1

BEST PRACTICES AND BOLTING MACHINE INNOVATIONS FOR ROOF SCREENING

S. B. Robertson 

G. M. Molinda

D. R. Dolinar

D. M. Pappas

NIOSH 

Pittsburgh, PA 

G. E. Hinshaw

J.H. Fletcher and Company 

Huntington, WV 

ABSTRACT

 Rock falls in coal mines cause many injuries each year. 

Most of these injuries are not caused by major roof collapses, but 

from falls of smaller rocks from the immediate top or roof skin. 

Even though safety professionals and mine operators strive to 

reduce these types of injuries, there has not been a substantial 

decrease in the rock fall injury rate over the past decade. Various 

surface controls are used in mines to control the roof skin. One 

control that has proven to be very effective is roof screening. 

Roof screen effectively controls roof skin and provides a high roof 

surface coverage. Although many mines are reluctant to use 

screen for primary skin control because of the additional costs of 

time and materials, others are having great success at controlling 

both the costs and surface rock. In this study, injury data are 

presented that show a dramatic reduction in roof skin injuries 

when screening is used. Much of this success is due to the 

protection from falling rock that the operators receive from roof 

screen. Five case studies in which roof screen was used are 

presented along with the associated costs of materials, impact on 

bolting advance rates, and potential ergonomic risks. The effects 

of roof screening on skin control and safety are also included. 

Finally, this paper provides information about best practices and 

features of roof bolting machines that affect production and 

safety. 

INTRODUCTION 

 Rock falls in coal mines cause serious injuries to miners 

every working day. From 1995 to 2001, an average of 650 

reported injuries per year resulted from rock falls of coal mine 

roof. The majority of these injuries, about 99%, were not caused 

by a major roof collapse, but from falls of smaller rocks from the 

immediate roof where the average rock size was 76 x 46 x 10 cm 

(30 x 18 x 4 in) (Molinda, 2002). This type of rock fall, which does 

not extend more than one or two ft into the immediate roof, has 

also been termed skin fall. Most of the injuries caused by these 

skin falls happened under supported roof. 

 Even though the mining profession has become safer 

throughout the years, there was no significant decrease in skin fall 

injury rates from 1995 to 2000. Fortunately, there was a slight 

decrease in roof skin injury rates during the year 2001. Safety 

professionals and mine operators should continue to strive to 

reduce skin fall injuries. 

 One way to prevent these injuries is to better support the coal 

mine roof surface. The supports used to prevent skin falls are 

called surface controls. Greater use of surface controls has been 

found to maintain the integrity of the immediate roof, decrease the 

likelihood of an injury, and possibly reduce clean-up time due to 

less rock on the coal mine floor. Coal mines make use of various 

surface controls such as large roof bolt plates, steel straps, 

header boards, large bearing plates, and steel screen or mesh. 

Each control has its own application and effectiveness depending 

on the geology of the roof, moisture sensitivity of the rock, and the 

life of the entry in which it was installed. Some roof is of such 

poor quality that nothing short of full coverage will provide the 

correct protection. 

 The purpose of this investigation is to educate the mining 

industry about roof screening and the current related practices 

used today. Issues that prevent or support the use of roof screen 

as a skin support, routinely and on-cycle, will be addressed. 

Analyses of injury data pertaining to roof screening, along with a 

review of five case studies of roof screening, are included in this 

report. In addition, machine design innovations and a state-ofthe-art roof bolting machine with a new material handling system 

(MHS) are discussed. Finally, a list of best practices for roof 

screening is provided. 

Figure 1. Roof Screen effectively maintaining weak roof 

skin. 
 

2 

ROOF SCREENING: SKIN CONTROL 

 Figure 1 shows how roof screen can effectively control weak 

roof skin and support large pieces of rock. Unfortunately, roof 

screening is not a widely utilized skin control, especially when 

installed on-cycle. Only a handful of coal mines are routinely 

screening in the Eastern U.S. compared to a greater acceptance 

of screening in the Western and Central U.S. 

 A number of mines were visited to document their experience 

with controlling poor roof conditions and the methods used to 

control it. The immediate roof commonly consisted of clayshale, 

soapstone, or highly laminated shale; clay veins, slickensides, 

and/or potting were problematic. Roof surface fell immediately 

during mining or soon after between bolts because of three 

primary factors: weak rock, horizontal stress, and weathering. 

Partial surface controls such as pizza pans, roof straps, or 

Monster1

 MatsTM did not provide the protection desired by the 

mine operators. To achieve such protection, the mines began 

routinely implementing the use of roof screen. 

 An advantage of using roof screen over other supports is the 

large amount of roof coverage – close to 100% of the roof can be 

covered. A recent study conducted by the National Institute for 

Occupational Safety and Health (NIOSH) has shown that in a 

mine with 5.5-m (18-ft) entries, installing a 4.9 m x 15.2 cm (16 ft 

x 6 in) steel strap every 1.2 m (4 ft) of advance achieves an 11% 

roof coverage. However, if the mine used 4.0 x 1.5 m (13 x 5 ft) 

sheets of screen, it could achieve 72% coverage. Therefore, the 

use of roof screen would provide 61% more roof coverage 

(Molinda, 2002). 

