Publisher’s version / Version de l'éditeur:
Construction Technology Update, 1997-05-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
NRC Publications Archive
Archives des publications du CNRC
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Ways to reduce blistering in built-up roofs
Paroli, R. M.; Booth, R. J.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
NRC Publications Record / Notice d'Archives des publications de CNRC:
https://nrc-publications.canada.ca/eng/view/object/?id=683f5276-f9ef-4db9-9211-c142cfc3c9d6 https://publications-cnrc.canada.ca/fra/voir/objet/?id=683f5276-f9ef-4db9-9211-c142cfc3c9d6b y R.M. Pa roli a nd R.J. Booth
Blistering is a common problem w ith field-applied built-up roofing systems.
This Update explains w hat blistering is, describes the causes and recommends
w ays of dealing w ith it.
C o n s t r u c t i o n T e c h n o l o g y U p d a t e N o . 4
Statistical d ata from th e Un ited States sh ow th at blisterin g w as th e m ost com m on p roblem w ith bu ilt-u p roofs from 1983 to 1992 (1). Blisters are raised su rface areas over void s in a roof system th at con tain en trap p ed air an d / or m oistu re. As th e roof su rface tem p eratu re rises, th e p ressu re in sid e th e blisters rises an d th e ad h esion of th e asp h alt w eaken s. Usu ally n oticeable on h ot an d su n n y d ays, blisters ran ge from sm all sp on gy sp ots to large p ron ou n ced areas.
It h as been d em on strated th at a p erfect void -free roof can n ot be laid even u n d er id eal con d ition s (2). Void s can resu lt from skip s in bitu m en m op p in g, en trap p ed d ebris, cu rled felts, u n even su bstrates, an d from m oistu re on th e roof or in th e m aterials lead in g to en trap p ed gases an d bitu m en bu bblin g. Blisters d evelop from void s bu ilt in to th e roof.
On e m iscon cep tion abou t blisters is th at th ey are p erfectly sealed . Th is is n ot th e case, for if it w ere, th e blisters w ou ld n ever grow beyon d th e lim its d ictated by th eir gas con ten t an d th e m axim u m tem p eratu re of th e roof su rface.
Type s of Blist e ring
Blisterin g of bu ilt-u p roofin g takes tw o m ain form s: blisters betw een th e roof m em bran e an d th e su bstrate, an d blisters betw een th e m em bran e p lies.
Blisters Betw een the Roof Membrane and the Substrate
Pockets of air an d m oistu re, trap p ed betw een th e m em bran e an d su bstrate, exp an d in th e su d d en h eat of th e su n an d d isp lace th e m em bran e to form a sm all blister. Th e blister w ill on ly be form ed w h en th e su bstrate m aterial h as low p erm eability; u n d er th ese circu m stan ces, tem p eratu res from th e h eat of th e su n can rise too qu ickly for trap p ed air an d w ater vap ou r to escap e th rou gh th e su bstrate. Pressu res w ill th en d evelop in th e air
Ways to Reduce Blistering
in Built-Up Roofs
Figure 1. Blistering occurs when pockets of air and moisture trapped between the plies of the membrane, or between the membrane and substrate, expand and displace the membrane.
2
p ockets, w h ich can cau se d isp lacem en t an d stretch in g of th e roof m em bran e, in creasin g th e size of th e air p ocket/ blister.
If th e roof m em bran e su ffers an irre-versible stretch , su bsequ en t coolin g w ill n ot cau se th e air p ocket to retu rn to its origin al size an d a p artial vacu u m w ill be created in th e n ow p artly d evelop ed blister. Air an d w ater vap ou r m ay be d raw n slow ly th rou gh th e su bstrate to refill th e origin al p ocket of air, n ow sligh tly in creased in size an d read y to start an oth er cycle of d evelop m en t of th e blister w h en th e su n ap p ears again .
