Introduction
Have you ever found yourself at a hardware store searching through a larger variety of caulking sealants than you expected – confused about what you need exactly? In plumbing, there are a lot of sealants and a range of applications, but selecting what you need is simpler than you might think.
To caulk something according to Merriam Webster is “to stop up and make tight against leakage.” In our case, that something is most often the gaps and spaces that are apart of plumbing fixture installations: the gap between a tub spout and the wall of the tub, for example. The word “caulk” has its origins in shipbuilding, first appearing archaically in Thomas Wright’s 1800s print of a 1500s manuscript titled The Chester Plays1:
“And I will goe gaither slyche,
The shippe for to caulke and pyche,
Amounte yt muste be with stiche,
Borde, tree, and pynne.”
Without analyzing each of these obsolete words, a loose, more modern rending of this would read something like:
“I will go gather a sealant
with which to caulk and pitch [resinous, sealing material] the ship
amount must be with [join together]
every part of the ship.”
Here, “to caulk” meant to make the ship watertight. And while the word caulk was originally referenced in the waterproofing of a ship, it would of course find wide usage not only in the construction of a ship but in construction generally.
So, what is caulk?
What
If the act of caulking or to caulk something means to “stop up,” caulk itself is simply the material to do so. But what material is used to caulk? Obviously, there is a history on the subject that is wider than needs be gotten into here. More narrowly, what materials are commonly used for caulking in plumbing?
I’ll keep this list to 7 mostly common sealants and try to avoid getting too technically into the chemistry and manufacturing of these products.
Acrylic latex
If there is poster child for caulk, it would be acrylic latex. This is the common, relatively cheap caulking you find everywhere in your home: along trim, cabinetry, etc. Rivers of this stuff are pressed and squeezed every year in wide application. Acrylic is a synthetic resin and is processed into a kind of plastic. In its usage for caulk, acrylic latex is water-based, fast drying, and paintable. It bonds with porous material such as wood and concrete, as well as non-porous materials such as tile and metal.
Siliconized acrylic latex
A little dab of silicone will do in the acrylic latex. Silicone is a different synthetic creation than acrylic in that it is less of a kind of plastic and more of a kind of rubber. Silicone is more water and heat resistant than acrylic as well as more flexible. Acrylic latex caulk is sometimes modified with silicone to capture a measure of these qualities.
100% silicone
More silicone means more water and heat resistance as well as flexibility. It is harder to clean up than acrylic latex, harder to remove, and cannot be painted over. Silicone only bonds reliably with non-porous materials.
Polyurethane sealant
Polyurethane is an organic polymer that is manufactured in many, many forms. When it is manufactured as a sealant compound, it has flexibility and waterproofing qualities like silicone, though not as good. It is also paintable and cheaper like acrylic latex, and it bonds with both porous and non-porous materials.
Hybrid/MS (modified silicone) polymer sealant
Modified silicone is newer than the previously mentioned sealants (though it has been around since the 1970s), and it is another creation designed to capture some of the strengths of the previous sealants while minimizing some of their weaknesses. Modified silicone bonds to both porous and non-porous materials, often adheres to damp or wet surfaces, is paintable and rubber-like when it cures, and is easier to clean than polyurethane or silicone. It is a more premium product.
Fire caulk/fire stop sealant
Fire stop sealants are really their own category. There are a variety of these sealants (acrylic, silicone, and intumescent sealants that expand under heat) that are created with fire-resistant additives.
Roof sealant
Roof sealants are also their own category. There are a wide range of roof sealants as well as categories and applications. Generally, roof sealants are formulated in their chemistry to withstand a more hostile environment (weather exposure, extreme temperature swings, UV exposure) over many years.
Now that you have a basic overview of what material you might use to caulk your plumbing in with, the only obvious thing may be where the roof sealant is used. So when and where, exactly, would each of these sealants be used?
When and Where
There is overlap on the application of several of these sealants, but there are some general guidelines on usage, and understanding the possibilities of each one will empower your decision-making. One principle that is almost always fool proof is simply reading the manufacturer’s recommendations for the product.
