap and not a good canvas for this purpose, in fact, our 12-ounce cotton/polyester canvas is stronger than a 16-ounce all-cotton canvas but far less bulky.
Our sewing thread
We use polyester/cotton core spun which expands when absorbing moisture, thereby preventing water from leaking through the seam. Joints are double-sewn using a shorter stitch on the second run. This is important because it prevents the needle from following its own track. The joints are constructed so that water runs over them (like on roofing tiles) and not into them thus it is not necessary to soak the seams before use to make them leak proof.
All foam mattresses usually lose some height due to compression. When the swag is not in use for any length of time, storing the mattress in a flat state will help it to bounce back.