Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~0~6~L~
METHOD FOR DETECTING ~IOISTURE IN MULTIPLE LAYER ROOFS
This application is a divisional appllcation of
application Serial No; 235,986, filed September 22r 19750
(1) Field of the Invention'. This invention per-
tains to methods for detecting the presence, location, and con-
centration of moisture in roof coverings.
~ 2) Description /of Prior Art: ~arge industrial and
commercial buildings often have flat roofs which require a cover-
ing for protection from the elements. This covering often con-
sists of multiple layers of tarpaper or felt which are bonded
together, usually by covering each layer with hot tar or bitumen
before emplacement of another layer. A layer of heat insulating
material is commonly provided between the structural roof and
the multiple felt layers.
Over a period of time, natural weathering forces re-
sult in a deterioration of the integrity of the roof covering.
Alternate heating and cooling of the roof causes cracks to appear
in the tarpaper or felt. Moisture seeps between the layers and
expands by freezing during the winter and evaporation during the
summer, with consequent further separation and cracking of the
layers. Eventually the covering deteriorates to the point that
water leaks into the insulation and through the roof, necessi-
tating replacement of the damaged portion o the covering.
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For purpo~ of ConVQnienCe in lllustr~tion, the
deterloration of multiple layer roo~s in wet are~s ~ay b~ ~,
clas~ified a~ flr~t stage penetrat:ion~ ~econd stage pen~-
tratlon, and ex~ensive water penetration ox third ~tage
S penetration. First ~tage penetrat:ion 1~ the initial
phase of water penetration into the roGf. I'h~ top lAyers
of felt may be ~omewhat porous from asphalt ~d tar
breakdown from normal weathering, or water may be found
between two layers arrivlng by capillary action from a
surface imperfection. There are no leaks into the
build~ng at th~s stage of detexioration, but roo~ damage -~.
may continue at a xapld rate unlass th~ ~ituat~on i8 cor- ~'
rected and water entry poin~ ~ealed. S~cond stage pene-
tration i8 similar to first ~tage penetxation ~xcept that
deterioration has progressed into the lower layers of
rooflng elts. Water may now be sandwiched between thr~e
or our elt layers instead of ~u6t the top ~wo as in ~irst
stage penetration. While ther~ 8t~11 may be no water leak-
age into the building at the ~econa staqe, the proteotive
layers of ~he roof are in-very bad shape and leaka~ into
the ~uilding can be expected at any time. The felt~
bitumen layers must be raplaced. The insulation layer
in mo~t ca~es can be ~alvaged, but should be inspected to
determine it~ condltion~ At th~ third or exten~ive watex
penetration s$age, water ha~ penetrated all tha pro~eotive ~5
felt layers and the insulation a~ well. Since tha binders
u~ed $n mo~t in~ulatlon are water soluble, the roof water
often dissolves the binder~, and the insulation deteriorates
and becomes muehy and also looses its h~at ~n~ulation ability.
In thi~ third ~tage of exten~ive water penetration, both
_
-
the felt layers and ~he ur~clerLyirlg in~ulatlon are ~oaked,
and the ~ntlre roo covering mu~t be xemoved and rebullt
-from the ~tructural roof on up u~ing new materials.
In gen~ral, first 6tage penetration area~ can be
5 most easily r~palred by top-coating with a cold applie~
roofing maEtic or an ela~tomaric Coat~ng ma~eriAl . If
the penetration i~ indtcated a~ com~ng from a ~la~hing
crack or tear, th~ repair should be made with an ela~to-
meric material applied over a poly~thylene me~h tape.
Asbestos fillad bituma~tic m~erlals work for temporary
repalrs. Thes~ ar~ not ~atis~actory ~or long-term repalrs:
becau~e they become brittle and cracked ~o again allow~
water entry.
Second stage penetration ~ndicates much more 6evere
roof deterioration. Only temporary repair3 ca~ be made
with the top layers. Permanent coxrective repair~ fox ~econd
8tag9 penetration areas require~ the x~mo~al of existi~g felt
plys down to the insulation to completely remove all trapped
water. Where vi8ual in~pection indicates excesslve deteriora-
t~on of the insula~ion~ it should be removed. The roof~hould be rebu~lt from the lnsulation on up, or a~ required
~rom the decking on up, u~ing standard roofing techni~ues
and new dr~ mater~als. Hot tar and hot a~phalt felt laminat~
ara ~a~is~actory here becau~e all roof water has ~een rèmoved.
