Sewing - Tarp

tarp front view tarp rear view
Front view shows bugbivy cord attached to ridgeline.

tarp front polecup tarp read polecup
Front and rear pole cups (older design for front pole cup, but new design similar). Front pole cup holding hiking staff. Note reinforcements for loops that secure s-hooks to underside of ridgeline. Note that all guyline attachment reinforcements both sewed and glued (using Silnet).

tarp ridgeline hooks tarp silfix repair
First photo shows closeup of s-hooks attached to underside of ridgeline. Cord between s-hooks for hanging wet socks. Front s-hook for hanging poncho, so as to block rain from blowing in front of tarp. (Older version. Current version doesn't use s-hooks, just loops of cord, with mittenhook on bugbivy cord.) Second photo shows field repair to tarp (using scrap of silnylon and Silfix) after wind pulled stake out of ground. Stake flew back towards tarp like missile because still attached to guyline, causing gash in tarp.

Further discussion of tarp here.

Features:

Finished size of flat tarp about 104" long, 90" wide at front, 50" wide at rear. If tarp pitched with sides 4" from ground, with front height of 37" and rear height of 22", then tarp will have walls sloped at 46° and will cover area about 103" long by 64" wide at front and 36" wide at rear.
To pitch at height just specified, using pole cups, and assuming pole tips are level with ground surface (as opposed to sunk below surface or elevated by rocks above surface) requires front pole 41" long and rear pole 23" long.
Final weight of tarp, seam-sealing, guys, polecups, and stuff sack, assuming 1.4 oz/sqyd (after coating) silnylon is used for fabric and 2mm braided dacron for guys, is about 320 grams (11 oz).
Loops on underside of ridge-seam can be used as attachment point for raising head end of bug bivy, for hanging poncho or rain jacket, or for hanging socks and other items to dry.
Rear panel provides full protection from rain blowing in rear of tarp.
Original design included front doors, to provide protection from rain blowing in front of tarp. However, these doors proved to be both unnecessary and something of a nuisance, since they just got in the way unless tied up, and then had to be untied to allow them to dry out. Current design relies on hanging poncho or rain jacket from ridge-seam. Doors added about 35 grams of weight.
Some designers put a beak on front of their tarps, to serve same purpose as doors. Namely, to protect from rain blowing in front of tarp and to deflect wind. Sloped beak deflects wind better than flat doors, but can't be rolled up, and thus will get in the way and cause heavy condensation on still humid nights.
With or without doors or beak, tarp pitched normal height will fare poorly if there is strong wind blowing directly at front. So in case storm is anticipated, best to face tarp away from wind and towards natural windblock of some sort, in case wind changes during night. If this is not possible, then pitch front of tarp lower than normal. Also important during storms to fix stakes securely in ground and put heavy rocks on top to keep them in place. Make sure rocks only touch stakes and not guylines, since otherwise rocks will cause abrasion as guylines move in response to wind pushing against tarp.
In extreme storm conditions, where no natural windblocks are available, pitch tarp very low (like 20"), then crawl under and hope for the best. There will probably be heavy condensation, but this should not pose a problem with synthetic quilts.
Guylines should be rated for at least 200 lbs (2mm braided dacron is rated for exactly 200lbs), both for wind gusts and to handle forces associated with tripping on the lines. 200lbs may seem excessive, until one considers that knots and abrasion can easily reduce guyline strength by 80%.
Pullouts attached to triangular reinforcement patches which are glued (with silicone seam-sealer) as well as stitched to body of tarp. Gluing spreads load out much better than stitching and also adds to strength of fabric. Line of stitching is weak point, especially for thin fabrics like silnylon, and thus reinforcement patch which is only attached by stitching doesn't really provide that much reinforcement.
