I wish to add a new cold water pipe?

Hi ... first I would try to identify that the two pipes do the job that you think they do and where they come from and where they go. Will taking a flow of water out of the prospective supply pipe have an underiable consequence somewhere else. Then you need to think about the water which is going to the garden ... this will need to pass through a check valve to ensure that under any conditions water will never siphon from the garden supply back into your drinking water.
If you decide that joining the pipes is the correct thing to do you can make plumbing T joints using a screw on fitting which does not require soldering. If any soldering is undertaken remember not to use a lead based solder.
You also need to think about stoptaps so that it is possible to contine to use the house water supply even if the garden supply is turned off ie big freeze!

Nyge
March 2011

compression tee and elbow will be the easiest push fit plastic will probably be too bulky

brian
March 2011