 While some mines use plastic geogrid screen or chain-link 

fence as a screening material, most mines that routinely roof 

screen install steel wire sheets with a 10.2-cm (4-in) grid pattern. 

A smaller grid size can be used if rock flaking is a problem. 

These sheets can be ordered in many sizes depending on the 

coverage desired. Roof screen can also be ordered in different 

steel gauges, determined by the diameter of the wire, and each 

gauge of screen has a particular strength. These strengths along 

with the rock load height they support are shown in figure 2. The 

rock density used to calculate rock load height is 2,595 kg/m3

(162 lb/ft3

). All the mines visited during this investigation, except 

for one, used 8-gauge steel sheets; these sheets can support 

0.64 m (2.1 ft) of roof skin. 

 

1

Mention of any company name or product does not constitute 

endorsement by the National Institute for Occupational Safety and 

Health. 

 The deflection of roof screen varies depending on the rock 

load height, bolt spacing, and steel gauge of screen. A Canadian 

study determined that the center deflection of an 8-gauge sheet of 

screen installed on a 1.2-m (4-ft) bolting pattern with a rock load 

of 10.2 kN (2,300 lbs) or 0.3 m (1 ft) of rock was 16.0 cm (6.3 in). 

For a rock load of 20.4 kN (4,600 lbs), which is close to the 

strength limit for an 8-gauge sheet of screen, the average center 

deflection is about 25.4 cm (10 in) (Pakalnis, 1983). 

 Roof screen can be ordered with additional reinforcing wires – 

extra rows of wire – along the width and length of the screen in 

locations where roof bolts are installed. For example, a typical 

piece of screen is designed with a 10.2 x 10.2 cm (4 x 4 in) wire 

grid pattern. With reinforced screen, extra rows of wire are 

fabricated into the screen causing a 5.1 x 10.2 cm (2 x 4 in) grid 

pattern. This reinforced screen serves two purposes: it can 

prevent wires or welds from breaking around roof bolt plates and 

it provides a visual location where operators should install roof 

bolts. The latter helps operators to achieve the desired bolt 

spacing. 

ROOF SCREENING: SAFETY AND ERGONOMICS 

 Rock fall injury data from four mines in the eastern U.S. were 

analyzed to determine the effect of screening on safety. A mine 

in northern Maryland had roof conditions that deteriorated to a 

point where roof screening was necessary everywhere. After 

screening was implemented, injuries dropped from an average of 

14 to 2.2/yr (figure 3). At another mine in the Illinois basin, similar 

geologic circumstances were encountered that required roof 

screening. Injuries there subsequently dropped from an average 

of 8 to 0.25/yr (figure 4). Clearly, the use of screening dramatically 

reduced skin fall injuries at these mines. 

Figure 2. Different strengths and rock load heights for 

four steel gauges of screen (3). 

Figure 3. Number of skin fall injuries per year at a northern 

Maryland coal mine before and after the implementation of 

roof screening. 

Figure 4. Number of skin fall injuries per year at a mine in 

the Illinois basin before and after the implementation of roof 

screening. 
 

3 

 Rock fall injury data from two longwall mines in the western 

U.S. that screen routinely were compared to data from all other 

longwall mines. These two mines commonly encountered poor 

geologic conditions. Figure 5 shows that the skin fall injuries at 

these two mines were lower each year compared to all other 

longwall mines. According to mine management at both of these 

mines, roof screening was the reason for such low injury rates. 

 One reason roof screening provides a reduction in injures is 

because roof screening protects roof bolter operators while 

drilling. During this drilling process, rocks often become 

dislodged from the roof due to disturbances caused by the drill 

itself. When roof screening occurs, the screen is placed against 

the roof when the Automatic Temporary Roof Support (ATRS) is 

raised before the drilling operations begin. Hence, immediate 

protection is provided to roof bolter operators from rock falls. A 

study conducted by the U.S. Bureau of Mines in 1993 has shown 

that the activity of drilling is the most hazardous job task 

performed by the bolter operator, accounting for 31.4% of all roof 

bolting injuries. Out of these drilling injuries, almost half (45.9%) 

were due to falling rocks or coal from the roof (Klishis, 1993). 

Therefore, to receive the full benefit of roof screening on injury 

reduction, roof screen should be installed, on-cycle, during normal 

bolting operations. 

 Even though roof screening has been shown to reduce skin 

fall injuries, a legitimate concern remains during screen 

installation that material handling injuries may increase due to 

increased handling of materials and supplies. Additional 

materials and bolts, used in conjunction with roof screen, offer a 

challenge for the roof bolter operator because most roof bolting 

machines are not designed to accommodate the sheets of screen. 

