Seems like a great idea, right? Fit everything together for your sprinkler system (or laundry-hamper-holder) so you know exactly how long the pipe needs to be cut, then take it apart and glue back together permanently.
Unfortunately, it doesn’t work that way.
If you dry fit PVC plumbing connections, you’ll wind up with PVC fittings that only allow the pipe to go in part of the way, rather than all the way like they will when you’ve got them all gooey with glue.
Why not, you ask? It works with Legoes, wooden dowels, puzzles, the tile you set last weekend, and all sorts of other stuff you’ve done before. What’s so special about PVC plumbing fittings that they won’t let you test out your design precisely? What happened to measure twice, cut once? Are you doomed to have to hire a pro or screw up over and over?Not necessarily, but let’s explain a bit about PVC plumbing fittings and then go from there.
(Hold on, need a disclaimer here: If you don’t know what you’re doing, a pro really might be the best idea – they can usually do it faster, better, and with far less frustration than a newbie. If you do hire a plumber instead of tackling the project all alone this time, take the time to watch what they do and you might learn something new – hey, you could consider it an educational investment! And, as always, consider the bad things that could happen if the project fails…. )
PVC fittings are designed to connect to PVC pipe by interference fit, also known as a press fit or friction fit. What this means is that the fitting is designed so that it does not exactly match the pipe – the hub of the PVC plumbing fitting is actually tapered a bit to make an extremely snug fit so there’s no gaps for PVC glue/cement to fill in. This is a good thing when you think about it (hmm… incomplete seal = leaks = bad).
Why is this important? Despite the name, PVC glue is not glue at all. It’s actually a solvent that kind of liquefies the PVC it touches so the pipe and the fitting will essentially melt together where they are “glued”. Think of it like welding metal – same idea but with chemicals instead of a torch (and less chance of catching things on fire). Once cemented together and the connection has cured, the pipe and fitting are no longer separate pieces, which is what makes a properly cemented PVC connection virtually leak-proof. Also, since you’ve got cement gooped on, the fitting slips in much easier. Learn more about PVC and PVC cement here.
Okay, so we know why dry fitting PVC plumbing fittings won’t work the way you want, but what to do about it? Surprisingly, the answer is incredibly simple: use a ruler or measuring tape.No, seriously – check out the inside of a PVC fitting (or click on the picture to your left); you can see where the end of the pipe will be once properly cemented and inserted. Simply measure from that point and you’ll quickly find out how much pipe you’ll need to get to the next fitting. Some people have a good eye for how the pipe will fit by just looking at the outside of the fitting and can simply line up the pipe and fitting right next to each other and go from there.
Please note, different PVC fitting manufacturers might have slightly different hub lengths, so if you’re sourcing your fittings from multiple vendors and need a high degree of precision, you might want to measure each fitting. If you’re sourcing all fittings from the same manufacturer, you shouldn’t need to check every fitting, but we would recommend checking at least one from each different type of fitting (90 degree elbow, tee, side outlet 90, coupling, etc) and size. It’ll only take a minute and will really help make the project go much more smoothly.
Now that you know how to “dry fit” your PVC plumbing fittings, get out there and finish those projects!
Your turn: what’s the most useful thing you’ve learned about working with PVC?