Hot xoofing materials do not work well over ar~s ln a r~o~
containing moi~ture, and therefore ~hould not be used ln
these area~. The repair area~ are built up to the level
o~ the axisting ,older; bullt up roo~lng, and ~roperl~ over-
~ 5~lapped. Cold procas~ roof ma8~ or el~3tom~rlc coa~ing~
are applled a3 a flnal protective top coat.
Generally, roof coverlng doe~ not detariorate ov~r an
~ntlre roof at once, and repair or r~placement of the
entire covering i9 not justifled. Ilo~ever~ it may be dif-
flcult to dein~ tha areas of the cov~rlng ~hat need repair
or replacement, ~ince water may txavel a con~iderable di~-
tanee between ths layer3 to the point where the leak in
the roof appear~. The traditional method of determinlng
areas of we~nes~ in roof covering~ has been to tak~ core
sample~ of the covering. Thi~ method 18 time ~on~uming
and ha~ tha obvious disadvantage o~ xi~king destructlon
of sound rooE covering if too great an area is te~ted,
whlle deteriorated covering far from the polnt of leakage
may be missed if extensive samples ar~ not takan. More~
over, core ~amplinq is an impractical method for detecting
the f~rst ~tages of roo~ deteriora~ion, which a5 noted above
can o~ten b~ repaixed without replacement of the ao~erlngO
Several non-destructiva roof wetn~ss testing technique~
have been de~eloped in an a~t~mp~ to reduce the time and
expen~e of testing roofs. The~e technlques include in~ra-
red scanning o roof~ to detect cool moist area~, and
nuclear particle bombardment to determine hydrogen ion
coun~ in the roof covering. Such techniques generally
requ$re expen~lve and delic~te equipment, and do not di~-
crimlnate the-flr~t stage and ~econd stage penetration wet
areas from dry area~ with as hi~h a degree o~ accuracy a~
is de~irable~
~o~ s
SUMMARY OF ~'~IE INVENTION
I have invented a new method for detecting the pres-
ence of moisture in a multiple layer built up roof, and for deter-
mining the location and the relative concen-tration of this mois-t-
ure in the roof. My method is capable of distinguishing the areas
of first stage, second stage, and third stage moisture penetration
into the roofs, thus acilitating the most efficient and economic-
al repair of the roof. The method does not require the use of
complicated equipment, or highly skilled operators, yet the re-
lative concentration of moisture in the roof is measured to ahigh degree of accuracy.
In applying my method, a .number of spaced points are
first established on the area of the roof to be tested. These
points may be located by laying out a number of spaced lateral and
longitudinal intersecting grid lines. The intersection points of
: the grid lines form positions in a matrix, each intersection point
on the roof being locatable as a position in the matrix. For con-
venience the matrix may be reproduced in scaled down form on a
sheet of paper prepared by the operator. I have found that the
moisture concentration in built-up roofs is proportional to the
dielectric constant of the roofing material. Thus, by measuring
the dielectric constant at each of the grid line intersection
points r it is possible to determine the relative concentra-tion of
moisture in the roof at the various points spaced over the area of
1 the roof~ ~t is not necessarv to measure the absolute dielectric
- constant of the roof, since it is the relative concentration of
moister in the
- ~ ,
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,
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wet portions of the roof as compared to the dry portions of the
roof that is most significantO I have found that the relative
dielectric constant of the roof may be effectively
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.
.
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.
: . . . . ~ .
m~asured u~ing a spray field ~apacitance meter, in which the
plates of th~ cap~citor are eo-planar. Tha me~urements 50
obtain~d are recorded and are asslocia~ed with the polnt ln
the matrix from which they were taken.
