For mixes with a 1/2 in (12.5 mm) nominal maximum aggregate size (NMAS) or larger, the working rule from paver OEMs and paving schools is to set the bottom of the auger 2 in (roughly 50 mm) above the loose mat before screed strikeoff, then iterate upward in small increments until the surface closes up [S1][S3][S4].
Coarse-graded base and binder mixes (large-aggregate, 19 mm NMAS and up) magnify two failure modes: auger flight dragging rocks to the surface, and an over-stuffed head of material forcing the screed to ride high. Both are corrected by the same lever, auger elevation, which is why the spec is the same for the mix but the tolerated error band is tighter than for a 9.5 mm surface mix [S4].
The 2 in (50 mm) baseline and why it exists
The Caterpillar paving guide and Roads & Bridges' operator reference both define the 2 in (50 mm) number as the distance from the bottom of the screed plate to the bottom of the auger segments, measured on a cold, parked machine before pulling onto the mat [S3][S4]. Caterpillar frames the procedure as part of the screed-floating factor chain: paver speed, head of material, screed adjustments, mix type, mix temperature, and grade conditions. Auger height is the variable the operator directly controls to keep the head of material at roughly 50% of the auger diameter [S3][S5].
Asphalt Pro's training column drives the same point with a tape-measure test on a new starter mat: if the crew is matching a 2 in compacted mat, the bottom of the auger should sit 4 in off the ground (2 in of new loose mat plus the 2 in standoff); 6 to 7 in puts too much mix in front of the screed and the crew will fight the head all day [S1].
Coarse mixes shift the rule toward "higher, not lower"
Roads & Bridges is explicit that with base-course mixes (large aggregate), augers set too low leave two visible texture stripes behind each auger flight because the turning flight pulls oversized rock up to the surface [S4]. The fix is iterative: raise the augers in small increments (1/4 in to 1/2 in per step) until the surface is tight and uniform, and do not chase this with more auger speed, which only compounds segregation [S4].
Practical offset used by paving crews on 19 mm and 25 mm base lifts: 2.5 to 3 in (about 65 to 75 mm) above the pre-compaction mat, still measured from the bottom of the screed plate to the bottom of the auger segment. This keeps the head of material constant while moving the flight face clear of the larger aggregate. Crews running a 37.5 mm stone matrix asphalt (SMA) or large-stone base will push to 3 in plus; if the flight has worn, the offset must grow, since a worn flight increases the auger-to-pre-strikeoff gap from a healthy 5 to 6 in to 6 to 7 in, which is itself an over-feeding indicator [S1].
Head-of-material target: half the auger diameter
Independent of mix size, the head of material should cover one-half of the auger shaft from the center out to the auger ends; that is the equilibrium condition a floating screed is tuned for [S4]. Haitian Casting's auger-parts reference, summarizing OEM service data, sets the standard coverage at approximately 50% of auger diameter measured from the top of the shaft, with most standard pavers offering about 5.75 in of total vertical adjustment on the auger lift cylinders [S5].
The head-of-material rule interacts with auger height. Lower the auger and the effective chamber volume shrinks, so the same conveyor output produces a higher head; raise the auger and the chamber volume grows, so the head drops. On a coarse mix, the right answer is to keep the head at 50% by raising the auger, not by trimming conveyor feed, because trimming feed starves the screed extensions and produces the same end-of-auger starving pattern that shows up as a center-line hump [S3][S4].
Comparison of options by mix size and paver class
The four common operating points, lined up against nominal maximum aggregate size, head-of-material target, measured auger-to-screed-plate gap, and screed response, are as follows. Numbers are taken from the operator sources cited; the auger gap column is the value crews actually tape-measure on a cold machine [S1][S3][S4][S5].
9.5 mm surface mix: NMAS 9.5 mm, head target 50% of auger diameter, auger gap 2.0 in (50 mm), screed response standard, no texture striping if matched. 12.5 to 19 mm binder: NMAS 12.5 to 19 mm, head target 50% of auger diameter, auger gap 2.0 to 2.5 in (50 to 65 mm), screed response standard with marginal striping risk if set at 2.0 in. 19 to 25 mm base: NMAS 19 to 25 mm, head target 50% of auger diameter, auger gap 2.5 to 3.0 in (65 to 75 mm), screed response shows two texture stripes if set too low, resolved by raising in 1/4 in steps. 25 mm+ SMA or large-stone base: NMAS 25 to 37.5 mm, head target 50% of auger diameter, auger gap 3.0 in plus (75 mm plus), screed response is highly sensitive; lift the auger first, then recheck head of material, never the reverse.
The decision rule is short: pick the auger gap by NMAS, then verify head of material at 50% of auger diameter. If the head is high, raise the auger; if the head is low, lower the auger; do not change conveyor feed rate to chase the head while leaving auger height fixed [S3][S4].
Failure modes: too low vs too high
Too low (auger 1 in or less above the loose mat) produces two parallel texture stripes, especially behind the right and left augers, because the flight physically drags oversized rock upward [S4]. Coarse mixes show this faster than fine mixes; an inexperienced crew will often try to fix it by tamping harder or slowing the paver, which is the wrong lever. The correct fix is auger height, raised in 1/4 in increments [S4].
Too high (auger 3.5 in or more above a thin mat) over-fills the head of material, forces the screed to ride up, and produces an open-textured mat across the full paving width with large temperature differentials across the mat [S4]. On coarse mixes this combines badly with the mix's already high internal friction, because a riding-high screed has less compactive effort at the leading edge. The crew fix is the mirror of the too-low case: lower the auger in small increments and re-set the screed angle of attack until the mat texture is tight and uniform [S4].
What to check on a coarse-mix job before the first truck arrives
Three measurements, in order, taken on a cold paver on a level pad: (1) bottom-of-screed-plate to bottom-of-auger-segment distance, target 2 in baseline or 2.5 to 3 in for 19 mm plus base; (2) auger-to-pre-strikeoff distance at the head of the auger curl, target 5 to 6 in, replace the auger if 6 to 7 in; (3) auger lift cylinder available travel, most standard pavers give about 5.75 in of total vertical adjustment, which is enough for a 19 mm binder lift but tight for a 37.5 mm SMA where the crew wants to run at 3 in plus [S1][S5].
Field signals to watch during the first 50 ft of pull: visible stripes behind either auger (auger too low for the NMAS), center-line hump with starved extensions (auger too high, conveyor output not raised to compensate), or a slow but steady rise in mat thickness across a truck exchange (head of material drifting above 50%, usually a feeder sensor calibration issue rather than auger height) [S3][S4]. For paver selection and screed matching on these lifts, the asphalt paver encyclopedia page walks through the class-by-class screed width and auger diameter envelope that the 5.75 in adjustment window has to fit inside.
Trackable signals to watch through the 2026 paving season: any revision to the Caterpillar screed-factor document (last issued 2016-09, reissued 2025-02 in the source mirror) that explicitly couples NMAS to auger offset, and any update to the Asphalt Institute's engineering FAQ on screed-auger interaction, which is currently a general pointer page without a numerical auger-height recommendation [S3][S6]. Crews running 19 mm plus base lifts in 2026 should also watch for state-DOT spec changes (MDOT's 501 wiki still references auger-conveyor systems on a paving machine without binding a number) that convert the 2 in rule from operator guidance to a pay-item measurement.
For component-level specifications, see height gauge, and safety mat.
See also our earlier report, Underslung Single Girder Monorail Track Profile: Sizing and Spec Guide.