All pipe is to be made of a long hole, surrounded by metal or plastic centered around the hole.
All pipe is to be hollow throughout the entire length - do not use holes of different length that the pipe. All pipe is to be supplied with nothing in the hole so that the water, steam or other stuff can be put inside at another date. All pipe should be supplied without rust - this can be more readily applied at the job site. Some vendor are now able to supply pre-rusted pipe. If available in your area, this product is recommended as it will save a lot of time on the job site.