In applying my m~thod~ th~ measurement~ obtalned from
portlon~ of the roof having fir~t ~age pen~tr~-ion may be
quita close in magnitude to the normal ~catter of readlngs
~rom the dry roof portlons. The measurements indlcating
first stage penetration may be separated ~rom the dry portion
mea~urements by several technique~, including taklng core
sample~ ~t a number oE the spaced point3 on the roof and
examining the core ~amples to determlne i they ara from
wet or dry portion~ of khe xoof. The smalle~t reading
obtained at ~oints having wet cora samples can be used to
des~gnate all points havlng mea6urements hlgher than such
~malle~t measurement a~ wet poi~ts. Similarly, all point~
having measurement~ smaller than the large~t measurement at
a dry poin~ can ba designated as dry poin~s on the roof~
Since mo~t multipl~ layer roofs are similar in construction,
and ~hu~ have ~imilar dielec~ric constant properties~ it ls
also possible to use the measurements obtained ~rom ~ ~econd
roof which is known to have wet portiona and dry portion39
as a ~tandard again~t which the mea~urement~ obt~in~d ~rom
the xoof being tested can be compared.
~urther ob-~ect~, feature~ and ad~antage~ of my lnvention
will be appa~ent from the following data~led description
tak~n in con~unctlon with the accompanying drawings ~howing a
pr~ferred methocl for detecting molsture in multiple layer
roof~ exempli~yi~g the prinoiple~ of my inventlon.
~s~
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a perspecl:ive view of a building having a
multiple layer roof with a roof capacitance measuring unit in
position to take measurements thereon.
FIGURE 2 is a scaled drawing of the roof of th~ build-
ing shown in FIGURE 1.
FIGURE 3 is a cross sectional view of the roof of the
bullding in ~IGURE 1, with a capacitance meter in position there-
on to take relative capacitance--relative dielectric constant
measurements.
FIGURE 4 is a graph showing the frequency of the re-
lative capacitance readings taken on the roof of the building
shown in FIGURE 1.
DESCRIPTION OF A PREFERRE:D EMBODIMENT
Referring now more particularly to the drawings, where-
in like numerals refer to like parts throughout the several views,
a roof capacitance measuring unit used in my method for detect-
ing moisture in multiple layer built-up roofs is shown generally
at 10 in FIGURE 1. The capacitance measuring unit 10 is shown
in FIGURE 1 on the substantially flat roof 11 of a building 12
such as an industrial plant or commercial facility. The capa-
citance measuring unit consists of a capacitance meter 13 elect-
rically connected by a cord 13a to an ammeter reading dial unit
14, both o~ which are mounted for convenience on a wheeled cart
15.
.',
; ~ - 7 -
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.s
Referrinq to FIGURE 3 the capacitance meter 13 is
shown in operating position on a cross section of the roof 11.
For
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.. . .. . .
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illustrative purposes, the roof ]1 i5 shown as having a smooth
surface. However, the presence of gravel, pebble~, etc., on the
surface o~ the roof does not affect the application of my method.
The roof 11 is composed of multiple layers of felt or tar pap~r
lla which are bonded together by an adhesive layer llb, this
adhesive usually being bitumen or tar. The layers of felt lla
and tar llb are layed over an insulation layer 11¢ which is on
the structural roof lld of the building 12. The structural roof
lld is shown for illustrative purposes in FIGURE 3 as composed
of concrete, but my capacitance method for detecting moisture
can also be used with roofs having any other structural material
such as steel or aluminum. Although a conductive structural
decking may increase meter readings, the increase in readings is
subs~antially uniform in the range of readings near the dry roof
portion rPadings, so that wet areas will still yield relatively
higher readings as compared to the dry areas. As described above,
the layers of felt lla are capable of absorbing water where
cracks or separations have occurred in the felt and tar layers.
Water may also accumulate between layers, causing further layer
separation because of freezing and evaporation forces. As a xe-
sult, water will seep through the protective layers of felt and
tar to the insulation layér llc and will probably leak through
the structural roof lld, which usually is not constructed so as
to be water-tight. In addition, there are various openings in
~ - 8 -
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the structural roof such as the skyliyht 16, roof access open-
ing 17, plumbin~ vents 18, and drains 19 as shown on the roof 11
in FIGURE 1. The presence and relative concentration of moistuxe
absorbed by the felt or insulation,
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or between tl~ layer6 i9 detecte~ by the capacitance meter
1~ a~ explalned belo~. It i8 important that the xelative
conc~ntra~on of moi~ture ln the layers b~ de~ermlned b¢cau~
those areas o ~he roof which have first or econd ~tage
moisture penetration may need to be replaced or repaired
even though not presently vi~ibly cracked.