Acrylic latex
This is the stuff that’s all over your home (the trim, the doors, etc.). Mostly, you’re not going to put this stuff too near your plumbing as it’s not as water resistant as siliconized acrylic latex or 100% silicone. What you might use it for is cosmetic applications to fill gaps around penetrations in dry locations, loose escutcheons in dry locations, access panels to drywall, or a gap from a vanity cabinet against drywall. This is a caulk that a painter is using more than a plumber.
Siliconized acrylic latex
Toilets. All day, every day. 100% silicone is messy and difficult to apply cleanly. While it is not as water resistant as 100% silicone, you don’t need to waterproof a toilet as you would a shower, unless there are larger issues going on in your bathroom. It looks good when it cures if done properly, is easier than silicone to remove, and it creates an adequate seal that you can clean around.
There are other plumbing features that are more exposed than a toilet to water but are not necessarily exposed constantly. Drop-in sinks and tub spouts are two good examples. Intuitively, you may think jumping up to 100% silicone is the answer for these examples, but it might not necessarily be, and here’s why:
Sealants are a game of chemistry. So, ultimately reading specific product information is going to be your best bet when deciding what to use.
For example, Hercules® Plumbers Caulk, manufactured by Oatey, is a siliconized acrylic latex sealant that is marketed as “waterproof” and is used by plumbers all over the country in residential and commercial applications. It’s a good, easy-to-apply, one-stop product to use during fixture installations. It is adequate for a wide range of situations without needing to switch back and forth between multiple kinds of sealants.
Finding and understanding the uses of a go-to brand for your plumbing needs like Oatey’s Hercules can help simplify a lot.
100% silicone
Generally, the more exposed to water the area is going to be, 100% silicone is going to be a great choice. And that’s a great way to think about it, just remember it needs a dry environment long enough to cure. It is often used inside showers on joints, corners, frames, etc. Some drains require silicone rather than plumber’s putty.
One more important point of consideration: silicone has lower absorption characteristics than acrylic latex. This is important when thinking about chemical cleaner or urine exposure – silicone is more resistant to color change and degradation.
There are not pin-point regulations on exactly when to use siliconized acrylic latex and when to use 100% silicone. Some of that judgment should be rendered by you in the field or at home when thinking through the application and manufacturer recommendations. The goal is that this gives you proper considerations to make that decision.
Polyurethane sealant
We’re moving away from fixture caulking towards penetration sealing with these next four kinds. Polyurethane sealants work great for exterior penetrations: a pipe sleeve coming through concrete and filling annular spaces, a vent pipe coming through a metal wall, an irregular exterior gap that needs painted, etc.
Hybrid/MS (modified silicone) polymer sealant
You’re probably not stocking a hybrid/ms sealant on your work truck along with a polyurethane sealant as there is a lot of overlap, and the polyurethane sealant is going to be cheaper, but there are certain conditions you may consider using a hybrid/ms sealant over a polyurethane one.
A hybrid sealant is usually going to have better compatibility when bonding if you are applying the sealant in an area where there are different kinds of construction materials meeting together – including damp surfaces. It is generally easier to apply than polyurethane if you need a cleaner look, and it is more tolerable to UV exposure.
In premium locations where the exterior appearance matters, the more premium product might be your best bet.
Fire caulk/fire stop sealant
In many localities, after plumbers, HVAC workers, and electricians have finished roughing in a home and are done making penetrations, locally adopted codes may require fire caulking to be done to prevent fire, smoke, and fumes from traveling along those penetration paths.
Sometimes, an intumescent additive, which causes expansion under heat, is required of the sealant in the event that the pipe, which is now usually ran in PVC (plastic), doesn’t melt and open up a new channel. I have typically seen that the job of fire caulking is contracted out in residential jobs but is often part of the plumber’s scope of work on commercial projects.
It’s easy to treat fire caulking as an afterthought when doing a little home renovation, but it is an important consideration to be aware of.
Roof sealant
I have seen many plumbers skillfully punch a vent through the roof of a residential build cleanly without applying any kind of sealant under the shingles or leaving nails exposed to weather. It’s still not the best method.
Learn to punch the vent out skillfully and cleanly, and then also apply some sealant that won’t stand out against the color of the roof to better secure the shingles and seal the vent.
Guidelines for when and where
Here are some simple ways to think about when and where to apply caulking:
Always reference install and manufacturer recommendations first – from drains to fixture installations to roofs, these standards will usually give you the clearest, exact information you need.