 Also, operators contend with not having enough storage area to 

easily retrieve supplies. 

 In addition, screen handling is also a challenge for the 

operators. The hand-loading of roof screen onto the machine 

often occurs on uneven and muddy floor conditions. If the screen 

can not be stored on the rear of the bolting machine, it is hooked 

to the bolting machine by chain and pulled on the ground. Thus, 

operators must exit the machine to retrieve each piece of screen. 

 Overhead lifts and awkward positioning, along with lifting, pulling, 

and twisting movements may have negative ergonomic 

consequences on operators. 

 Finally, a sheet of screen can be cumbersome for operators 

to handle. A 4.9 x 1.5 m (16 x 5 ft) sheet of 8-gauge steel weighs 

approximately 13.6 kg (30 lbs). Therefore, handling screen may 

increase the chance of musculoskeletal injuries such as 

sprains/strains. As many as 29.7% of all mining industry lost-time 

days (1997 to 1999, excluding fatalities) were from strain/sprain 

injuries of the back, knee, or shoulder. Also, 27% of all lost-time 

days were from material handling injuries (Sacks, 2000). 

Therefore, an increase or difficulty in material handling is a major 

concern for mine operators. 

 A final safety note: It is a good practice to order roof screen 

so that the edges of the screen are flush-cut with no wires 

protruding from the ends. These wires create a potential for cut 

and puncture injuries. Also, if the screen is not flush-cut, the 

wires get tangled, making it difficult to separate the sheets. This 

can lead to possible sprain/strain types of injuries. 

ROOF SCREENING: MATERIAL COSTS 

 Material costs for roof screen is one barrier that inhibits mine 

operators from purchasing the material. The cost of the roof 

screen will vary depending on the size of the screen and the 

gauge of the steel. To assess these costs, NIOSH recently 

conducted a study comparing the use of steel straps and roof 

screen (Molinda, 2002). Roof straps were chosen because they 

are another surface control used to combat poor roof conditions. 

The cost of a 4.9 m x 15.2 cm (16 ft x 6 in) steel strap was $8.00, 

and the cost of a 4.0 x 1.5 m (13 x 5 ft) sheet of 8-gauge steel 

screen was $10.32. Assuming a 1.2-m (4-ft) advance for each, 

the material cost per 0.3 m (1 ft) of advance for steel straps was 

$2.00; the cost for steel screen was $2.58. It should be noted that 

this study did not include costs of the roof bolts and plates needed 

to install roof straps and screen. 

 For the above scenario in a 5.5 m (18-ft) wide entry, the roof 

coverage would be 11% for steel straps and 72% for screen. 

Roof screen provided a significant increase in roof coverage 

(61%) for only a $0.58 per 0.3 m (1 ft) increase in cost over the 

use of steel straps. Moreover, the spacing between straps is 

commonly decreased when bad roof is encountered. For 

example, if the spacing is decreased from 1.2 to 0.9 m (4 to 3 ft), 

the cost per 0.3 m (1 ft) of coverage would increase to $2.67. In 

this instance, the cost of using steel straps would actually exceed 

the cost of using steel screen, without providing significantly 

improved coverage. 

ROOF SCREENING: BOLTING ADVANCE RATES 

 Another barrier to the widespread use of screen as a skin 

control installed on-cycle is the additional time and labor required 

to handle and install the material. Five in-mine studies to 

document the impact of screening on-cycle on bolting advance 

rates are presented below. For these studies, only activities that 

delayed the bolting advance rates were classified in terms of time 

to handle roof screen. For example, a scoop operator delivering 

screen to a mining cut does not slow down the process of bolting. 

 However, the time taken by the roof bolt operators to load the 

screen onto the bolter and then handle these sheets does slow 

down bolting activity. 

 When considering these five cases, the reader should keep in 

mind that the following five factors influence the bolting advance 

rate and labor costs as well as safety when handling and installing 

screen: 

1. Design of the bolting machine in relation to screen storage 

and maneuverability, positioning of the roof bolt operators, 

material handling innovations, and machine height. 

2. Installation procedures for handling/installing screen and 

loading it onto the roof bolting machine. 

3. Number of persons involved in handling screen. 

Figure 5. Number of skin fall injuries at two longwall 

mines in Colorado and Utah that routinely roof screen 

compared to all U.S. longwall coal mines. 
 

4 

4. The amount of training and experience the operators have in 

installing roof screen. 

5. The amount of mining height available to handle screen. 

Case 1

 A mine in central West Virginia recently began roof screening 

because of difficulties with a weak claystone, weathering, clay 

veins, and potting. In general, the newly exposed roof skin shows 

little damage, but can be expected to deteriorate in 3 to 4 months 

after an area has been mined. This mine installs screen on the 

intake, belt, and track entries. 

 Two roof bolter operators and a helper install the screen. 