. The capaci.tance meter 13 prefe~ably utili2e~ two co-
planer ~lectrode~ 20a and 20b which ar~ moun~ed ~t the
bottom of the meter 13. The~e electrode3 are ele~trically
~olated from one anothar, and are connected to a source o~
constant fr~quency alterna~ing current ~AC~ powerO A ~pray
or fringe electro-~tatlc fleld will exl3t between the two
electrode~ and ~hey will in effect form two plates of a
capacitor, with a small amount of electric current flowing
through the capacitor~ Thi3 cuxrent will be measured by the
ammeter xeading dlal unit 14. Th~ amount of current flowing
in the circuit will b~ proportional to the capacltance of the
- ~ two electrode~ 20a and 20b, which i8 in turn proportional to
the dielectric constant of whatever materlal i8 in the spray
; 20 field between these two el~ctrodes. By de~in$tion, a
' dlélectric mat~rial b~tween the plates of a capac~tor
increases the capacitance thereof over the ~n-~acuum
capacitance by multiplying the in-vacuum capacitance by the
dielec~r~c cons~ant of the material. I have found that the
~ 25 diel~ctr$c constant of falt and tar layera in a buil~-up
; roo in~raa~e~ wlth an increa~e ln the conc~nt~atlon of
~ ; : moisture .in the layer~ and between the layers. Thus, it ~s
: po~sible to obtain a read~ng o the relativ~ diel~ctric
con~tant and hence the relatl~e concentration o moisture in
the roo. layers by placing the electrode~ 20~ and 20b of
~ ~S~6~5
the capacitance me~r 13 ac3ain~t ~hs roof ~urface or agalnst
the gravel or pebhle~ on the ~u:rface, and xe~ding on the
readlny dlal unit 14 the current th~t 1~ flowing in th~ circuit~
It i6 to be expectad th~t the dlelectric properties
of roofin~ mat~rial will vary ~:rom buildi.ng ~o building,
and from polnt to point on the xoof of a sinqle building~
These vari~tlon~ can occur becau~e of diEf~rence~ in the
material itself, in the number o~ layer~ of materi~l that
have been placed cn the buildin~, in the thlckn~8~ o~ the
: 10 roof layer~ and the gra~el layer, s~paration of the layer~,
varla~ion~ ln surface temperaturo, and 60 forth, including as
. one variable the relative concentration of moisture contained
within the layers of roofing material. In addltion, where water
i~ trapped or absorbed only in a single layer or in very deep
lS layers of roo~ing materlal~ the readings tha~ are obtalned
. . may not vary greatly from the normal ~cattQr of readlngs
o~ained from dry roof portions. However, the presence of
moisture in a ~ingle layer or in deep layexs must be ascertalnsd
~ince the roofing wlll eventually begin to deter~orat~ in
.the~e ar~a~. It is po~sible, by us~ng my method, to.
- dlscrlm~n~te b~tween th~ dry portion~ o~ the roof and the
wet portions wh~ra first or se~ond stage moi~ture penetration
has occurred, as will be developed below.
:: In ~pplylng my method o de~ecting moisture i~ roofs t
the roof 11 ~o be measured ls fir~t marked o into a grid or
matrlx whi~h~E)referably ha~ uniformly s~a~ed 1nter~cction polnt~
- at the corner~ of grid ~quare~ that are approx~mat~ly 5 feet
. on a side. The later~l and lon~tudinal intersecting grid lines
.
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21, as shown in l'IGURE 1, may be phy91c~11y marked on the
roof 11 of the building hy ch~lking the l.ines on the
bulldlng or ~y mar~lng them off with tape, or by ~erely
marking the intersection point~ 21a of the gridlines.
A scaled drawi~ of th~ roof 11 with the ~rlcllines 21,
as shown ln FIGUR~ 2, 19 then made. Each gridline
intersection point is located b~ assigning the point a
lateral and longitudinal poqition in the matrlx, a9
(0,0~, ~A,l), ~B,5), etc. in the manner ~hown in F~GU~E 2.
