What is the difference between SRT and DdRT in tree climbing?

Ask this question in a tree care yard and you’ll get two answers, because the industry changed the names and not everyone came along. SRT and DdRT described how many ropes were in play. SRS and MRS describe what the rope is doing. The second pair is what the standard prints.

That matters more than terminology usually does. The two systems load the tree differently, wear the rope in different places, and — the part that decides a purchase — are approved on different hardware.

What do SRT and DdRT stand for, and what replaced them?

SRT is single rope technique and DdRT is doubled-rope technique, and both were renamed after the old acronyms proved ambiguous in tree work. Tree Care Industry Magazine records the reasoning: SRT arrived from caving and rope access, but “in tree work, we also use a single rope for other climbing methods, such as DdRT,” which made a name built on rope count useless as a description. The new acronyms, the article states, “are SRS (stationary rope system) and MRS (moving rope system).”

The standard uses the new names. ANSI Z133-2017, approved 5 October 2017, writes Subsection 8.2.4 as “arborist climbing lines used for moving rope systems” and Subsection 8.2.5 as “arborist climbing lines used for stationary rope systems.” DdRT and SRT do not appear in those clauses. Plenty of people still say the old words, manufacturers included — ISC’s Rope Wrench manual, revised October 2023, scopes the device to a “Single Rope Technique (SRT) configuration.” The TCIA author quoted above rejects the change outright: “there is nothing stationary about SRS except the fall of the line (standing part) from your anchor point in the canopy.” Write SRS and MRS on paperwork, expect SRT and DdRT on site.

Why does a moving rope system take half the effort and twice the rope?

Because the rope runs over the union and comes back down, so the climber hangs between two legs and each one carries roughly half the load. The friction study published in Arboriculture & Urban Forestry builds on that assumption — each leg of the rope holds one half the climber’s weight — the 2:1 that makes MRS forgiving on a limb walk.

You pay for it in rope. Tree Care Industry Magazine puts the arithmetic plainly: in a doubled system climbers “go 6 inches for every foot of rope,” against a single-rope rig where they “go up one foot for every foot of rope.” Sherrill frames the same trade as “half the weight, twice the rope.” SRS is 1:1 — full body weight, no wasted travel.

What does the anchor actually see in each system?

A moving rope system and a canopy-anchored stationary system load the tie-in point about once; a base-anchored stationary system turns that union into a redirect and can load it about twice. Sherrill states that in MRS “the tree is experiencing 100% of our body weight on ascent and descent,” and that an SRS canopy anchor sees the same forces. The rope splits the load across two legs, but both terminate at the same union.

Base anchoring is the case to understand first. Jeff Jepson’s single-rope technique write-up works a 200-pound climber out to 200 pounds on the load leg and 200 on the tension leg, “a total of 400 pounds of static load on the limb redirecting the rope,” and says base anchoring “exposes the redirecting limb to twice the load that would occur if the rope was anchored to the branch itself.”

Two credible sources describe that anchor differently, and it’s worth naming rather than smoothing over. Jepson gives a flat doubling. Bartlett Arborist Supply makes it depend on the angle between the two legs: about 2.0x with the legs parallel, 1.9x at 45 degrees, 1.4x at 90 degrees and 1.0x at 120 degrees. Both describe the same physics — Jepson’s is the parallel-leg case — but a climber who reads only one carries the wrong model into a redirected setup. Bartlett adds the caveat that matters most: “these figures apply when the load is at rest; when the load is in motion, the force at the anchor fluctuates higher.” Neither publishes a measured dynamic figure, and we won’t invent one.

Which system is your device approved for?

Read the manual: devices are approved for one system, the other, or both, and the difference is sometimes a separate accessory you have to buy. This is the whole practical content of the SRS/MRS split.