Operators utilize a twin boom bolting machine with mast feed that 

enables angle bolting. Mining height is 165 to 190 cm (65 to 

75 in) and entries are 5.5 m (18 ft) wide. The miners install 8-

gauge steel sheets of screen that are 4.0 m (13 ft) long and 1.5 m 

(5 ft) wide that are fabricated with flush-cut edges. The effective 

roof coverage is 72%. 

 

 The sequence of activities to handle and install the roof 

screen are the following: The scoop pulls up behind the bolting 

machine loaded with screen. The roof bolter helper and scoop 

operator then load approximately 10 sheets (enough for 1 or 2 

mining cuts) onto the rear of the machine on top of bolting 

supplies. During this time, primary roof bolting operations and 

screen installation are not interrupted. Before a sheet of screen is 

installed, the ATRS is lowered and a drill steel is placed into the 

chuck to help control movement of the screen on the ATRS. 

Operators then lift and carry the sheets from the rear to the front 

of the machine, then onto the ATRS. Operators center the screen 

across the ATRS, maneuver into position for a slight overlap 

between sheets, and raise the ATRS. At this point, the screen 

handling time is completed unless the screen needs to be 

adjusted. Only screening activities that delayed the bolter 

operators from advancing were considered to be time delays. 

Therefore, no time delay was attributed to screen loading onto the 

rear of the machine because the bolter helper and scoop operator 

performed this task. 

 

 On two different occasions, a time study was performed 

observing the same roof bolter operators with the same bolting 

machine. The first investigation occurred 18 months before the 

second, when the roof conditions were poor. Roof conditions 

during the second study were fair to good. During the first study, 

it took an average of 1.72 mins to handle each sheet of screen. It 

took an average of 0.85 mins to handle each sheet of screen for 

the second investigation (table 1). 

 During the latter study, operators installed screen twice as 

fast than in the previous study. This can be explained by a 

combination of two reasons. First, the operators had more 

practice and became more efficient with their installation 

procedures over the 18-month time interval. Also, during the first 

study, irregularities from the bad top made it difficult to raise the 

screen against the roof without the screen sliding. Even though 

the operators placed the screen over the top of the drill steels to 

control movement, the uneven roof caused the screen to tilt and 

shift. Therefore, operators repositioned the screen numerous 

times. The operators also bolted from the side of the machine, 

and reaching out from this location to reposition the screen was 

sometimes demanding. Additionally, the sheets were sometimes 

cumbersome to handle and involved lifting, pulling, reaching, and 

twisting motions. Occasionally, the edges of the sheets that hung 

off the back of the machine became damaged when tramming 

around corners, making it difficult for operators to pull the sheets 

apart. 

 

 In this case, the screen effectively controlled the roof. The 

timely use of screening minimized skin hazards in high-travel 

entries and reduced clean-up time. However, even with a threeperson crew, the miners struggled with the installation procedures 

during the first study. This was partially due to the operators’ 

inexperience with screen installation at this mine and the poor 

roof conditions. Practice and better roof conditions contributed to 

quicker installation times during the second study. 

Case 2

 A mine in the northern Maryland has been screening because 

of difficulties with clay shale roof, which is highly slickensided and 

brittle. It appears wet during visual inspection, but is actually 

glassy due to the extreme number of slickensides. Loose rock 

typically falls immediately upon mining and also spalls out 

because of mine humidity. Miner operators cut up to 76.2 cm 

(30 in) of drawrock and try to hold the rest with screen. They 

install screen mine-wide. Mine management claims that without 

the use of screening, mining would be very difficult due to 

increased clean-up time, higher risk of injury, and slower bolting 

advance rates. 

 Entries average 5.0 m (16.5 ft) wide and the mining height is 

close to 2.4 m (8 ft). Operators install 8-gauge steel sheets of 

screen that are 4.3 m (14 ft) long and 1.5 m (5 ft) wide. The 

effective roof coverage is 85%. The sheets extend to within 0.5 m 

(1.2 ft) from the rib after screen installation; some guttering occurs 

along the ribline. The sheets are fabricated with flush-cut edges. 

Two roof bolter operators and one bolter helper, also serving as a 

rib bolter, install the roof screen. In the mine’s approved roof 

control plan, roof screening and rib bolting must occur on-cycle. 

The rib bolter operator installs two rib bolts for each advance and 

still has time to help the operators handle roof screen. The 

bolting machine is a walk-through bolter with a rib bolting machine 

retro-fitted onto the rear. Therefore, there is no storage space on 

the bolting machine for roof screen. 

 The sequence of activities to handle and install the roof 

screen are the following: The screen for each cut is placed and 

hung along the rib by the scoop operator and rib bolter operator. 

