TM 5-6635-386-12&P sample  of  the  material  into  the  pan  and  taking  a  tester reading.    It  may  not  be  possible  to  obtain  135  PCF  dry material  In  the  pan,  however.    Any  error  resultant  from an   inadequate   compaction   of   the   dry   material   will   be negligible. h. This curve IS accurate as long as the same 135 PCF is produced in the field.  If other weights are desired for  field  compaction,  then  a  separate  curve  should  be produced  for  each  desired  field  weight.    Obviously,  It  is desirable to attempt to select only one or two appropriate field weights to minimize curves and effort The  pan  can  also  be  set  on  legs,  anything  at  least  two feet   from   the   ground   and   five   feet   from   surrounding objects Watch out for the tailgate of a pickup truck due to the  presence  of  the  gasoline  tank  and  spare  tire,  both heavy in hydrogen. 1-32.  ROOF MOISTURE TESTING The  soil  tester  becomes  a  most  useful  tool  when  it  is used to measure the trapped moisture in a built-up roof. It  is  always  difficult  to  locate  a  leak  and  to  estimate  the area of roof damaged by a leak  Built-up roofs generally consist  of  a  waterproof  membrane  sandwiching  a  thick layer  of  insulation  between  it  and  the  roof  structure  If moisture becomes trapped In the insulation, the insulating   value   is   damaged   and   the   roof   structure   is subject to deterioration, dry rot, or other injury. Any   attempt   to   puncture   the   roof   to   investigate   the degree  of  moisture  intrusion  is  undesirable  because  of the  additional  damage  the  testing  introduces.    The  non- destructive    nature    of    the    tester    provides    for    rapid, accurate determination of roof moisture without penetration of the roof. 1-33. ROOF MOISTURE IS A RELATIVE MEASUREMENT All    roofs    are    not    constructed    alike.        They    vary    in thickness, type of insulation, type of underlying construction, and type of waterproof membrane. Some    will     have     hydrocarbons     associated     with     the construction   materials   and   Insulation,   others   will   have little    Internal    hydrocarbon    structure.        The    moisture channel is affected by the internal hydrogen in the same manner as it is with bound water in soil.  Thus, a certain background reading will be present even though the roof is "dry". A  roof  with  wet  insulation  4  inches  thick  will  provide  a higher moisture count rate than will a roof with the same wet insulation, but only 2-inches thick. Thus,   the   reading   of   a   moisture   tester   on   a   single location  on  a  roof  cannot  be  used  alone  as  an  index  of whether the roof is wet or dry. A series of readings over a representative sample of the roof must be taken and readings compared to each other to  produce  a  profile  which  can  then  be  interpreted  as  a moisture profile. 1-34.  PROCEDURE Starting   from   a   corner,   take   readings   on   a   10’   grid pattern  over  the  roof.    1/2  minute  readings  should  be quite adequate.  Record the actual counts Do not bother with computing a ratio and do not attempt to use the soil moisture chart to determine actual moisture in engineering units. It will be noted that a majority of  the  readings  will  fall  at some   minimum   value   for   this   particular   roof   This   will correspond  to  the  dry  readings.    Other  readings  will  be higher.  The highest readings should be further explored with additional readings taken on 5’ grid to establish the boundaries of the higher readings. These  highest  readings  represent  the  wettest  portion  of the roof It may be necessary to make a test penetration to  determine  the  actual  moisture  present  at  this  highest value    By    observation    this    can    be    classified    as    an arbitrary "moist", "wet", or "saturated" value. Readings  in  between  the  dry  bottom  readings  and  the moist high readings can be classified proportionately. Extra readings should be taken around chimneys, vents, stairwells, cornices, and other protuberances.  Wet areas should be tracked to their perimeters. 1-35.  PLOT ROOF MOISTURE PROFILE On   a   drawing   of   the   roof,   plot   the   observed   values 1-22