How important is elongation where flashing is concerned?
I have a flashing that has 200% elongation and substitution request for a product that has 16% elongation. Is the 200% elongation equivalent to a tricycle that can go 300 miles per hour?
Flashing for what? Roof? through-wall?
What was the “base” flashing: copper, stainless steel, rubberized asphal?
Some context would be useful.
The flashing is through-wall, 40 mil self-adhering thru-wall flashing composed of a 32 mil self-adhering rubberized asphalt membrane laminated to a 8 mil high densitypolyethylene film.
elongation for what you are using the product for is unnecessary. yes, it is one of the product attributes, but so what? you don’t need it. We routinely specify stainless steel for thru-wall conditions - stainless steel has zero elongation. It doesn’t need to move.
“stainless steel has zero elongation. It doesn’t need to move.”
But it moves, indeed. It’s something to do with changes in temperature, not only in a day but over the seasons of a year. And when the sun shines on the flashing (or the finish covering it), the flashing gets much warmer than the ambient air. Also, the flashing moves when the wind blows on the building or the ground shakes.
Being algebraically-challenged, I resist calculating something like thermal movement but I’m sure there’s someone smart who can find the formula and plug the numbers into an equation that gives an answer to the original question. Maybe a product rep (not merely a peddlar).
Otherwise, follow the maxim, “some is good, more is better and too much is probably just right.”
BTW, not all self-adhering rubberized asphalt sheet membrane is equal. The kind that’s for below-grade waterproofing is not suitable for the high-temperature environment of a roof. Consult with a manufacturer’s rep to ensure proper selection … or at least read the manufacturer’s selection guide.
According to the SMACNA Manual [Appendix A-1], the Coefficient of Thermal Expansion of stainless steel is 0.0000096 which is almost identical to copper [0.0000094]. Stainless and copper are about mid-way between lead [highest] and steel [lowest].
IMHO, malleability is a more important characteristic for a concealed flashing. Cavities are more highly engineered today than 20 years ago as are the components that are in them. A properly construction cavity requires well constructed end dams and other flashings which need to be properly formed to fit the field conditions. If a concealed flashing is to contain metal, it would best used with a self-sticking modified asphaltic membrane.
Yolanda,
I like the tricycle analogy! We have several equal products for rubberized asphalt wall flashing in our master. 200% elongation appears to be standard.
or what I typically call for in a cavity flashing: dead soft temper for stainless steel, and it not be “work tempered” before installing. You want that stuff to just lay in there as if you sprayed it onto the surface. (lead sheet used to be really good for this). I agree that elongation may not be that necessary a criteria. You want something that will not keep trying to go back to its original shape, but rather conform exactly to the location.
There are a good number of similar self-adhering bituminous/plastic sheet flashing products around with similar performance characteristics. Why is the proposed substitute product different? Different materials? (Doesn’t sound like it.) Clerical error somewhere between test report and data sheet? Very common.
But you are asking the right question, even though most of the comments above don’t address your question. Because there is modest thermal movement and (hopefully) very little structural movement within a wall assembly, ultimate elongation may not be a critical characteristic, though you don’t want flexible flashing to sag. Although harder to find in test data, puncture resistance is probably more important.