The rib bolter operator lifts the screen onto the machine from 

either the back or the side – during this study, he chose to carry 

the screen along side of the bolter. He then lifted the screen up to 

the roof bolt operators where they grab and pull the screen up 

and across the ATRS. Operators center the screen across the 

ATRS; maneuver into position for a six-inch, screen overlap; and 

then raise the ATRS. At this point, the screen handling 

operations are complete unless screen adjustments are required. 

Persons Bolting Machine Load Time Install Time Total HandlingTime

Installing (min. per sheet)(min. per sheet) (min. per sheet)

Case 1 3 Twin Boom with 0 1.72 / 0.85 1.72 / 0.85

Study 1/Study 2 mast feet drill

Case 2 3 Walk-through (Centered) 0 0.42 0.42

Rib bolter attached

Case 3 3 Walk-through 0.40 0.50 0.90

(Centered)

Case 4 2 Walk-through 0.73 0.72 1.45

(Off-Centered)

Case 5 2 Fletcher CHDDR 0.16 0.68 0.84

walk-thru with MHS

Table 1. Roof screen installation information including 

the number of persons involved, bolting machine type, 

and screen handling times for five case studies. 
 

5 

 For this case, it takes an average of 0.42 minutes for roof bolt 

operators to handle and install each piece of screen per 1.2-m 

(4-ft) advance (table 1). Any adjustment of screen position is also 

included. The total bolting time per advance, including drilling, 

installing, maneuvering, setting the ATRS, and handling screen is 

8.36 minutes. Screening time takes up only 5% of this total. Mine 

management feels that the benefit of superior roof coverage and 

a major safety improvement (figure 3) outweigh the cost of the 

additional time. 

 Some concerns are that the screen edges can get damaged 

when moved around by the scoop, making it difficult to pull apart 

the sheets. Also, the rib bolter operator chose to lift the screen up 

to the other operators while standing along the rib. This subjected 

him to a possible rib fall injury. Carrying the screen onto the 

bolter from the back of the bolting machine would decrease this 

risk. Whether lifting the screen from the side or the back, the 

screen must be lifted from the ground to an elevated or overhead 

position, which puts the person at risk of musculoskeletal injury. 

 Roof bolter operators at this mine have over six years of 

practice installing roof screen and are comfortable with the 

installation procedures. Operators do not have difficulty keeping 

the screen from sliding on the ATRS because of the position of 

the operator on the machine. While under support, one operator 

holds the screen in place from the center while the other raises 

the ATRS. This is an advantage of using a walk-through bolter. 

In Case 1, operators had difficulties holding the screen in place 

because of positioning in relation to the machine; they operate 

from the sides of the roof bolting machine. 

Case 3

 A longwall mine located in Colorado has been routinely roof 

screening, on-cycle, for over ten years. This mine has had great 

success in maintaining very low skin fall injury rates. From 1995 

to 2002, there were just two injuries due to skin falls (figure 5). 

The immediate roof is 1.2 m (4 ft) of mudstone followed by 10.7 to 

12.2 m (35 to 40 ft) of stack rock. Miner operators try to leave 

0.5 m (1.5 ft) of head coal. When the roof is exposed, the 

mudstone slakes and the roof is difficult to maintain. Roof screen 

is the preferred surface control at this mine to maintain good roof 

conditions. 

 

 The mining height averages 3.0 m (10 ft). Entries are 5.0 m 

(16.5 ft) wide and the roof screen dimensions are 4.2 x 1.7 m 

(14 x 5.5 ft) for an effective roof coverage of 85%. The sheets are 

made with 9-gauge steel and fabricated with flush-cut edges. 

They are also reinforced with extra wires where roof bolts are 

installed. Two bolter operators and a bolter helper handle and 

install the roof screen. The bolting machine has a centered 

walkway which prevents screen storage on the machine. Instead, 

the sheets are hooked to the back of the roof bolting machine by 

chain and dragged to the bolting face. 

 The sequence of activities to handle and install each sheet of 

screen is the following: The bolting machine is trammed to an 

outby location where a bundle of roof screen is located. 

Operators then lay 4 to 6 sheets of screen on the ground behind 

the machine, then hook them up with a chain. The bolting 

machine is then trammed to the next location pulling the roof 

screen behind it. When ready for a sheet of screen, the ATRS is 

lowered and one bolter operator (the one who finishes installing 

his bolts first) exits the machine to retrieve a piece of the screen. 

This operator and the bolter helper lift and carry the screen onto 

the machine shown on figure 6. The other operator then grabs the 

screen and both bolter operators lift and place the screen across 

the ATRS. Next, the operators bend a set of heavy wires over the 

screen which prevents the screen from moving when the ATRS is 

raised. This wire assembly, which is a homemade machine 

innovation, is shown in figure 7. Once the ATRS is raised, the 

screen handling operations are complete. 

 Once the bolting machine is trammed to where the bundle of 

screen is located, it takes about 0.40 minutes to hook up each 

sheet of screen behind the bolter. It then takes an average of 

0.50 minutes to handle and install each sheet. This total time of 

0.90 mins makes up 17% percent of the total bolting time for a 

1.4-m (4.5-ft) advance as shown in table 1. 