The measuring unit 10 i~ then placed on the roof 11
and the capacitance meter 13 is call~rated ~o a
conV~nien~ value with the capacitance meter in the
air away from any di~lectr~c mat~rialO Thus the measuring
unit will not measure the absolute dielectric constant o~ the
roof coverin~, but will read the dielectric constant relative
to air, or for that ma~ter, relative to any other oonvenient
mater1al~ Alr provides the most convenient reference material,
BinCe its dielectric properties are airly constant and close
to that of a vacuum, and it is availahle anywhere. The operator
then pu.~h~s ~he measuring unit 10 along the gridlines 21 and
8tops to take relative capacitance or rslative dielectric
constant mea~Uremen~s with the meter 13 at each gridline
intersection point 21a. Ile marks down the value of the
relati~e d~lectric constant re~d~ng as shown on the
25 reading dial Uilit 14 on the scaled drawing shown in FXGURE 2,
; wi~h each rè~ding b~ing marked next to the intersection
poin~ 21a which is associ~t~d with that reading.
Alternatlvely, the drawing of the roof 11 with the
gridlines 21, as shown in FIGURE 2, may be prepared initially
. . .
.. . .. . .
. . .
~ror~ measurement~ taken o~ the cllm~nslon6 o~ the roof. The
actual rea(lin~s with tl-~ measuring unit 10 may then b~ matle
by pushing ~he unit alon~r a st:raisht llne and taking
measuremenk~3 every S feet, and continulng ~chls pattern back
and forth across the roof until readings hEIve been taken
over ~che entire roof. 1~ flve-fvot square gr~.d has been
chosen front experience as yielding the maximum probabili~y
of detecting wet areaR o~ the roof, while mlnimi~ing the
amount of work required to t~ke measurements over the Qntire
roof area.
'rhe readings which, for illustrative purposes, have
he~n marked ~o the upper right of the inter~ectlon points
of the gridlines on the drawing in FIGURE 2, were obtain~d
with a par~icular model of capac~tance meter 13 calibratecl
to a p2rticular reading ln air, wherein a capacitance raadln~
in air would he approximately " O " . The relativ2 capacitan~e
< readings markea on th~ drawlng in FIGUl?E 2 ar~ collected and
shown in graphlc form in the bargraph of FIGU~E 4. A~ shown
- in FIGURE 4, the relative capacltance readlngs t~nd to cluster
~ 20 around a xeading of 5 or 6~ with only scattered r~adings
.. ,~ .. , :. . ....... .... . . . ....... . . .............. . .............. .
occurrlng out at higher valu~s~ It has been found lthat
wi~h the particular mcdel of capacl~cance meter 13 calibrated
in alr as above, that dry roof ~ generally tend to read
between 3 or 4 and 6 or 7. It has al~o been found that the
- 25 standard doviation of rQadings 'caken on dry roofs seldom.
~ . .
exceeds.l ~r ~ where the meter 13 ~ ~1mllarly calibrated
'' in air. ~lëncQ, read$ng~ of more than 12 or 13 are almo~t
~urely not readings taken at dry portion~ of the roof.
The mean and standard deviation o all readings excluding
r. . ~ ... . - -, . ..
: ' ' .
12
.. . .
6~S
those readin~s over 13 can then ~be calculated. The standard
formulas for the sample mean M and sample variance S are, of
course,
M ,- 1 n
- ~ Xi and s2 = 1 (xi-M)2, where x
~ i-l
is the value of readiny "i". The sample mean and sample stand-
ard deviations so calculated are considered the estimators of
the mean and standard deviation of the population samples from
dry portions of the particular roof being measured. There is
thus a high probability that any reading that exceeds the dry
roof mean plus 3 dry roof standard deviations is a reading taken
at a portion of the roof that is wet. Readings beyond 14 or 15
can be conclusively assumed to be at wet portions of the roof.
For the readings shown in FIGURE 2, the dry roof sample mean is
e~ual to 6.75 and the dry roof sample standard deviation is equal
to 2.135, and thus the sum of the sample mean and three times the
sample standard deviation is 13.155, which agrees with the origin-
al estimate.
The concentration of water in the layers can vary great-
ly, and water may be contained in one of the deep layers but not
in the upper layers, or in fact in more than one layer. The
readings taken at wet areas of the roof will thus tend to vary
greatly and will not cluster around a single value, but rather
Il~
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will cluster around several reading ~alues. The reason for such
clustering can be s~en from an examination of Tables l-S below.