An in creased occu rren ce of blisterin g betw een cellu lar foam in su lation s an d roof m em bran es w as in itially th ou gh t to be d u e to gases escap in g from th e cells of th ese in su lation s. Alth ou gh th ere is little evid en ce for th is, it is clear th at th e im p erm eable n atu re of cellu lar foam in su lation s can in crease th e in cid en ce of blisterin g becau se of th eir in ability to ven t gases both d u rin g an d after th e in stallation of th e roof. Th e u se of a fibrou s coverboard (w ood fibre, glass fibre, or p erlite board ) over cellu lar in su lation s is recom m en d ed to red u ce th e occu rren ce of blisters in bu ilt-u p roofs (3,4).
Blisters Betw een Membrane Plies
Th e trad ition al con cep t of in ter-layer blisterin g assu m es th at blisters are cau sed by air an d m oistu re trap p ed betw een th e layers d u rin g ap p lication . In reality, su bstan tial blisters can n ot form w ith ou t a top p in g-u p p rocess to ad d m ore air an d m oistu re betw een th e layers. Pressu re levels m easu red in sid e blisters are low er th an th ose exp ected from sealed system s, an d it is clear th at blisters in h ale an d exh ale d aily (5).
In a p erfectly closed an d elastic system , a void w ith in a roof w ill grow becau se of th e exp an sion of air an d / or w ater vap ou r by d ay, bu t it w ill revert to its origin al d im en sion s at n igh t. Bu ilt-u p roofin g m em bran es are n ot p erfectly elastic; th ey exp an d easily w h en th ey are w arm bu t becom e stiff an d resist retu rn in g to th eir origin al sh ap e w h en th ey are cooled . An y p erm an en t d eform ation of th e m em bran e w ill create a vacu u m w ith in th e blister an d
air w ill fin d its w ay in to th e en closed sp ace th rou gh m icroscop ic cracks in bitu m en m op p in gs or alon g th e felts th em selves.
Blisters grow w h en :
• th e volu m e of air su cked in to th em at n igh t exceed s th e volu m e of air forced ou t of th em by d ay, an d
• th is in creased p ressu re overcom es th e p erip h eral bon d of th e blister.
Wa ys t o Re duc e Blist e ring
A few sm all void s d o n ot p ose a p roblem w ith resp ect to m em bran e blisterin g as m em bran e elasticity an d ad h esion can accom m od ate sm all m ovem en ts. Nu m erou s large voids, however, will result in membrane blisterin g p roblem s.
Th e follow in g action s w ill lim it th e n u m ber an d size of void s bu ilt in to roofs: • Use d ry m aterials in d ry con d ition s.
- Seal con crete d ecks w ith good qu ality air/ vap ou r retard ers.
- Store m aterials u n d er tarp s an d off th e roof su rface.
- Avoid an y w ettin g of m aterials, as th is brin gs m oistu re to th e roof.
- Make su re su bstrates are d ry before com m en cin g w ork.
• Use sp ecial p recau tion s in w in ter. Days are sh orter, th e air is cooler an d asp h alt stays h ot for a sh orter p eriod . Th is m ean s th ere is m ore ch an ce of h avin g m oistu re p resen t an d less ch an ce of it d ryin g. Wh at is m ore, “h olid ays” (void s in asp h alt) are m ore likely to occu r w h en asp h alt cools qu ickly.
- Start th e w orkd ay later an d fin ish earlier.
- Use h igh er asp h alt tem p eratu res an d in su lated equ ip m en t.
• En su re in tim ate con tact betw een m aterials. - Fit in su lation board s sn u gly an d m ake
su re th ey ad h ere w ell to th e su bstrate. - Use fibrou s coverboard s over all
cellu lar p lastic in su lation s to allow ven tin g of gases d u rin g ap p lication . - Broom -in organ ic felts to force an y
gases ou t of th e roof before th ey becom e trap p ed as th e asp h alt sets.
bou n d ary be m arked so th at it can be ch ecked p eriod ically to d eterm in e if it is gettin g larger.