 There are some safety concerns when dragging the roof 

screen behind the bolter. Operators must exit and enter the 

machine for each sheet of screen. When doing this, the bolting 

machine cable and the chain that hooks up the screen to the 

machine must sometimes be stepped over, creating a slipping or 

tripping hazard. Exiting, entering, and walking across the 

machine repeatedly will also induce fatigue. To combat this 

problem, NIOSH personnel recommend that anti-fatigue mats be 

fitted on top of the walkway. The mats are rubber cushions 

designed to more evenly distribute a person’s weight on the foot. 

They provide a comfortable surface causing less fatigue in the 

feet and shins. 

 Similar to Case 2, roof screening at this mine has been 

proven to dramatically decrease roof skin injuries. It also 

effectively controls the roof surface. The reinforced screen helps 

Figure 6. Roof bolting operators handling roof screen on a 

walk-through bolting machine. 

Figure 7. Wire assembly retrofitted onto an ATRS to control 

the movement of roof screen. 
 

6 

to maintain the integrity of the screen and maintain correct bolt 

spacing. The positioning of the operators on the walk-thru bolter 

and the wire assembly on the ATRS help operators to better 

control movement of the roof screen. This mine has been 

screening for over 10 years and installation procedures have 

become routine. Installation time was not as low as in Case 2, 

0.90 versus 0.42 mins per sheet, mainly due to the time spent 

loading and retrieving the screen. As in the previous cases, the 

screen can get damaged when tramming, especially when being 

dragged on the ground, making it difficult for operators to 

separate the sheets. Also, operators must lift the screen from 

ground level to an elevated position causing a potential for 

musculoskeletal injury. 

Case 4

 This mine, located in the Illinois basin, has a weak and 

laminated shale roof, and degrades from horizontal stress and 

weathering. The mine has found that steel screen helps maintain 

the integrity of the roof and dramatically reduced injuries due to 

rock falls from an average of 8 to 0.25/yr (figure 4). 

 Two bolting operators load and handle the roof screen. The 

bolting machine is a walk-thru bolter with a walkway off-centered 

to the right. The left-side rear compartment, used as a storage 

area for bolting supplies, is wider than the right side. It is wide 

enough for screen to be loaded on top of the bolting supplies. 

Most J.H. Fletcher and Co. walk-through bolting machines 

manufactured after 1995 have this off-centered, walk-through 

design. 

 The mining height ranges from 2.3 to 3.0 m (7.5 to 9 ft). 

Entries are 4.9 m (16 ft) wide and the dimensions of the screen 

are 4.5 x 1.5 m (15 x 5 ft). Roof screen extends to within 15.2 cm 

(6 in) from the rib for an effective roof coverage of 94%. The 

sheets are made with 8-gauge steel and fabricated with flush-cut 

edges. To avoid the screen wires from breaking or being cut by 

roof bolt plates, the mine installs 30.5 x 30.5 m (12 x 12 in) 

plywood boards between the roof bolt plates and the roof screen. 

 

 The sequence of activities to handle and install the roof 

screen is the following: The scoop operator places screen at an 

outby entry. Bolter operators back up the bolting machine to 

where the screen is located, then load enough sheets (5 to 7) to 

bolt the next cut. Before each sheet is installed, the ATRS is 

lowered and one bolter operator walks back to the screen 

location. Next, the operator lifts and pulls the screen towards the 

front of the bolter, swings the screen towards the other operator, 

and they then place it across the ATRS. One operator holds the 

screen in place from the middle of the machine, while the other 

raises the ATRS. At this point, the handling time is over unless 

the screen needs adjusted. 

 For this case, it takes an average of 0.73 mins to load each 

sheet onto the rear of the bolter. It then takes 0.72 mins to place 

the screen onto the ATRS, including any adjustment for sliding. 

The total time for handling screen is 1.45 mins per sheet (table 1). 

 With the walk-through bolter, the sliding of screen is minimal 

because an operator is able to hold the screen in place. Case 4 

differs from Cases 1, 2, and 3 in that only two operators are used 

to handle the screen instead of three. 

 Some safety concerns are noted with screening at this mine. 

Even though the left rear compartment on the bolting machine is 

large enough to stack sheets of screen, the screen still hangs off 

the ends and can get damaged while making a turn. Sometimes 

the screen gets caught on the bolting materials that are stored 

underneath. This makes it difficult for operators to remove the 

sheets from each other or from the supplies. In addition, it is 

strenuous to lift and pull the screen to the front of the bolter. The 

operator must lift the screen overhead and then twist before the 

other bolter handles the sheet. The screen must be lifted over the 

bolter canopies and on top of the ATRS. These actions increase 

the potential for musculoskeletal injury. 

