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Re: Re-routing Copper Plumbing Manifolds in Concrete Slab

From: dknorr2@earthlink.net
Category: Plumbing
Remote Name: 24.129.154.197
Date: 14 Dec 2005
Time: 02:57 PM

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Question: I just purchased a 12-year old house in Clearwater, Florida. I plan to convert the formal dining room into a third bedroom, since it will never be used as a dining room as it’s too far away from the kitchen. Anyway, the only place I can install a closet in the “bedroom” falls right where there are copper manifolds and tubing coming up from the concrete slab. My question is, can I chip away the slab around the copper pipes, lay the manifolds down below the surface, re-route the tubing out of my way, and then re-cover the manifolds with concrete? This would mean that solder joints would be below the later-applied concrete. Would there be any problems with doing this?

Builders Websource® Answer: Generally, Builders Websource recommends against embedding plumbing joints of any kind in a concrete slab. Ground movement, thermal expansion, compromised slab reinforcement, and workmanship considerations all factor in to the potential for future leaking. Furthermore, most building and plumbing codes prohibit direct burial or soldered copper joints. However, it is possible to use silver brazing of copper joints under a slab. This is common in the hydronic or radiant heating industry where copper manifolds are sometimes installed under the slab. Silver brazing is different than soldering. Unlike soldering which bonds together a copper joint with a low melting point alloy (typically in the 400-500 degree range, silver brazing bonds the copper joint together with high-temperature process (typically above 900 degrees). A properly brazed joint is as strong or stronger than the pipe itself. Always use at least type "L" copper for any embedded application and be careful to avoid denting the pipe (particularly if you're using soft annealed copper tubing). Before you decide to embed any copper joints in your slab, consult with your local building department for all approvals (in writing).

Furthermore, when brazing copper tubing or pipe, using standard "solder type" couplings and elbows available from home improvement warehouses and typical plumbing supply stores could unwittingly result in a weak joint. According to the American Welding Society Standard C3.1-63 and an excellent article in PMEngineering by Watler Sperko, "overlap equal to [at least] two times the thickness of the thinner member (2t) is all that is required to develop brazed joint strength equal to the tube strength." To develop comparable tensile strength, typical copper fittings designed for soldering have an overlap length of 7-10 times the wall thickness of the copper tube. Since brazing temperatures are roughly three times that of soldering, it is more difficult to heat a longer overlap of concentric tubing/coupling, particularly with larger diameters. As a result, it is easy to inadvertently form a poor joint when brazing, which will lead to premature failure in service.

Rather, by focusing the intense heat on a smaller overlap area, it is possible to develop a superior bond with better consistency while also using less filler material. While a longer overlap area is not prohibited, studies have shown that the longer the overlap, the more likely a a poor joint can form. Furthermore, be vigilent when selecting your brazing alloy. Just as lead has been banned from all solders used with potable water supplies, many silver brazing alloys contain high concentrations of Cadmium (to enhance fluidity), a known toxic element. Do not use alloys containing Cadmium. Rather, consider using a product such as Safety-Silv® 56 or Safety-Silv® 45T from J.W. Harris Company. Safety-Silv® 56 has 56% silver (Ag), 22% copper (Cu), 17% zinc (Zn), and 5% Tin (Sn) while Safety-Silv® 45T has 45% silver (Ag), 27% copper (Cu) and 25% zinc (Zn), and 3% Tin (Sn). These alloys offer superior properties and fluidity that produce top-notch joints. Both products are also NSF (National Safety Foundation) certified to NSF C2, suitable for use in food processing applications.  These alloys become fluid at 1200 and 1265 degrees Farenheit, respectively.

If you cannot find copper couplings specifically designed for brazing, a handy dimpler tool is available that creates a mechanical stop in a traditional sweat-style copper fitting. Visit http://www.brazingdimpler.com for more information. If you are limiting your joints to just a few and you are not a professional plumber, consider trimming the fittings in the field, being careful, however, not to distort the fitting, which could lead to an inferior joint.

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