Want to run pex to shower/tub valve hot and cold. Single shower head. City water pressure 70psi. Questions: Should 1/2 or 3/4 pex be used for supply lines? Can 3/4 pex be used from valve to shower head?

It would depend on the valve. 1/2" pex is run to 1/2" shower valve inlets all the time, no problem. 1/2" is sufficient to the shower head also.

Some valve manufacturers say do not use pex on valve to spout. Is that because of a back pressure restriction?

Yes, the reason is that it's "possible" because the slight difference in size that you "could" get water coming out of the shower head when the tub spout is running. Frankly myself I have NEVER seen it happen, yet.
It's advisable to run full port 1/2" copper or brass to the spout to be safe, I guess.
There is not a code that says you "have to" do it that way. Only some manufactures that address the idea. I have used pex to the spout usind a drop ear 90, never had a problem with water coming out of the shower head while the spout is on. It's not my normal way of installation, I usually use 1/2" brass nipples and 90. Just didn't have them with me on a particlular job.

A good many people run into problems with PEX to the tub spout...
The higher the water pressure the more likely it is to happen.

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yes, we have had many people post here about water coming out of the shower head when it should be coming out of the spout and it is almost always pex from the valve to the spout even though the directions on every shower vavle i have ever seen specifically says DO NOT USE PEX FROM VALVE TO SPOUT. and i would think to the shower head would also create a problem.

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PLUMBERS "Protecting The Health Of The Nation"

To the showerhead does not create a problem. It is only created on tub spouts where a diverter closes off the easier path out the tub spout to send it to the shower. When PEX is used on those tub spouts the excessive back pressure sends water up the shower.

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Is 3/4 pex from valve to shower head the way to go?

depends on the shower head you want to use??

The question would depend on the valve as far as the supply is concerned .You may supply a 1/2" valve with 3/4" but, may not see any difference .you may not supply a 3/4" valve with 1/2" and not expect to reduce flow ,per the amount it could have provided.The 3/4 riser would be in the same ,/as the valve size is concerned\The one area you must find concern is you must use at least 3/4" poly to the tub spout from the valve.to control pressure backing when the tub spout divertor is not in use.Good Luck.If you can say what valve number and pressure your lines run at ,well or city .we may be able to get deeper into the exact size desired to get best resultes.

You're not going to have a problem with 1/2" pex to your valve or shower head. If you're worried about the back pressure (which you are because you posted) then use full size copper or brass. It's not complicated.

"Author: ironranger (MN)
Yes, the reason is that it's "possible" because the slight difference in size that you "could" get water coming out of the shower head when the tub spout is running. Frankly myself I have NEVER seen it happen, yet.
It's advisable to run full port 1/2" copper or brass to the spout to be safe, I guess.
There is not a code that says you "have to" do it that way. Only some manufactures that address the idea. I have used pex to the spout usind a drop ear 90, never had a problem with water coming out of the shower head while the spout is on. It's not my normal way of installation, I usually use 1/2" brass nipples and 90. Just didn't have them with me on a particlular job."

I have seen it; it is a problem that is reported frequently on this and other forums. It is all about pressure, and even with copper down and brass nipple, high system pressure can cause it.

There are many reasons it could happen, not just because of pex.

Delta Tub shower valves say right in the instruction manual not to run pex to the tub spout. Condo unit with 26 units, Plumber piped all the spouts with 1/2" pex. All 26 had to be re-done. He is now working for "others"

Moen says it too!

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I will use a Delta single handle valve with 1/2 pex supply lines and 1/2 brass from valve to spout. Question: Should 1/2 or 3/4 pex be used from valve to single shower head? City pressure is 70-80psi.

1/2"

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John William Gardner 10/8/1912 - 2/16/2002

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This slow drip will waste 7+ gallons of water per day.

IF this is a shower only, you do not have a spout, and yes that is the reason.

It also depends on the distance between the valve and the spout.

Have never seen it with copper or brass drop to the spout.

Use whichever you want, you will not know, or feel, any difference.

IF it is, you may be the first one who has ever done it. You could use 2" pipe to the shower head, but the flow limiter in the head is going to govern how much water comes out, NOT the size of pipe to it.

the flow restrictor will limit the maximum but has nothing to do with the minimum.
i trimmed a shower friday, damm if the flow restrictor didn't fall out and rolled away. i'm calling al gore's office to order a new one.

you know packy....that happens to me a lot

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PLUMBERS "Protecting The Health Of The Nation"

hj you have never seen it with copper or brass drop to the spout. I have read to use brass nipples and 90 to the spout for strenght so if someone slipped and grabbed the spout it would not be ripped out the wall.

Has never seen 'it' happen, (water from the head spill while the spout was running) when copper or brass has been used for the spout stub, me either.

If someone slips in the shower and grabs the spout on the way down, the spout is probably going to give no matter what is used...

That's what grab bars are for.

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"The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an
exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." -
John William Gardner 10/8/1912 - 2/16/2002

Repair your leaking Plumbing fixtures ASAP [www.theplumber.com]
This slow drip will waste 7+ gallons of water per day.

Thats amazing I thought I was the only one. I have a customer call and complain about a shower head and when I check it out the flow restricter somehow falls out and I can't find it. Actually come to think of it the customer never seems to call and complain about it again.

I'll leave it up to the customer to alter it.

e-plumber
-----------------------------------------------------------------------------------------------------------------------------------

"The society which scorns excellence in Plumbing as a humble activity and tolerates shoddiness in philosophy because it is an
exalted activity will have neither good Plumbing nor good philosophy: neither its pipes nor its theories will hold water." -
John William Gardner 10/8/1912 - 2/16/2002

Repair your leaking Plumbing fixtures ASAP [www.theplumber.com]
This slow drip will waste 7+ gallons of water per day.

That is the only way I do it. I was responding to the statement that "dripping from the shower when filling the tub can also happen when copper or brass are used to the spout." THAT is what I have never seen happen.

Amazing how thoses flow restrictors pressed in by machine can fall out just by tipping them over, or the hole mysteriously grows larger between the time the head is removed and reinstalled.