Polecups included for front and rear poles. Polecups allow ordinary smooth poles to be used for supporting tarp, rather than requiring poles with ski-tip end or poles with rough sides so as to allow tying a clove-hitch around pole. Possible to setup tarp without polecups, such as by tying ridgelines to tree or using clove-hitch on pole with rough sides.
More information about tarp configurations here.
Silnylon stretches, which is an advantage, since it means that tarp designed with straight edges (such as pattern below) can be pitched taut, whereas a tarp made of non-stretch fabric would require catenary curves for a taut pitch. Also, slightly stretchy fabric like silnylon is more tolerant of small errors in cutting and sewing than would be a non-stretch fabric (stress tends to be concentrated at point of imperfection, while stretching tends to mitigate this effect). Finally, stretchiness of silnylon means that sudden forces (such as from gusts of wind or from tripping over guylines) less likely to tear tarp than with non-stretch fabric. Like all nylons, silnylon stretches even more when wet. So tarp which is pitched taut when dry will sag when rain starts. Best way to deal with this is pitch tarp initially with front pole at slight angle, then later, as fabric stretches and tarp begins to sag, straighten pole and thus raise tarp a few inches.
In order to keep weight down, only a small amount of silicone is used to make silnylon, and this is not sufficient to keep water from penetrating under pressure. In particular, pressure of falling rain drops is often enough to cause small amount of water to spray through silnylon. Amount of water that gets through quality silnylon will not be enough to damage insulating properties of either down or synthetic sleeping bags, and thus poses no safety risk. However, even a tiny amount of water can damage paper items, so books and maps should be kept stored away when rain is pounding down hard on tarp.
Silicone slightly soluble in water, and thus coating will gradually wear away. It might be possible to retreat fabric with spray-on silicone, though I have not yet tried this.
Like all nylons, silnylon weakened by exposure to ultraviolet radiation from sunlight, which is especially intense at high altitudes in summer. Supposedly, silicone provides some protection from UV radiation. On the other hand, base nylon fabric used to make silnylon is delicate to begin with (1.1 oz/sqyd), so best to minimize UV exposure.
Alternative to silnylon is cuben, which is made of polyester film with dyneema (polyethylene) reinforcing grid. Cuben neither stretches when wet nor allows water to spray through under pressure and weighs less than half what silnylon weighs (CN2K.08 cuben weighs about .6 oz/sqyd, while standard silnylon weighs about 1.4 oz/sqyd). Cuben has excellent tensile strength due to dyneema reinforcing grid, but film between grids is very thin and much more easily punctured and abraded than silnylon. (My own experience using cuben for stuff sacks is that film developed holes within less than a month, and I was not abusing these stuff sacks. I suspect tarp would develop holes in even less time when used outdoors, due to inevitable contact with branches and rocks.) Also, because of its delicacy, cuben is weakened more by needle holes than silnylon, so lines of stitching necessary to construct seams tend to be weak points. It is possible to use tape rather than stitching for ridge and edge seams on a cuben tarp, so as to avoid needle holes in these areas. However, pullouts will still need to be attached with stitching. Because cuben fabric doesn't stretch, sudden impacts to tarp (such as from gusts of wind or tripping over guyline), more likely to rip seams between pullouts and tarp than would be case with silnylon. Lack of elasticity also means less tolerance for imperfect cutting of pattern and imperfect seams than with silnylon.

Materials:

7 yards of 60" wide silnylon (only 6 yards required if 65" wide—requires careful cutting).
1.5oz silnet seam sealer and brush.
100% polyester thread, 80/12 universal needles.
4 feet 3/4" nylon webbing.
1 foot 1/2" nylon gros-grain.
50 feet 2mm braided dacron polyester or equivalent for guy-lines.
Cord lock.
Scrap of 1000d nylon cordura, about 5" x 12".
Fray check.

