Even though most tents come with a waterproof floor, experienced campers know to use a ground cloth/tarp as part of their tent-pitching routine.
I use an old space blanket, stuffed into the top of my tent bag. It’s the first thing out, first thing on the ground always. I use it for the moisture blockage, but more so to minimize the wear and tear on my tent floor. It must work; the floor is still in great shape after 30 years of casual camping!
The point of putting a ground cloth beneath the tent is apparent and obvious. In addition to a barrier against water from below, the ground cloth acts as a buffer against abrasion and punctures from objects on the ground. Some may not agree that this is actually effective; it has worked for me, however.
My older tent, a classic Eureka Timberline, received the same care and its floor works so well at restricting moisture passage that I had to slice an emergency drain in the corner during a water build-up from a serious wall panel seam leak during a downpour in Alaska’s Denali National Park.
Even tents with tent flies should be pitched with a ground cloth. The key here is to make sure that cloth is slightly smaller than the outside perimeter of your tent. Many tent flies don’t extend all the way to the ground. Rather, their lower lip sometimes ends several inches above the lower edge/seam of the tent walls. Water runs down the fly, onto the tent wall and down to the ground. If your ground cloth extends out beyond that bottom edge – water hits the ground cloth and flows back under the tent.
A second opinion on ground cloth placement makes good sense as well. Place it inside your tent so it forms a protective barrier between you and the tent floor. In case the latter leaks allowing water to enter, the interior ground cloth is going to keep you dry.
A key to it being effective inside your tent, the ground cloth should be slightly bigger than your tent floor. Turning up that additional material along the edges forms a small rim up along each wall – like the “bathtub” floor incorporated into most tents. Even if the floor fails to keep water out, you have a dry layer beneath you.
Both methods have merit. For light, occasional rain, the exterior ground cloth has served me well for decades. During downpours and extended showers, the addition of the interior tarp has kept everything dry. If weight is an issue, bring along at least one tarp and make the priority call as needed.
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