Device	System the maker states	Published rope range	What the manual says
Petzl ZIGZAG	MRS; SRS only with CHICANE	11.5–13 mm, EN 1891 type A	“Can be used in a moving rope system, as well as a stationary rope system when paired with the CHICANE accessory”
Petzl CHICANE	SRS (single ropes)	11.5–13 mm	“Auxiliary brake for mechanical Prusik on single ropes, for tree care”
ISC Rope Wrench	Single rope only (ISC writes “SRT”)	11–13 mm (RP280, RP285); 13 mm only (RP283)	“Meant to be used by Arborists… in conjunction with a Single Rope Technique (SRT) configuration”
Rock Exotica Akimbo	SRS primary, MRS also permitted	11.1–12.5 mm approved-rope list	“The primary application for the Akimbo is on a Stationary Rope System (SRS)”
DMM Hitch Climber	MRS first, SRS in practice	not published on the page cited	“Primarily developed with moving rope systems in mind, the TAP has also been adopted at the core of stationary rope systems”

Rope ranges are each maker’s figures for that device alone; a system is only legal on the narrowest overlap of every component.

Two details carry weight. Petzl publishes the CHICANE as the part that “allows ZIGZAG and ZIGZAG PLUS mechanical Prusiks to be used on single ropes” — so buying a ZIGZAG for SRS work means buying two things. And Rock Exotica publishes a working load limit of 100 kg (220 lb) or 130 kg (286 lb) depending on the approved rope, and states the Akimbo “is not a fall arrest device, and should not be shock loaded.” A device that clears both systems still doesn’t clear every use.

The hitch-versus-device question sits underneath all of this — see friction hitch vs mechanical device. MRS grew up on a hitch tending a doubled line, SRS on something rated to hold a single strand, and both have since crossed over.

Where does each system wear the rope?

A moving rope system wears the line at the union, because that’s the one place it runs under load every time you move; a stationary system wears it at the device and wherever the standing part touches bark. The MRS line saws across whatever it’s draped over, which is why friction savers exist at all.

Published wear data is thinner than you’d hope. The Arboriculture & Urban Forestry study measured friction coefficients for 12 climbing ropes through raw aluminum, polished aluminum and steel cambium saver rings, and found used ropes exhibited greater surface areas and produced greater friction coefficients than new ropes — wear compounds. It is candid that it never tested rope over bark, recommending future work “on branches with varying degrees of bark roughness.” Nobody has published the figure a climber wants.

A friction saver moves the problem rather than deleting it. Treemagineers’ hitch climbing guide notes that once the rope runs on rings instead of bark, “much more heat is now concentrated at the hitch,” and that the effect worsens with smaller-diameter line and hitch cord.

Rope choice follows the system. Sterling gives its 12.5 mm Scion a minimum breaking strength of 7,193 lb / 32 kN, EN 1891 Type A and ANSI Z133, and states it for “MRS, and SRS climbing techniques.” Teufelberger lists its 11.5 mm Tachyon as a 24-strand certified “to EN 1891A as well as ANSI Z133-2012,” though that product page publishes no breaking strength. Which construction suits which system is its own decision — see arborist rope vs climbing rope, and static vs dynamic rope for rope types and fall factor.

How do the two systems differ on getting down and getting the rope back?

A moving rope system is already rigged to descend the moment you stop ascending; a stationary rope system usually requires a changeover, and a base anchor has to be dealt with from the ground. In MRS the hitch that advanced you is the one that lowers you — an advantage on a job with constant repositioning.

SRS splits into two habits. Bartlett states a canopy anchor “doesn’t tend to double load the branch union but advancing your system higher might not always be so simple,” while on base anchors “advancing… is quite simple, no need for extra tools and you don’t fight too much friction on retrieval.” Jepson’s canopy-arrival routine is to tie in with a lanyard, then either have a ground worker clear the anchored rope or leave it as an access and rescue line. That second option is the quiet argument for base-anchored SRS on removals.

Harness attachment differs too — SRS devices generally hang from one point while MRS loads a bridge from both sides. That belongs to arborist saddle standards.

The short version

SRT and DdRT became SRS and MRS because the old names counted ropes instead of describing behavior, and ANSI Z133-2017 prints the new ones in Subsections 8.2.4 and 8.2.5 — expect the old ones on site anyway. MRS gives you a 2:1 and roughly half your body weight on the hitch, at six inches of height per foot of rope; SRS gives you 1:1 and a faster ascent at full weight. A canopy anchor carries the climber once, while a base-anchored SRS turns the top union into a redirect that published figures put between 1.0 and 2.0 times the climber’s weight depending on leg angle. Neither is safer in the abstract — they load the tree differently and fail differently. Buy the system your device is approved for, because Petzl, ISC and Rock Exotica each draw that line differently in writing.