DIY catenary-line-suspended dome shelter: background - Frank Groffie's miscellany

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This do-it-yourself (DIY) lightweight wilderness shelter sleeps two to three. It's a rainfly plus floor. The fly is made of waterproof, 1½-oz.-per-sq.-yd., coated taffeta nylon, which sheds wind and rain nicely. It's held up by a line draped over a stout tree branch. No surprises in any of this.

Surprise: The rainfly is a tight-pitching, wind-spilling dome shape maintained by an external web of lightweight braided-nylon lines. It has floor sills. Yet, there are no poles. Instead, there are eight catenary main lines, each of which holds five vertical suspender lines. Think Golden Gate Bridge but without the towers. (Well, there is one support tower, a tree, but the forest supplies that.)

It's been given a “bathtub” style floor and other accessories. Note that in reality the lines are nearly invisible, as in the photo above, whereas the diagram below shows the lines very prominently.

The diagram above is a perspective view from eye level and about 12 feet away. Three panels of the rainfly are clearly shown, and a fourth panel is barely visible on the right. Four more panels, the other half of the shelter, are hidden behind. The rainfly is dome shaped in side view and octagonal viewed from above. Notice how the main lines are catenary: they hang as would a chain suspended from both ends. Also, the rainfly's edges are cut catenary style to even out stresses and minimize sags. Forty-nine suspension points maintain the fly's dome shape.

Weight. 4.1 pounds total. That's as-built, all inclusive, my style, with accoutrements: rain gutters and collector tubes, bathtub floor plus pole-stakes, and nine aluminum guyline stakes (you might lose one).

Interior floor space. Forty-five sq. ft.. Eight ft. long by 6 ft. wide. The recommended hexagonal floor gives copious room for two plus their packs. Or comfortable sleeping space for three, with packs outdoors.

No tangling problems. The flysheet and its lines unfurl as does a parachute. There are no free ends in the lines to create tangles. Whenever I've unwrapped the package, the lines quickly present themselves in obvious layouts and arrangements.

Additional views:

It works. Photos here show the shelter protecting me from light rain and light wind beside Natalie Lake (39.277 °N, -120.472 °W) on a solo fly-fishing jaunt in the summer of 2009. This little lake was once reputed to hold a magnificent population of fat trout, though they didn't show for my visit.


This particular dome shelter may constitute a best of both worlds: the realm of sturdy tents and the realm of lightweight flysheets suspended from trees and using guylines.

Compared to a tent (or any shelter rigged with poles), the shelter yields these flysheet-style benefits:

Lower weight. Its suspension system, composed of thin nylon lines, weighs far less than tent poles.
Compactness. (Again, thin nylon lines serve in lieu of metal poles.)

Compared to a conventional rainfly, strung from trees, its dome shape presents a low-profile, smooth, uniformly convex roof against the elements, with an even stress distribution and a lower surface-area-to-volume ratio. These tent-like characteristics yield

Strength, i.e., resistance to strong rain and wind.
Low deformation in high wind.
Rapid runoff of rain, without pooling on roof panels.
Lower weight (less fabric).
Elbow room and interior space to move around and sit up in general.


Three-season use only: this shelter has not been tested in high winds or snow and it is not intended for such conditions.
 There’s no barrier against mosquitoes, a common issue with a conventional rainfly or any nonenclosed shelter. My countermeasure, highly recommended, is to sew bug protection to the head opening of one's sleeping bag.
Guylines are many, and they pose a trip hazard.
At least one tall tree is usually needed to easily erect this shelter, which tends to limit selections of shelter placement. However, there’s a work-around using a pole support, as explained below.


Center pole. This DIY line-suspended wilderness shelter is capable of doing without tree support. A central support pole may be inserted. I provided the flysheet with a small (3-in.-diam.) opening near the center. This opening is provided with a 4-in.-long tube with drawsting closure. A pole may be placed in the center of the shelter and extending out of the opening to support the main suspension lines. Some sort of base for the pole (my preference), such as a book or slab of tree bark, or opening in the floor so that the pole can rest on the ground, would be needed.

One may assemble the needed pole by lashing together two hiking staffs, or trekking poles. Or, combine the stick you carry with one you find at your campsite. Whichever, I've developed a dedicated lash, shown below, weighing 1 oz., out of nylon webbing, buckles, and rivets. The lash allows one to quickly attach together two 3- to 4-ft. sticks to make a centerpole for the DIY shelter in case one wants to erect the shelter in a meadow or on a treeless ridgecrest.


Rain gutters.
A downpour of ½ inch will drop 0.3 gallon of rain per sq. ft of ground outside your shelter. That's unavoidable and perhaps OK: the ground may be able to handle this, by way of natural infiltration. However, huge amounts of water shed off any rainfly. Your shelter will impose an additional load of ½ gallon of runoff along each lineal foot of rainfly. And that's an average: some spots may concentrate runoff in amounts of 1 or 2 gallons per foot. This rainfly runoff likely will create streams or ponding or some combination thereof next to or under the shelter unless one is extremely lucky or careful in selecting a campsite.

Here — with this domed rainfly shelter system — rain gutters along the rainfly edges collect this runoff and direct it to low points you select away from the shelter. These gutters consist of flexiible plastic tubing. The rainfly edges are turned into the tubing and connected using lightweight aluminum pop rivets at 6-in. spacings.

Above. This (superior) DIY line-supported dome shelter includes edge gutters and bathtub flooring. Note minimal water infiltration from sides.

Below. A conventional (inferior) rainfly. Note infiltration from outside via wind-driven rain, splash, flysheet runoff, and ground ponding. In addition, assuming the user has selected a site without perfect slope characteristics, external water can flow into the shelter and saturate sleeping bags and clothing.

Bathtub floor. A so-called bathtub floor is nothing special. This term indicates only that waterproof shelter floor panels have been extended upward some 1 foot to give the occupants protection from water entering from the sides and below. Nearly all tents incorporate this strategy.

What's critical here is equipping a rainfly style shelter with a bathtub floor. This is done using so-called pole-stakes (see upper diagram, above). These are essentially tubes made by cutting 17-inch lengths from discarded (and straightened) aluminum tent poles. The lower 4 inches are driven into the ground to spread out and hold down the floor. The upper 13 inches extend vertically to hold up the sides of the floor. The floor sills of the "bathtub" also give the shelter's occupants a measure of protection against cold breezes.

The 17-in.-long pole-stakes, being hollow, are surprisingly lightweight: a mere 23 g (¾ oz.). In comparison, conventional best-available aluminum-rod tent stakes come in at 7 inches long and 17 g (½ oz.): they're lighter, but they do only one job rather than two.

View from above

The diagram below shows the shelter as if you could see through the rainfly. The rain gutters collect not all but about ¾ of flysheet stormwater runoff and direct it away from the shelter. Two downtubes on high sides of the shelter send runoff sideways. Ground stormwater runoff is shown flowing toward the lower left corner in this diagram. The downtubes are detachable and may easily be clipped to whichever corner where they're needed.

The emergency ground gouge is a tiny (1 in. deep) trench that one scratches in the earth using a stick. It's generally out of bounds in leave-no-trace practices, but it can be critical when a heavy downpour threatens the watertightness of a shelter and the safety of occupants.

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