How much waterproofing is enough?

"What is enough waterproofing to prevent moisture seeping through the floor when I put a knee down on it?"

Divide your weight by the surface area of your knee.

Sheild silnylon from thru-hiker has been waterproof for me for floor. Richard Nisley measured HH 3500 mm.

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You can have 100000000000 waterproofing and still be soaking wet, Technique and common sense beat numbers.

True enough but I've had floors from other silnylon that leaked, which I managed with, but it's better if the floor doesn't leak when you're on wet ground

@ Nathan I understand the math behind what you're saying, but how does that translate to the HH of a fabric? ie the maximum pressure exerted by my nee is roughly 20ish PSI, I would have no idea how many mm of water in a column that translates to (all manufacturers use the HH spec).

@ Michael That may be true, but the opposite is also true. Technique and common sense will never be able to make up for a fabric that is underspec'd. I figure a tent is the one piece of gear that I really need to count on. If my shelter fails at its primary purpose, there is no point in bringing it along. There is no technique that will overcome a tent fly that mists water when heavy rain and hail hit. That would be like saying the size of the pores on your filter make no difference as long as you know how to use it and you follow all the rules of filtering. In both cases, if those microscopic holes are too big and things get through, you're going to have a bad day!

HH is just another way of expressing pressure. 20psi is about 14,000 mm of water.

Thanks Jerry, but I don't think I'm ready to take on building my own shelter right now. Will be looking to purchase something pre-made.

1 psi = 27.707 inches of HH
20 psi = 554.14 inches of HH

1" = 25.4 mm
554.14" =14,075 mm HH

..more or less

The one step conversion is 1 psi = 703.758 mm HH

Jesse,

Regarding floor HH requirements, I will use my 190 lb and 70" tall body as an example to answer your question regarding knee pressure. When kneeling, ~310.09 inches are supporting my weight; the pressure exerted on the fabric is thus 18.42 PSI (12,794 mm H2O or 125kpa).

No manufacture, that I am aware of, uses that high of a floor HH. Look at the Hilleberg expedition tent floors; they use material that tests 7,000 mm H2O (68.6 kpa and 10 PSI).

There is a simple reason why Hilleberg can use lower HH fabric / coating than the pressure exerted from my knee. The water under my knee / fabric would need full containment to generate 18.42 PSI pressure. In the vast majority of the cases, the ground displaces some of the water away from the fabric.

I'm curious as to the answer to what (I think) is a relatively simple question.

Is there a consensus on what the most practical HH is for tent floor and fly?

Jesse,

The international standard for classifying a tent fly as rainproof is 1,500 mm H2O MINIMUM. USA manufactured silnylon averages 1,200 mm H2O for the best material commonly available. That is close enough for most people doing UL backpacking.

Again I will use Hilleberg expedition tents as an example of what is required to address the worst case (maximum required). Their expedition tent flies test 3,000 mm H2O. There are some US vendors and material combinations being sold for UL backpacking that I tested > or = 3,000 mm H2O for the fly.

Max,
I knew I would be stirring the hornets nest with my question. It was inevitable. :) One could hope it would be an easy question, but I've found that on this forum, there is no such thing. That's just the way it is when you have so many different backgrounds and experiences.

Richard,
Thanks for the info. If the standard says 1500mm is waterproof, that sounds like a good place to start for me. The tent I retired was a nearly 15 yr old Kelty that finally gave all she had. It wasn't near as light as what is available today. Even if it means a few extra ounces I think I'll verge on the safe side and go with something on the higher end of the range.

Well, it sort of depends on what you do, and, where you are.

For State/National parks, there is often not a lot of choice in camping spots. Low-lying areas are usually prevalent where people and cars have compressed the soils. In a heavy rainstorm, the site is often one big puddle. In this case I use an Exped Sirius. It has a 10,000mm bathtub style floor. It has been through 5 days of continuous rain, but never leaked. It was sitting in a 2" deep site-wide puddle and didn't leak. Would I take it backpacking? No. It weighs 5.5pounds. This is for getting to the ADK's and for comming home, often at a state campsite.

I also have a mesh tent and tarp that weigh a little under two pounds (including lines, but no stakes.) Stakes bump the weight up to just over 2 pounds. In spring/fall (when the weather says cooler nights) I often just take the tarp. Backpacking is different because you can select the site you want to set up on. A slight mound, or raised area will keep the grass/duff floor dry, generally. If you anticipate a hurricane, a larger mound works better. (Yes, I have been out in a hurricane and stayed dry at night.) I recommend a small can of bug spray to keep ground insects down. Saturating a bandana with bug spray will last about a week if you keep this in a baggie, taking it out and spreading it under your bag before going to bed. Fleas, chiggers, spiders, etc all can be an anoyance at night and if they get into your bag. Mosquitoes, and the rest are usually covered by permethrin in my cloths, often, these are placed on top of my bag to dry.

Hiking in the rain, at least in the ADK's, is difficult. Slipery roots and rocks can cause all sorts of falls. It alsways slows you down. Usually, by 1000-1100 things have dried up some. Just in time for the afternoon rains at around 1600-1700. Being the first one on the trail means wet pants, then socks, then shoes on a slipery trail. I often wait till 0900 to get going…

The UL polycro or similar (painters drop cloths, tyvec) have a way of becomming punctured easily. I quit using them about 5-6 years ago. My pad works well enough as a ground cover. This year especially, the heavy and frequent rains have left the woods quite wet. Everything is damp, all the time, when I am out. My pack weight often increases for a day or two after I leave, due to this. Water to drink is NO problem this year.

Soo, it really depends on where you are camping. Often a pine tree wiill have a dry area near the root bole. I like to set up there. Sometimes you can find a dryer area in the woods to set up. This often means raking up some duff to fill in low spots for my bed. Downed smaller branches can be used, too. Just cover them with leaves/duff. For the canopy, 1500mm is not enough, sometimes misting right through the fabric. New tarps will often get a coating of 20-30/1 mineral spirits/caulk. I believe this raises the HH to about 2500-3000mm for a couple ounces in weight.