Case 5 

 Case 5 is from the same Illinois mine in Case 4, but in a 

different section. All the mining conditions are the same except 

that this section uses a Fletcher CHDDR walk-through bolter 

equipped with a material handling system (MHS) shown in figure 

8. This state-of-the-art system has many machine innovations 

that reduce material handling and address ergonomic concerns. 

Some examples are a mechanized screen tray and detachable 

material pods (figure 8), a winch rope built onto the machine, and 

remote control operations. 

 

 The sequence of activities to handle and install the roof screen 

is the following: The scoop operator drops off a bundle of screen 

containing 25 sheets behind the bolter. A mechanized screen 

tray, located above the left material pod, is extended toward the 

rear of the machine by remote control operations. The front and 

rear lift cylinders decline the tray towards the mine floor. The tray 

is placed at a ramped angle making it easier to pull screen onto 

the tray. A winch, which is also operated remotely, is used to pull 

on the bundle. The screen tray can move in eight directions so 

that the bundle is pulled on straight. The tray is then secured in 

place with a pin and sleeve assembly and the winch attachment is 

then disconnected. Before tramming, the tray is lowered and 

moved toward the center of the machine. This protects the 

screen from being damaged. At the beginning of the next cut, the 

screen tray is raised to an ergonomically friendly elevation 

(around shoulder height) so that each sheet is pulled straight off 

without the operator having to lift the sheets. The screen is then 

handled and installed exactly as described in Case 4. 

 

 For this case, it takes a total of 3.9 mins to load a bundle of 

screen onto the tray, which is an average of 0.16 minutes per 

sheet. This does not include cutting off the metal straps on the 

bundle that secure the screen. It then takes 0.68 mins to handle 

each sheet of screen. The total time for handling and installing is 

Figure 8. The J.H. Fletcher material handling system, 

featuring left and right material pods and roof screen 

tray. 
 

7 

0.84 mins per sheet (table 1). Compared to Case 4, the time for 

installing the screen, once loaded onto the bolter, is almost the 

same. But because of the MHS, load times are lower by as much 

as 0.6 mins per screen. In addition to the saved time, the 

operators’ exposure to ergonomic injury is significantly decreased. 

 The strain/stress is significantly reduced in handling screen 

because the operator can pull off the screen at a more 

comfortable level. Also, the screen has less chance of becoming 

damaged because the bundles are handled less and do not get 

damaged when turning corners. Roof bolters do not have to 

struggle to retrieve bolting materials because the screen tray is 

separate and elevated. Fatigue is reduced because operators do 

not hand-load the screen. During installation, the elevated tray 

minimizes lifting. Finally, the tray can also be positioned so that 

the screen does not cover the walkway, providing a clear exit off 

of the machine. 

 For this case, roof screening effectively controls the roof 

surface and has been proven to dramatically decrease roof skin 

injuries. The material handling system practically eliminates the 

hand-loading of screen and provides an easier method for screen 

handling, reducing ergonomic risks to the operators. The screen 

handling time was lower in Case 5 than in Case 4 from 1.45 to 

0.84 mins per sheet. This is due to the quicker loading procedure 

of the screen using the MHS. At the same time, the handling time 

was slightly higher in Case 5 than in Case 2 from 0.42 to 

0.84 mins per sheet, but only two roof bolter operators were used 

instead of three. 

ADDITIONAL FEATURES OF THE MATERIAL HANDLING 

SYSTEM 

 The MHS has many additional features beyond those 

described above. Bolting supplies are not hand-loaded onto the 

bolter while underground, but are loaded outside into material 

pods (figure 8). These pods are then taken underground on 

supply cars. A larger left pod and smaller right pod contain 

supplies such as bolts, plates, cap board, and boxes. At the face 

area, material pods are pulled onto the bolting machine by a 

winch that is operated remotely. For roof bolter operators, the 

only hands-on activity is to hook up the winch rope to the material 

pods and operate the remote controls. 

 Time can be saved when loading bolting supplies with the 

MHS. To demonstrate, a comparison was made of operators 

loading supplies in Case 4 and Case 5. In Case 4, it takes 3 men 

approximately 13 minutes to hand-load a shift’s worth of supplies 

onto the bolter. In Case 5, it takes 2 men about 6.0 minutes to 

load the left and right material pods onto the bolter, including the 

removal of the empty pods, based on accounts of bolter operators 

and mine personnel. Each pod contains a shift’s worth of 

materials. Also in comparing Case 4 and 5, there is a significant 

decrease in exposure to injury due to the MHS. The risk of 

sprain/strain, slip/trip, crush, or cut type injury is reduced because 

trips made to the roof-bolting machine while hand-loading 

supplies are eliminated. 

 Material pods are easily pulled onto the scoop bucket because 

the pod height is low enough to be pulled under the scoop ram. 

Both the left and right pods can fit into the scoop bucket. The 

pods are pulled onto the bolting machine directly off the scoop. 