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5~96~Si
Table 1. Effect of Layer Position on Meter
Readings for one Wet Felt Layer
and Dry Insl~r~tion
(Meter reading on dry insulation ~ 3.0)
Meter Readings
. ,_ __ _ _ _ _ _
Wet Fe:Lt Wet Felt Wet Felt
- Number of Wet Second Third Fourth
Felt Dry. Bottom From From BottomFrom Bottom
Layers Felt Felt Bottom
1 3.5 7.8
2 4.0 7.8 7.8
3 4.3 7.7 7.6 7.8
4 4~5 7.7 8.0 8.g 8.1
4.6 7.7 8.0 8.0 8.2
i
: Table 2. Effect of Layer Position of Meter Readings
: for two Wet Felt Layers and Dry Insulation
(Meter Reading on dry insulation = 3.0)
. Meter Readings
: __
..
Wet Felt Wet Felt Wet Felt ` A
. Wet Second Third and Fourth
: Number of Bottom and Third Fourth and Fifth
: Felt Dry Two from from from
Layers Felt Felts .Bottom Bottom Bottom
:' . __ _
1 3.5
2 4.011.0
~:~ 3 4.310.5 11.0
. - 4.510.~ 10.~ 11.4
5 . 4.610.0 10.3 10.9 10.5
` .
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Table 3. Effect of Layer Positlon on Meter Readings
for three Wet Felt Layers and Dry Insulation
(Meter Reading on Dry Insulation ~ 3.0)
Meter Readings
,., , ~ ~
Number of Dry Wet Bott:om Wet Felts Second
Felt Layers Felt Three Felts Third and Fourth
from Bottom
.
1 3.5
2 4.0
3 ~.3 13.0
4.5 1~.1 11.4
4~6 11.6 10.5
Table 4. Effect of the Number of Wet Felt Layers
on Dry Insulation
(Meter reading on dry insulation =3.0)
Number of Wet
Felt LayersMeter Readinq
1 8.0
2 11.0
3 12.5
4 13.0
13.1
- ~ ~ 15. _
.
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Table 5. Effect of the Number o~ Wet Felt Layers
on Wet Insulatlon
(Meter read.Lng on dry insulation = 3.0)
Number of Wet
Felt La~ers Meter Reading
0 16.0
1 18.0
2 19.5
3 20.1
4 . 21.0
21.5
'''' ' - 1 ~ -
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Where one felt only is wet, the meter readings (relative
dielect~ic constant measurements) tend to cluster around 8.0, as
shown in Table 1. Where two or more felts are wet, the meter read-
ings tend to remain within the ranye from 10.0 to 13.0, as shown
in Tables 2-4. Large meter readings of 16.0 or more are obtained
when the insulation layer is wet, as demonstrated in Table 5. It
is thus seen that for the type of roof being tested here, readings
clustered around 8.0 indicate stage one penetration~ readings with
in the range of 10~0 to 13.0 indicate stage two penetration, and
readings over 16.0 indicate stage three penetration
Thus it would be difficult, if not impossible, to esti~
mate the sample mean and sample standard deviation of only the
population of readings from wet portions of the roof that are close
to the dry roof portion readinys. Certainly the parent popula-
tion of all wet roof portions would not be normal, since the pop-
ulation obviously has several modes. It is, of course, desirable
to ascertain as many of the wet roof points as possible without
including therein, as error, an unreasonable number of dry roofing
points. It may be assumed here that the dry roof readings are
taken from a population that is approximately normal, which cor-
responds to previous experience. The mean of this population can
be estimated by the sample mean M of 6.75, and the population
standard deviation can be estimated by the sample standard devia-
tion S of 2.135. If such a normal population is assumed, only 10%
of the dry roof readings should be greater than 9.5 and only 5% of
the dry roof readings should be greater than 10.27. Thus, if all
~ - 16 -
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of the xeadings which are greater than 9 are con~idered to be wet
readings, at a maximum it is to be expected that only
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.