Pattern:

tarp pattern

Sewing instructions:

Cut 2 roof panels, 1 rear panel.
1/2" flat-felled seam between roof panels, along straight edge, to produce trapezoidal roof.
Trim from front and rear of roof as indicated in pattern diagram above. Exact amount to trim depends on angle at which tarp will be pitched. Angle fixed for rear (due to sewn-in rear panel), but variable for front. In practice, difference between exact amounts and amounts shown in diagram offset by cutting/sewing tolerances for amateur sewer and stretching of tarp fabric, hence exactness doesn't really matter. Trim after sewing ridge seam, rather than before, in case roof panels not cut exact same length.
Glue triangular pullout reinforcement patches to tarp using seam sealer, 1" from edge to allow for later hemming. Patches for ridge seams will need to be trimmed slightly to match tarp shape. Patches for sides should be about 5" closer to front of tarp than rear, since the tarp is wider and will be pitched higher at the front, and hence has more need for side stabilization there. Approximate sizes of triangles: 7" wide by 3.5" high for ridge seams, 5" wide by 2.5" high for corners, 4" wide by 2" high for sides. Easiest way to perform gluing is first draw outline of reinforcement patch on tarp, then spread small amount of sealer within outline, then apply patch and press to ensure good seal. For the front ridge seam, trim patches to allow 1" from edge, then apply two patches to underside, with sealer between these layers. For rear ridge seam, attach patch close to edge so that multiple layers of sealed fabric when edge later folded to make hem.
1/2" hem on all sides. Also, stitch through reinforcement patches to further secure.
Attach 3/4" webbing pullouts. For corner and side pullouts, use 2" of webbing. Use 4" for front and rear ridge pullouts. 3/4" wider than ridge seam, so sew bartack in hem area, reinforced by grid of straight stitching along ridge seam. Fray check bar tacks.
1/2" hem on all sides of rear panel. Attach rear panel to roof. Probably will be necessary to leave small section unsewed near ridge seam.
Attach hanging loop. Cut 2 x 3" and 2 x 2" strips of 1/2" gros-grain. Stitch 2" strips to upper side of ridge seam as reinforcements, centered 6" and 12" from front of tarp. Form 1/2" loop in 3" strips, then attach to lower side of ridge seam below reinforcement strips, using 2 parallel bartacks. Attach 8" loop of 2mm dacron cord to one loop, for attaching cord to elevate bugbivy roof. Other loop is spare (or could be used for s-hooks, as in previous version).
Seam-seal upper side of ridge seam. Be sure to put a heavy coat of sealer over hanging loop reinforcement strips, since stitching here will be pulled from below, thus opening needle holes if these are not heavily sealed.
Attach guy lines to pullouts: 10' for front, 8' for rear, 4' for front corners, 3' for rear corners and sides.
Make large polecup, suitable for hiking stick with 2" diameter knob at top. Cut 5" x 7.5" scrap of 1000d nylon cordura, zigzag edges, and two pieces of 3/4" webbing about 3" long. Sew 1/4" hems on long sides, 3/4" hems on short sides. Fold so that coated side faces outside (uncoated side more resistant to abrasion) and sew up sides, with bartack near opening. Bartack short loops of webbing to 3/4" hems on either side. Apply fraycheck to all stitching. Tightly tie one webbing loop to tarp tieout with short piece of guyline. Tie main guyline to other webbing loop. Final size 3" high by 4.5" wide. Final weight 10 grams. (Older design of large polecup similar to that of short polecup, with 3/4" tunnel rather than 3/4" hem and no webbing loops. Two problems with older design: unable to quickly remove hiking stick, in case needed as weapon for defense against wild animal; caused abrasion to guyline.)
Make small polecup, suitable for pole .75" in diameter. Cut 3" x 5" scrap of 1000d nylon cordura, zigzag edges. Sew 1/4" hems on long sides, 3/4" tunnels on short sides. Fold and sew up sides, without sewing through tunnels, so that coated side faces outside (uncoated side will be more resistant to abrasion). Apply fraycheck to all stitching. To attach polecup to tarp, make small loop at one end of cord, then thread free end through polecup tunnel, tarp tieout, other polecup tunnel, loop. Final sizes 1.75" high by 2.5" wide. Final weight 4 grams.
Make medium size stuff sack from remaining scraps of silnylon, sewing details here.