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Sealants, compatibility and joint design · Lesson 1 of 4

Sealants, compatibility and joint design: principles and terminology

What a sealant is for, where it belongs on flashing work, how to read an ASTM C920 label, the parts of a sealant joint, and the safety data sheet rule.

You will be able to: Define the terms on a sealant label and in a joint detail, say where sealant belongs on flashing work and where it does not, and state the chemical-safety rule that applies at the joint.

11 min · 9 slides · 9 questions

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Slide 1 of 9

What a sealant is for

Sealant backs up a correctly lapped and sloped flashing; a detail that only works because of a sealant bead is not a sound detail.

UFGS 07 92 00 says not to install sealants in place of required weatherproofing parts such as flashing, drainage components and joint closure accessories.

Sealant closes a joint between parts. It does not replace a part.

Sealant used as a backup, against sealant used in place of a partTwo pairs. Top: a lapped base flashing and counterflashing with a sealant bead as backup, against a bead alone filling the wall to roof corner. Bottom: butyl tape concealed inside a lap, against butyl smeared across the exposed face.CorrectWrongLaps shed the water.The bead is the backup.A bead alone, no flashing.UFGS says do not.Butyl concealedin the lap.Butyl smeared on the face:sun degrades it.

Two pairs. Top: a lapped base flashing and counterflashing with a sealant bead as backup, against a bead alone filling the wall-to-roof corner. Bottom: butyl tape concealed inside a lap, against butyl smeared across the exposed face.

Sealant backs up a lapped, sloped flashing and never replaces one (AFS note, UFGS 07 92 00). MCA says butyl is concealed, with at most a small exposure at a lap end. Diagrammatic, not to scale.

Sources

Videos for this lesson

What this video shows: A metal panel supplier’s channel. Its description says sealant is a secondary and tertiary safeguard, not primary protection against water, and covers tape and gun sealants and when not to use sealant.

Video: When and When Not to Use Sealant on a Metal Roof Installation by The Metal Roofing Channel, on YouTube. AFS checked its description against this lesson.

What this video shows: A sealant maker’s short video, made for siding. Its description says most failures trace to joint design and that sealant should bond on two sides only, never three. Principle only.

Video: How to Install Sealant Correctly: Avoid the #1 Cause of Failure by Titebond, on YouTube. AFS checked its description against this lesson.

Glossary

Backer rod
A rod packed into the back of a joint so the sealant fills it to the specified depth. The federal sample specification calls it a backstop.
Bond breaker
A tape or coating at the back of a joint that stops the sealant bonding there, so it bonds only to the two opposing faces.
Three-sided adhesion
Sealant stuck to the back of the joint as well as to both sides. The sample VA specification says to prevent it.
Class (movement class)
The number in an ASTM C920 label that states how much increase and decrease of joint width the sealant withstands, as a percentage of joint width. Class 25 means at least 25 percent.
Grade NS and Grade P
Grade NS is nonsag, for vertical joints. Grade P is pourable or self-leveling, for horizontal joints.
Safety data sheet (SDS)
The product safety document OSHA hazard communication requires the employer to keep readily accessible during each work shift for each hazardous chemical.
Tooling
Pressing and smoothing the fresh sealant into the joint after it is applied, to ensure adhesion.