Being a thermoplastic, plastic piping will combust at 700º F, and will lose its structural integrity at just 250º, leaving an opening that will allow the spread of smoke and flame . To counter this, fire - stop materials are required to seal these penetrations . Installed in the space around the pipe, these materials ex - pand when exposed to high heat . A common fire - stopping assembly for plastic piping through concrete floors consists of a ring of intumescent material held in place around the pipe with a metal collar . This as - sembly involves shooting masonry anchors into the concrete, clamping the collar in place and installing a smoke seal with a bead of fire - resistant caulking . The expected service life of these intumes - cent materials varies significantly by manu - facturer . Some fire - stop manufacturers claim service life measured in decades, while some make no claims of an expected service life . Although a test method for aging of intumes - cent materials has been developed, there is no established method for using the results of these to calculate service life . As with any potentially life - saving product, these fire - stopping materials must be installed properly and tested and listed by an accredited third - party agency before use . Cast - iron piping will not melt or burn away in a fire, meaning engineers can avoid the added cost and complexity of adding fire - stopping materials to their piping assemblies for every penetration in a building . All that is required to seal a cast - iron penetration is some mineral wool batting and fire - resistant caulking around the pipe, which makes the process simpler, faster and more cost effec - tive . And the more floors and fixtures in the building, the greater the cost savings .   Underground Installation Proper underground piping installation is one of the most misunderstood piping appli - cations . Underground piping is expected to support not only the earth load, but the live (traffic) load above it as well, all while limit - ing deflections or obstructions that can cause joint leaks . Cast - iron soil piping can handle these loads with no deflection . Defined as flexible systems, plastic piping will deflect if not properly installed . The point of failure for plastic pipe is deflection in excess of just 5% of the inner diameter of the pipe, accord - ing to the ASTM D 2321 standard – or only a quarter - inch for 4 - inch pipe . With plastic installations, the sidefill stiffness of the trench is critical to support the pipe . For thermoplastic pipe, the trench is required to be the width of the pipe O . D . plus 16 inches or pipe O . D . times 1 . 25 plus 12 inches . For example, a 6 - inch pipe would require a 20 - inch - wide trench – that’s a lot of extra digging on the front end and extra compaction on the backfill . Compaction in 6 - inch maximum layers – done by hand – is required by ASTM D2321 . Depending on the soil type, minimum compaction density can range from 85% to 95% .   Because cast iron is up to 10 times stronger than most thermoplastic materials, it does not need the compaction of the sidefill to support the pipe wall, so the trench can be as narrow as the installer wants .   Also, the plastic trench involves spe - cial bedding requirements – a minimum of 4 inches of material, depending on the soil type, to support the pipe . Cast iron requires no such bedding, only that the trench bottom be flat so that the pipe is uniformly supported . Again, a lot of extra work and materials when it comes to plastics . Above - ground Installation Above - ground plastic DWV applications introduce a different set of issues that can add cost and complexity to high - rise com - mercial construction projects . For example, a lack of understanding of hanger require - ments can lead to improper installations . Plastic must be supported by hangers every 4 feet in horizontal installations . Cast iron, on the other hand, needs only to be support - ed within 18 inches of each joint and every 10 feet . The cost of hangers, anchors, con - nectors and extra labor in a typical plastic installation can be up to 2½ times that of cast iron in horizontal applications . And unlike cast iron, plastic expands and contracts between swings in heat and cold . The rate of thermal expansion for plastic is approximately . 36 inches per 100 feet of pipe for every 10 degrees F .   Installing plas - tic in high - rise construction requires the use of expansion joints to accommodate these thermal effects . One method for compensating for ther - mal expansion is to use an expansion cou - pling, which is a pipe - within - a - pipe that telescopes in and out as the piping expands and contracts . Expansion joints are required in vertical stacks at every other branch in - terval and on horizontal runs exceeding 20 feet – once again adding cost and com - plexity to the system . And expansion joints can create headaches during water testing of the system because they can leak if not properly installed . Creating A Quiet System With fire - resistive construction or under - ground installations, the differences between plastic and cast iron are hidden from view . But there’s no hiding the differences in sound Cast - iron piping is a cost - effective solution when dealing with fire - resistive construction because it will not melt or burn away in a fire .