Tee into existing stack-retrofit

Hi all. I need to tee a dry vent into an existing 3" vent stack just below the attic but there are fittings above and below where I need to tee in so i can't separate the pipe to fit the tee in. I found something on a UK site called a strap-on boss but I've never seen anything like that here; basically just drill into the pipe and strap the tee on.
Since there's no waste in this tee, is there something available/allowed here?
Thanks,

Saddles are not allowed anywhere in DWV system, although they will work. However, something from the U.K. will probably have the wrong radius for your pipe.

I figured as much. Thanks for confirming. Looks like I've got a lot of piping to redo
Cheers

Can you post a few pics of your situation?

In some cases it would be easier to take it across the attic to a different spot and run the new vent through the roof by itself.

Or tie into one of the existing vents, other than the main one.

Here's a couple of pictures. In the left picture, the main stack is shown going up through the attic. The horizontal portion is the waste from a toilet that comes down from about four feet to the right. This is shown in the second picture. The 1-1/2" pipe to the right (barely in view) is a vent from the basement that vents a laundry tub and washing machine and, possibly, a future floor drain. This ties into the stack a few feet above this picture.
The 1-1/2" pipe to the left is a shower drain that ties into the waste line and goes down to the basement.
I need to tee a vent into the stack in the first picture. There is about 8" there but I cant pull the vent down or push up because of the existing fittings.
My plumber wants to cut the horizontal waste line to the left and right and put a couple of flex connectors there but that sounds like waste could get hung up there.
My thought was to cut the shower line to the left, and the 45 coming down from the toilet (shown in the second pic) and pull the line down enough to get a fitting in. Then ABS couplings to fix the shower and toilet. I can't use the existing vent from the basement because I'm adding 4 fixtures down there and need a separate vent line.
Long winded but I tried to cover everything.
Thanks,

Let me start by saying that I'm not clear exactly what I am seeing in the pictures.

If the toilet waste is coming in from the right, any tee you install there would be below the flood rim of the fixtures served by the vent, which is not acceptable.

I think you're clear on what you are seeing. The vent for the toilet on the right joins the stack way up in the attic, so well above the toilet.
The vent I am adding will be from basement fixtures but, you're correct, this tee-in will be below the 2nd floor toilet. What you are looking at are the joists for the second floor.
This is where the plumber was going to tee in but I didn't like the way he was going to do it with the flex connectors (Fernco) and was trying to find another way.
Now you have got me concerned.....
Does the vent from the basement fixtures have to be above 2nd floor flood rims?
Thank you,

Doing some reading of Bert Polk's document and it looks like you're absolutely correct. Maybe I'll have to find a different plumber...
And find a way to get to that stack up higher
I am reading that the rule of being 6" above the flood rim doesn't apply to the main stack. Can you comment on that?

How many fixtures can I tie into the existing 1-1/2" vent there that comes from the basement?

how many, and which fixtures are already on the 1-1/2" vent line from the basment? what is approx length of this existing basement vent?

Right now there is a laundry tub, a washing machine and a floor drain. The vent is 1-1/2" about 20 feet with 5 elbows. I need to add a toilet, shower, vanity, and two more floor drains. The two extra floor drains are for 1) air conditioner, furnace and humidifier drain and 2) the bathroom where a small hot tub is, just in case it leaks.

so yoru total fixture unit load after installing new fixtures would be 18FU...your 1-1/2" vent can handle that for a length of 20 meters....so you can use your 1-1/2" vent

Mike: If that's the case, I owe you one. I have a couple of questions if you don't mind.
This vent will be all dry. Do elbows have an equivalent length? i.e. is one elbow worth 5 ft of straight pipe or does it not matter in venting?
Is there a table somewhere that I can show my plumber?
I found this one, http://www2.iccsafe.org/states/newyorkcity/Plumbing/PDFs/Chapter%209_Vents.pdf showing I can put 21dfus on a 3" stack with 1-1/2" vent up to 32 feet. Is that correct?
Thanks so much.

there may be charts showing equivalent lengths for fittings out there... but when we're talking venting to code you refer to developed length of the piping.
your link is for new york city...
all vent sizing and requirements can be found in section 7 of the OBC...specifically Table 7.5.8.3 "Sizing of Branch vents, headers, continuos vents and circuit vents.
your plumber should a) know these requirements or at the very least b) know where to find them.
if you, the customer, need to show him, the licensed professional, then something is wrong with this picture.

good luck with your project

Sounds good. Thanks for the help.
Cheers.