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10~ o~ the actual dry roof readings would be mistaken for wet
roof readings~
This error estimation m~thod can be generalized to re-
duce the maximum probability of e.rror (i.e~ designating a dry point
on the roof as a wet point) below any desired probability P~ The
relative dielectric constant measurements taken at several points
which are at wet portions of the roof are first determined. This
may be done in several ways, including taking core samples at
several of the spaced grid intersection points and determining from
inspection of the core samples whether they are from wet portions
of the roof. Where a wet core is found, that grid intersection
point from which the core is taken is designated as a point at a
wet portion of the roof. A second method of determining several
wet points utilizes the experience gained from measurements on
other roofs having similar construction to the roof being tested.
Construction of two roofs is similar if both roofs have coverings
having substantially the same number of layers of the same or
similar material. Most multiple layer roofs have at least four
layers of felt and bitumen over insulation and are thus substan-
tially similar, although the construction of the underlying struc-
tural roof may vary. However, as indicated previously, a conduc-
tive structural roof may add a substantially uniform increase in
readings to both dry and wet roof readings in the range near the
dry roof readings, and thus the effect of the structural roof can
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be compensated for. ~s demonstrated in Tables l-S fox the par-
ticular .roof being tested there~ with the dry roof readings tend-
ing to cluster around a reading of 5.0, wet roof readings should
begin at a reading of 8.0, and the roof is almost certainly
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wet where readings of 10.0 and above are obtained. Thus~ on a
roof similar to that for which the test results are shown in
Tables 1~5, relative dielectric constant measurements greater than
: 10.0 are almost certainly taken from wet portions of the roof and
may be designated as such.
T~e sample mean M and sample standard deviation S are
then calculated for all relative dielectric constant measurements
which are less than any measurement obtained from the points that
have previously been determined to be at wet portions of the roof.
The parent population of dry portion measurements is estimated as
having a normal probability distribution with estimated mean M
and estimated variance S2. The actual mean and variance of the
parent population will very likely be less than M and S respec-
tively, since only measurements which were almost certainly taken
from wet portions of the roof were excluded from the calculation
of M and S2, whereas it is quite likely that some high measure-
ments taken from wet portions of the roof were included in the
calculations.
For a random variable having a normal probability dis-
tribution of mean M and variance S , there is a number K such thatthere is less than a probability P that the random variable is
greater than or equal to K. Given M, S2, and P, the number K
can be calculated from tables of the standard normal distribution,
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using conventional techniques. For the numerical example given
above, where P 3 0.1~ M -6.75~ and Sa2.135, K is found to be 9.5.
Thus, any points on the roof having relative dielectric constant
measurements whlch are greatex than K can be designated as points
- from wet portions of the roof with less than a probability P of
designating a dry point as a wet point.
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In order to obtain readings that are even more accurate,
it may be desirable to take readings on a day that has followed
several days of dry weather to establish a good baseline for dry
roof readings. It would then be desirable to take readings again
after a heavy rain, or, if this is inconvenient, to irrigate the
roof over a period of a da~ or so to insure that a~y wet areas, or
areas that are susceptible to wetness, will have become thoroughly
wetted.
After measurements have been taken of the relative di-
electric constant at each of the spaced grid intersection points,the general areas of the roof where there is moisture in the roof
covering are known. However, it is often desirable to obtain a
more precise outline of the wet areas than that provided by the
grid intersection points. Thus, a relative dielectric constant
measurement can be taken at least one point which is between a wet
point and an adjoining dry point. If the measurement shows that
the point is dry, that point can be designated as an outer limit
of the wet area. If the point is wet, another measurement may
be made between that point and an adjoining dry point to better
define the limits of the wet areas.
It is also desirable to obtain a more precise moisture
profile of the wet areas between the points designated as at wet
portions of the roof. Relative dielectric constant measurements
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may thus be taken at at least one point between adjoining we-t
points. Several of these measurements between wet points are
shown for illustrative purposes in E'IGURE 2. This expanded
moisture profile allows a determination to be made of those wet
portions of the roof having first stage penetration, second
stage penetration, and third stage
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penetration, in accordance with the magnitudes of the relative
dielectric constant measurements. The nature of the repairs re-
quired on the roof in the wet portions can thus be determined.
It is understood that my invention is not confined to
the particular methods herein illustrated and descrlbed, but
embraces all such modified forms thereof as may come wi.thin the
scope of the following claims.
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