The machine has a rear lift system that places the rear bumper on 

the mine floor. This creates a ramp that allows the material pod 

to be pulled onto the machine. The pod is manufactured with 

guides on each side that run its full length and mate with the 

runners on the machine frame. These guides also secure one 

pod atop another for transportation into and out of the mine. The 

bottom of the pod is exposed to rollers on the frame which reduce 

the force required to pull the pod onto the machine. Once loaded, 

pins are placed through the pods and machine frame to keep the 

pod in place. These pods are loaded by remote control without 

the necessity of an operator being near the machine. These 

remote controls allow the operator better visibility because he has 

freedom to move around the machine. 

 Designed into the MHS are flat-top canopies and rounded 

ATRS edge pads. These features decrease the physical effort 

required to move the screen into place and reduce the likelihood 

of material snagging. There is a final safety feature for controlling 

the movement of screen: Clamps are built onto the ATRS which 

hold the screen in place while being raised. Once the ATRS is 

against the roof, these clamps retract. 

BEST PRACTICES FOR ROOF SCREENING 

 In the course of this investigation, a list of best practices has 

been compiled to assist mines in attaining safe and efficient roof 

screening installation procedures. These suggestions or 

practices were obtained through underground observations, 

meetings with mine management, and working with equipment 

manufacturers. 

1. Roof screen with flush-cut edges will help avoid material 

handling difficulties and lessen injury potential. 

2. Roof screen that is fabricated with reinforcing wires can help 

sustain the integrity of the screen and aid in proper bolt 

spacing. 

3. Sheets of plywood or matting can be installed between the 

roof bolt plate and screen to prevent the plates from cutting 

the screen wires. 

4. Anti-fatigue mats, fitted on top of the bolting machine 

walkway, will provide a more comfortable surface, resulting in 

less fatigue on the feet and shins. 

5. An ergonomic analysis of installation procedures will 

determine the safest and most efficient installation procedures 

(e.g., how many persons, best positioning, safest method). 

6. Design or engineering innovations of bolting machines will aid 

with installation speed and material handling. Some 

examples are the following: 

$ Rounded edges on bolter canopy tops to reduce material 

snagging. 

$ Clamps or a wire assembly on ATRS to hold the screen 

in place. 

$ The use of a separate tray to better handle roof screen 

retrieve bolting supplies. 

$ Remote control operations, wines, material pods, or any 

other machine innovation that will eliminate or reduce 

material handling difficulties. 

SUMMARY AND CONCLUSIONS 

 Roof screening has been found to be an effective method for 

controlling poor roof skin conditions. Compared to other skin 

controls installed on-cycle, roof screening provides the most 

coverage for the mine roof. Analyses thus far from mines using 

roof screen have shown that injuries from rock falls have been 

reduced dramatically. This may be due in part to the protection 

roof bolter operators receive during the hazardous drilling 

process. Increased acceptance of regular, on-cycle roof 

screening in adverse conditions should help reduce the number of 

skin fall injuries annually. 
 

8 

 Time studies documenting the handling and installation of 

screen have been presented. Results show significant variation 

in the additional time necessary for screening; however, this 

reflects the experience and resources of the individual mine. A 

lower bolting advance rate is a barrier that can be reduced with 

time and practice. There is a concern, though, that injuries from 

handling screen may increase due to the additional materials, 

lifting, and awkward positioning. 

 The design of the roof bolting machine affects the costs of 

loading and handling screen times, manpower requirements, and 

ergonomic exposure to risks. Material handling design 

innovations will make roof screening safer and more efficient. 

The new state-of-the-art J.H. Fletcher materials handling system 

decreases material handling of roof screen and supplies. It also 

significantly reduces the risk of injury, screening time, and 

damaged materials. In particular to reduce skin fall injuries, mine 

operators should continue to seek and implement safer work 

practices and apply machine innovations to create a safer 

workplace for miners. 

REFERENCES 

1. Klishis, M.J. and Althouse, R.C., 1993, “Coal Mine Analysis: A 

Model for Reduction Through Training,” Volume VIII – 

Accident Risks During the Roof Bolting Cycle: Analysis of 

Problems and Potential Solutions. Contract Report with 

Cooperative Agreements C0167023 and C0178052, West 

Virginia University and USBM. 

2. Molinda, G.M., Dolinar, D.R. and Robertson, S.B., 2002, 

“Reducing Injuries from the Fall of Roof in U.S. Coal Mines,” 

SME Annual Meeting, Phoenix, AZ, February 25-27, 2002. 

3. Pakalnis, V. and Ames, D., 1983, “Load Tests on Mine 

Screening,” Canadian Institute of Mining and Metallurgy’s 

Symposium on Underground Support System, Sudbury, 

Ontario, September 19-21. 

4. Sacks, H.K. and Pana-Cryan, R., 2000, “A Cost Model for 

Traumatic Injuries in Mining,” NOIRS 2000 Poster Session, 

Pittsburgh, PA, October.