Use regular acrylic latex for cosmetic and bonding features indoors in completely dry locations.
Use a good siliconized acrylic latex marketed for general plumbing fixture installation or re-caulking: there’s a measure of water resistance, and it’s easier to apply and remove.
If water exposure is high, consider 100% silicone.
If it’s exterior, use a polyurethane or hybrid/ms sealant.
Don’t overthink it. Many sealants will be labeled very specifically for what you need.
Why
Wasn’t the “why” of caulking already covered? To “stop up and make tight against leakage.” That’s true, but I’ll expand on this and show where this appears in plumbing codes.
First: simple aesthetics. Gaps generally do not have a pleasing appearance, and things just look better when seams come together. Of course, you don’t want water to pool up underneath a vessel sink, but once you’ve caulked the sink in, it looks complete.
When something is not watertight, major damage and health risks can occur. Proper caulking in plumbing is critical to prevent moisture and bacteria from creating moldy, unsanitary, and/or rotting spaces where water and bacteria would have otherwise traveled.
Plumbing and plumbing fixtures are connected to penetrations inside and outside of the home. While you won’t get the draft coming from under your uncaulked toilet as you would an uncaulked window, the principle is the same. Air sealing and insect control go hand in hand with proper caulking.
Pipes and fixtures can be subject to movement. While caulking should never be used as a primary means to secure, say a toilet or a vanity, caulking does provide ancillary support. Odor is also not primarily prevented through caulking but through proper fixture trapping and ventilation. On the other hand, proper caulking does prevent moisture from accumulating and odor from migrating.
Finally, although in my experience caulking is sometimes not enforced in residential construction on specific fixtures such as toilets, it is required by plumbing codes. The two most widely adopted codes here in the United States, the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC), state this explicitly:
IPC 405.6 “Joints formed where fixtures come in contact with walls or floors shall be sealed.”2
UPC 402.2 “Where a fixture comes in contact with the wall or floor, the joint between the fixture and the wall or floor shall be made watertight.”3
Caulking is not only an essential component on the job, but it is also a critical skill towards the goal of craftsmanship. How do you master it?
How
Like everything in the plumbing trade and in life, doing something intentionally is how you get better.
Most people at this point will need visual cues, and really nothing will substitute hands-on trial and error, so I won’t write a lengthy tutorial on caulking.
There are some excellent caulking tutorial videos online, like this one:
If you haven’t had much experience, or you’re just not great at caulking, these kinds of videos can be useful. Here are some short, general tips I think will prove useful whether you watch a video or not:
Experiment with a sponge, rag, and profiling tool, and then keep whatever you like to use on hand ready to go, so that you have everything you need to get a clean bead. When it comes to tools, ignorance is usually frustration rather than bliss. Find what makes the job smoother. Literally, in this case.
Always cut the tube tip at an angle, so the application is more controlled. Also consider the width of the gap you are filling before cutting.
Less is more, but stopping mid run will create ridges, and a bead that is too thin can leave small gaps as it cures.
All sealants have a cure time. Keep in mind that any water or moisture around the sealant before it’s dried will often washout the seal, though it may take minutes to see. Make sure the area is dry before you leave, and double check your work several minutes after applying the caulk to verify no washout has occurred.
When caulking in toilets, grout lines are not fun to deal with, but there are tools and tricks that will make those spots look clean (see video above). Don’t leave extra caulking smeared down the grout line – it looks terrible and unprofessional.
Conclusion
Knowing and understanding the little things such as the categories and applications of sealants, and then learning how to apply them skillfully, is what makes you stand out in your profession. It’s a field, like any other, where mastering the fundamentals and the basics will create success.
After selecting the right sealant to caulk with, the next biggest lesson before you go on to master the art of caulking, in one more sentence:
Select the right color.
Sources
- Wright, T. (Ed.). (1843). File:The Chester plays – a collection of mysteries founded upon scriptural subjects, and formerly represented by the trades of Chester at Whitsuntide (IA chesterplayscol00wrig).pdf – Wikimedia Commons ↩︎
- International Code Council. (2024). International Plumbing Code. Section 405.6. ↩︎
- International Association of Plumbing and Mechanical Officials. (2024). Uniform Plumbing Code. Section 402.2. ↩︎
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