function. For a pitched roof, the rafters, lining, tiles, ceiling boards, insulation and ceiling finish comprise a system, or a low temperature hot water heating system is formed from a boiler, pipework, tank, radiators etc. Products: finally, the individual products used to construct a system can be specified, e.g. joist hangers, terrazzo tiles, gas fired boilers. Using the classification system The tables need to be flexible and to be able to accommodate sufficient codings to ensure coverage, to allow for a multitude of items and circumstances, including new technologies and developments that are yet to emerge. Each code consists of either four or five pairs of characters. The initial pair identifies which table is being used and employs letters. The four following pairs represent groups, sub-groups, sections and objects. By selecting pairs of numbers, up to 99 items can be included in each group of codes, allowing plenty of scope for inclusion. For example, Systems are arranged in groups with subgroups which are sub divided, which leads to the final object code. For instance: SS_30 Roof, floor and paving systems SS_30_10 Pitched, arched and domed roof structure systems SS_30_10_30 Framed roof structure systems SS_30_10_30_25 Heavy steel roof framing systems or SS_50 Disposal systems SS_50_75 Wastewater storage, treatment and disposal systems SS_50_75_67 Primary sewage treatment and final settlement systems SS_50_75_67_46 Lamella tank systems Notes and references External links NBS Digital Plan of Work Trade and industrial classification systems Construction documents