I never cut the corners of the polycro. I fold them into that position. Look here in the Generic Tub Floor corner folding design.
http://www.ddhammocks.com/explore/mo.../tarp_tutorial
The "bellowed" flap consisting of two layers of polycro can be facing inwards, outwards or taped flat to create 3 or 4 layers of polycro. A small 1/16-1/8 " diam. hole is placed in the flap near the bathtub top edge that's reinforced with tiny 1/4" or so fiber stick on washers or fabric ring hole binder tab hole reinforcers. If I use the ones made for paper holes they are not a strong so I place one on each side of the hole to reinforce it. Look for the stronger fiber ones. Depending on the tape you're using if at all you may not need hole reinforcing washers. I like the added strength in these high force prone to being ripped areas so I add them. Wt penalty is less than 3 grams for 6 holes to be reinforced both sides.
From the places I have the small reinforced holes I add about 6-9" of 1.2 mm (super light wt) elasticized bungee that stretch to double their cut length that stretch out to the included mitten hooks on the inside corners of the MLD SoloMid and Duomid. The bungee allows some range of play for different set up dimensions(floor space, height, etc). If your tarps, mids, etc don't have interior lower edge attachment pts for the bungee you can attache to the tarp/tent stakes. BUT, if you do that make sure water doesn't run down it into the bathtub groundsheet. One way is simply too make sure the bungee is angled down and away from the top edge of the bathtub.
I like erecting mids with the ground edge 3" or off the ground to allow for greater airflow. I sometimes play with designs that vary the bathtub edge height 2-5" depending on what height I think needed to make it all work for me.
Another additional design feature using polyolefin that addresses the edge holding up concern is by creating a strong top edge around the entire perimeter by making a small 1" or less folded hem using the double sided tape included in window film kits, like Duck Brand, in unison with the folded corner as shown above. A good alternative that can possibly be lighter wt is using 1/4- 3/8" wide TUCK TAPE which is great for outside uses and has never failed despite being drenched and immersed in water. It comes out of Canada. I've seen some people use painters's or masking tape which is too heavy IMHO, not sticky enough, and prone to failure once wet, damp, or overly cold. Too much or too heavy a tape or too wide a hem used on the top hem of the bathtub floor and it gets material weighted down unnecessarily so. It's mostly the bungee that holds up the top edge.
Regardless of the design making bathtub floors out of polycro results in a UL/SUL TLC required groundsheet. They don't last forever and they will not function well if care isn't taken in the design and use.
For larger ground sheets for larger floor area shelters where the approach is a one stop polycro one and not everyone is on same extra TLC needed page that might use the shelter(children, dog, etc) I can recognize problems concerning concerning durability, holes, tears, dislodging of the polycro ground sheet, slipperiness, etc. Add really wet conditions, some lack of site prep(sand spurs/cockle spurs/manzanita spurs, sweetgum or sycamore tree fruit, a tiny pine cone, pine tree sticks, tree sap, sharp pebbles, bulky sometimes hard nodes of various ornamental turf grasses etc), mud, etc and polycro can become just more crap needed to be hauled out.
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Originally Posted by colorado_rob