Wh en th ey grow to abou t 1.5 m (5 ft), or if th ey occu r in a h igh -traffic area, blisters sh ou ld p robably be rep aired . To p reven t th e breakin g of blisters, avoid step p in g on th em , if at all p ossible, p articu larly w h en th e m em bran e is cold .
If a blister is broken or oth erw ise cap able of takin g in w ater, th e u su al rep air p rocess is to rem ove th e en tire raised p ortion an d p atch th e rem ain in g void w ith altern ate
Tre a t m e nt of Blist e rs
Sm all blisters, less th an .6 m (2 ft) lon g, are better left u n atten d ed . Th ey u su ally d o n ot cau se p roblem s, p rovid ed th ey rem ain w atertigh t an d d o n ot becom e too large. Larger blisters sh ou ld be rep aired p rom p tly, esp ecially if th e p rotective coatin g or gravel su rfacin g h as slu m p ed off.
Erod ed bare sp ots on sm all blisters can be treated w ith a cold -p rocess asp h alt coatin g an d sp rin kled w ith gravel. If a blister ap p roach es a d iam eter of abou t .6 m (2 ft), it is recom m en d ed th at its ou ter
3
Figure 2.Formation of roof membrane blisters Pocket of trapped air and moisture
Vapour pressure rises too quickly to escape through dense deck
Rapid expansion of trapped air and moisture forms small blister
Partial vacuum at night draws additional air and water vapour into blister from deck
Partial vacuum at night draws additional air and water vapour through micro cracks
DAY
NIGHT
Pocket of trapped air and moisture
layers of asp h alt an d su ccessively larger p ieces of felt. Altern atively, an “X” can be cu t in to th e blister, th e corn ers of th e cu t p eeled back, th e resu ltin g cavity filled w ith asp h alt, an d th e corn ers p ressed back in to th e bitu m en . Th e blister is th en p atch ed as above.
As an altern ative to trad ition al rep air m eth od s, con tractors sh ou ld con sid er u sin g sp ecially d esign ed ven ts d evelop ed by th e Cold Region s Research an d En gin eerin g Laboratory of th e U.S. Arm y Corp s of En gin eers. Th ese ven ts, w h ich allow for qu ick rep air, are available com m ercially (5).
Re fe re nc e s
1. Cu llen , W.C. Project Pin p oin t An alysis: Ten Year Perform an ce Exp erien ce of Com m ercial Roofin g 1983-1992. Nation al Roofin g Con tractors Association , Project Pin p oin t, 1992. 2. Cu llen , W.C. Th e Perfect Roof: Can It
Be Bu ilt? Roofin g ’87. Nation al Roofin g Con tractors Association , 1987.
3. NRCA Statem en t on Polyisocyan u rate, Polyu reth an e an d Ph en olic Foam Roof In su lation s. Nation al Roofin g
Con tractors Association bu lletin , Sep tem ber, 1988.
4. Fibreboard Overlays. Can ad ian Roofin g Con tractors Association bu lletin , March , 1993.
5. Korh on en , C. an d B. Ch arest. Roof Blisters, Cau se an d Cu re. US Arm y Corp s of En gin eers, Cold Region s Research an d En gin eerin g Laboratory, Rep ort 95-19, Ju ly 1995.
Dr. R.M. Pa roliis a research officer in th e Bu ild in g En velop e Program of th e N ation al Research Cou n cil’s In stitu te for Research in Con stru ction .
R.J. Boothis a p rin cip al scien tist at Han sed -Booth . He was a sp eak er at IRC’s 1996-97 roofin g sem in ar h eld in variou s cities across th e cou n try.
“Construction Te chnology Up d a te s” is a se rie s of te chnica l a rticle s conta ining p ra ctica l inform a tion d istille d from re ce nt construction re se a rch.
For more information, contact Institute for Research in Construction, National Research Council of Canada, Ottaw a K1A 0R6
Telephone: (613) 993-2607; Facsimile: (613) 952-7673; Internet: http://irc.nrc-cnrc.gc.ca
© 1997
Nation al Research Cou n cil of Can ad a May 1997
