b'absolute flatness more than 1/8 in/ft.lineal foot). its production.(10 mm/m) of length or width whenOnce again the roofing contractor isTo summarize, the knit line issue is examined in accordance with Practicefaced with a tough decision.Do youreported to be prevalent in all parts C550. proceed to install these boards or doof the country and coming from all Knit Lines you reject them?A professional designmanufacturers of polyisocyanurate Knit lines occur in theconsultant may reject them; one neverboards. The same national level impact manufacturing process wherein theknows what a future owner may thinkis true with the thickness variations individual lines of foam are laid downof a fully adhered roof filled with knitbeing seen in polyisocyanurate. Some on the bottom facer, which is movinglines. manufactures appear to be doing a into the laminator.As the foamBoard Thickness better job than others at this issue. It chemically reacts, it rises and will grow.The dimensional tolerances citedis recommended that you and your An upper steel platen stops the verticalabove in Section 8.1 of ASTM C 1289employees create a routine to inspect free rise, of the foam, at the specifiedstate that the thickness tolerance shallshipments of polyisocyanurate as it board thickness.However, the foam isis delivered for trueness (knit line also expanding horizontally.Where the two horizontally expanding continuousFigure 3. A close-up view of the board in Figure 2. buns of foam come together, it createsThe facer has delaminated from the foam core at a knit line.Here the foam cells are notthe knit line, and is now bridging at the knit line.in an ordered array because the bun growth is stopped by its neighboring bun, which is also expanding.Figure 1 shows knit lines on polyisocyanurate board being installed on a roof.Currently some manufacturers argue that this is only an aesthetic issue.But this is certainly not true in all cases.Figure 2 shows knit lines as apparent cavitation in the foam board.Figure 3 is a close-up of the board in Figure 2. In Figure 3 the facer has delaminated and is bridging between the foam buns, at the knit line.If you are installing anot exceed 1/8 inch and the thicknesscavitation) and thickness dimensions. fully adhered single ply membraneof any two boards shall not differFigure 1. A Fall of 2016 roof over the board shown in Figure 2more than 1/8 inch when measured ininstallation in the Midwest. The knit and 3, where would the uplift failureaccordance with Test Method ASTMlines on these polyisocyanurate boards occur?Right along the fractured knitC303.In Section 9.3 of ASTM C303are clearly visible at installation. The line; a progressive peel failure mayit states that the thickness on anyadhered membrane, to be installed, will then form going from side to side ifgiven board should be made at facermost likely telegraphs these through. gusting high winds are present.It islocations, generally near the facerKnit lines will then be visible on the suggested by some to use a cover board;corners, approximately 1-3 inches ininstalled roof. others would tell you to flip the boardsfrom the corner.This may be difficult upside down.As roofing contractors,to do in the field (measure 1-3 inchesFigure 2. A view of a you should not have to adapt yourin from the board edge).Take edgepolyisocyanurate board sample installation to manufacturingmeasurements along the perimeter andthat has undergone ASTM C1289 shortcomings of any roofing product. corner; if they are outside the 1/8- conditioning. Note the dimensional ASTM C 1289 only has one sectioninch tolerance have the manufacturerstability and amplification of the knit that may apply and that is Section 8.5perform the ASTM C303lines.Face Trueness which was cited above. measurement or have an independentFigure 3. A close-up view of the The ASTM C550 practice is usedlab run the test. board in Figure 2. The facer has to measure face trueness of 1/8 perRemember the owner is buying adelaminated from the foam core at the foot.This assumes the board is warpedspecified thickness (or R value) fromknit line, and is now bridging at the upwards at the center and not lyingyou.If the boards are of a lessorknit line.flat.Certainly, the board cavitation ofthickness than stipulated, you may the knit lines shown in Figure 1-3 arehave to defend an out of specification more than 1/8 wide (within any givenproduct when you had no control over www.mrca.orgMidwest Roofer 29'