Understanding Dry fit and glue up

[Isaac H]

This is a really noob question I suppose, but every thing I can find about doing this doesn't make sense. The steps are always

1. Measure and cut PVC
2. Dry fit
3. Mark the fittings and PVC
4. Prime and glue

Obviously that makes sense enough, but the few things I've done I've had issues. Mainly concerning the measuring and dry fitting.

My assumption is that you aim to get the pipe 100% of the way into the hub, so that's how I measure it, but 90% of the time when you dry fit you can only get it 50-75% of the way in. As you go along cutting and dry fitting, that throws off the measurements, and by the end you may be off by several inches. What is the correct way of doing this?

I've also assumed that you are supposed to have 100% of the pieces cut and dry fit before glue up, at least in cases where you're doing something with some complicated angles. So this causes a problem as stated, with the further away you get from where you start, you're measurements get further and further off.

Am I supposed to measure aiming to get 50% of the pipe into the hub so that dry fit and final are identical and then mark on the pipe how far in the hub the pipe should be? Or am I missing something else here in the process. Just trying to understand best practices and hopefully try to get a little faster at this for any other plumbing projects I do in the future. Thanks.

 

[oldVermonter]

Take a look at Oatey's instructions:

Link wasn't working.

They say "Pipe should easily go 1/3 to 2/3 the way into the fitting hub". I have always found if the pipes dry fit easily about 1/3 of the way, then a full fit works fine with primer and cement in place. (A full insertion is essential, you can't get by with anything less.)

 

[Sponsor]

 

[Isaac H]

Take a look at Oatey's instructions:

They say "Pipe should easily go 1/3 to 2/3 the way into the fitting hub". I have always found if the pipes dry fit easily about 1/3 of the way, then a full fit works fine with primer and cement in place. (A full insertion is essential, you can't get by with anything less.)

Thanks for the reply. That link seems to be broken. My problem isn't that I can't get them all the way in when I glue them, it's that because they don't go all the way in during dry fit, it throws off the measurements because when you glue it up they DO go all the way in!

For instance, pretend you have a 10' length you need to span from a drain, and you have 4 other lines connecting to it. You start at the stack, working your way back measuring and cutting to each fitting and then to the next, and you cut the last length and fitting so that it perfectly lines up and connects where it needs to. Then you mark everything and you start gluing up. But if all the fittings between points a and b are only 1/2 way in to each hub, then that's like 1/4" per hub that they pipe is not going all the way in, so when you glue them and do push them all the way in, when you get to the end, and you have glued up 4 fittings (8 hubs) that's 2" that will go "missing" from where your dry fit was at, and now the last length is 2" away from where it needs to be and you have to recut that last length. Does that make sense, and how do you do things in a way that avoids that mishap?

 

[Terry]

PVC needs to be primed first and then glue applied to the hub and to the pipe, and then quickly pushed together with a swirl motion to smear the fitting together. Wait too long, and it won't slide in.
Measurements are meant for full insertion. I don't assemble anything dry. You can't work that way. You need to visualize first and work with what is in your head. You can lay pipe and fittings on the ground and do your takeoff measurements that way, but don't even think about pushing pipe and fittings together unless you have primed and glued and ready to assemble for good.

THERE IS NO SUCH THING AS DRY FIT AND GLUE UP

 

[Reach4]

Couple of additional choices:
1. You can find the depth of the hubs. Measure. See page 12 of https://www.charlottepipe.com/Documents/DimensionalCatalogs/Plastic_Pipe_Fittings_DC-DWV(609).pdf
2.Make some pipes to experiment with with mock-ups. Cut some kerfs in the end of a test-fit pipe, and you can insert those all of the way. Then cut new pipe to the same length as the test-fit pipe.

You are not experienced, and you will be slow. If you make a mistake, you will have to saw stuff out. So when gluing, do it right the first time.

 

[Helper Dave]

Yeah, what you read about dry fitting is wrong. Plumbers don't work that way.

You can dry fit a fitting or two just to see if it looks about right, but you gotta trust your measurements, and glue as you go.

Once you've got a fitting where it needs to be, you glue, and carry on measuring further down the line.

It can help to have a slightly long piece to get another measurement (like the long piece on a stack to get a shorter connection up at a long sweep or 45 or what have you), but then you go back, make sure all your pipes are cut, and glue em up.

It's all about confidence ... then frantically trying to save a fitting, and ripping the pipes apart cause you messed up

 

[Isaac H]

Thank you everyone, this is really helpful. It sucks that there is so much information that is not just wrong but actually makes things worse. Thanks for your help.

 

[John Gayewski]

The only time you need to mark a fitting is in situations where you can't level it becuse a level won't fit in the space. Still if you are doing it right most times you don't need to mark a fitting.

Measure, cut, primer, generous glue, insert, twist into position. There are times when you need to glue two joints at once becuse or makes orienting the fittings easier.