BS 5534:2014+A2:2018, the UK code of practice for slating and tiling for pitched roofs and vertical cladding, contains rules for visual grading of tiling battens.\u00a0 This is not strength grading (in the sense of EN 14081) because the tiling battens are not normally structural timber (in the sense that the rafters are).\u00a0 But this doesn’t mean that the grading comes without assumptions of mechanical properties – because some design calculations are still necessary.<\/p>\n
Clause 4.11 gives a short list of permissible species and origin.\u00a0 This is presumably a result of these species and origin having been listed in BS 5268, the design code for structural timber prior to EN 1995.<\/p>\n
The visual grading requirements are in Annex D.<\/p>\n
Minimum sizes are listed in Table 3, which works on the basis of span and type of tiling.\u00a0 These minimum sizes apply regardless of the species and origin.<\/p>\n
Clause 5.2.5.2 goes on to say that the battens should have “adequate strength and stiffness to support the dead, snow and wind loads” and “provide adequate withdrawal resistance without\u00a0splitting and causing excessive bounce when fixing slates or tiles”.\u00a0 So when the spans, or loading, are greater than in Table 3 it is necessary to do “structural design calculation”.<\/p>\n
Annex F gives instructions for determining batten sizes by calculation.\u00a0 In Clause F.1 it explicitly says that the calculation should only be done when the spans or loading are different from Table 3 – so Annex F is to be used only<\/em> when the the spans or loading are greater than in Table 3.\u00a0 That implies that Table 3 sizes are sufficient for the types of loading being checked by Annex F, regardless of the timber species and origin. I say imply because the sizes specified by Table 3 actually pre-date these calculations.\u00a0 The 1997 version of BS 5534-1 has the same size requirements, but back then the timber species were simply divided into type A (those imported) and type B (those from UK and Ireland) and the properties given are different (the lowest mean modulus of elasticity, MOE, is 7.5 kN\/mm2).<\/p>\n Clause F.3 says that the short-term load sharing bending stress for timber battens should be limited to the “GS grade bending stress for softwoods of the appropriate species given in BS EN 1995\u20111\u20111”.\u00a0 This therefore assumes that the battens, graded to Annex D, have at least<\/em> the performance of the “GS grade bending stress”, but what is that?<\/p>\n Clause F.3 says that battens deeper than 72 mm should be designed in accordance with EN 1995-1-1.\u00a0 Given what is written in EN 1995-1-1 clause 3.2 (“timber members shall comply with EN14081-1”) this means that such large sized tiling battens need to be proper strength graded structural timber.\u00a0 It is not clear what requirements of BS 5534 still apply in this case.<\/p>\n But for more usual sized battens, the reader is directly referred to Table F.1 which gives “GS grade bending stresses and moduli”.\u00a0 Cross-checking with EN 1995 will result in immediate confusion because these values are very different, and seem to bear no relation to “GS grade” as we would see it now (between EN 1912 and EN 1995).<\/p>\n Table F.1 actually lists the design values for permissible stress design, whereas EN 1995 is a limit states code.\u00a0 To make sense of this we need to look back at\u00a0BS 5268 again.<\/p>\n Prior to the invention of the European strength grades “C” and “D”, BS 5268 directly gave design values for the visual grades.\u00a0 When the European codes were being developed, BS 5268 was modified to become compatible with the EN 338 strength classes, but the equivalence is not a simple matter – because of differences in the strength class definitions and in the different approach between permissible stress and limit states (the former putting safety factors within the design stress, the latter applying them separately).<\/p>\n Table F.1 is actually more of a direct lift from BS 5268 and when it says “GS grade” it means GS as it was then (prior to 2004) and not as it is now.\u00a0 Below is an attempt at summarising the difference for each of the listed species – but it should also be remembered that these sizes are too small to really be GS grade to BS 4978 (Clause 4.2 specifies a\u00a0minimum cross-sectional area of 2000 square millimetres, which is almost twice the size of the largest size in BS 5534 Table 3) and the assumption that they really would have those properties is questionable because of the relative size of the knots and the rules in Annex D being different to those in BS 4978 (but see addendum, below).<\/p>\n For all the species, except imported redwood and whitewood<\/em>,\u00a0 Table 10 of\u00a0BS 5268-2:2002 gives the same values as BS 5534:2014+A2:2018.<\/p>\n For imported redwood and whitewood, Table\u00a010 of\u00a0BS 5268-2:2002 gives higher bending stress (5.3 compared to 4.2).\u00a0The footnote of BS 5534 Table 1 says the value it quotes was derived from testing of battens.<\/p>\n Larch grown in UK and Ireland<\/strong> [WLAD] (Larix decidua, Larix x eurolepis, Larix kaempferi<\/em>) Pine grown in UK and Ireland<\/strong> [WPNN] (Pinus nigra,\u00a0Pinus sylvestris<\/em>) Spruce grown in UK and Ireland<\/strong> [WPCS] (Picea abies,\u00a0Picea sitchensis<\/em>) European redwood from Europe<\/strong> [PNSY] (Pinus sylvestris<\/em>) European whitewood from Europe<\/strong> [WPCA] (Abies alba,\u00a0Picea abies<\/em>) Spruce-pine-fir from USA and Canada<\/strong> [WPCE] (Abies balsamea,\u00a0Abies lasiocarpa, Picea engelmannii,\u00a0Picea glauca,\u00a0Picea mariana,\u00a0Picea rubens,\u00a0Pinus banksiana,\u00a0Pinus, contorta,\u00a0Pinus ponderosa<\/em>) Southern pine from USA<\/strong> [WPNE] (Pinus echinata,\u00a0Pinus elliottii,\u00a0Pinus palustris,\u00a0Pinus taeda<\/em>) Clause 4.11.1 references the four letter species codes to EN 13556, but since these are species combinations they are actually from EN 14081-1 (except PNSY)<\/p>\n That there is such a large difference in the GS C-class assignments could be due to the way the old visual grading assignments were transferred into the EN system.\u00a0 It does not necessarily mean that actual timber graded GS does not have that higher stiffness in reality.\u00a0 But again, proper GS is not such small pieces.<\/p>\n Fortunately, the limiting performance of all these is the same in both the older and current senses – C14 for British spruce.\u00a0 So if C14 spruce is thought enough for the regular tiling batten specification (BS 5534 Table 3), the other species would be too.\u00a0 There is, of course, still the open question as to whether the visual grading in\u00a0BS 5534 Annex D actually produces battens with C14-like properties.\u00a0 This is additionally complicated by the different size adjustment factor (BS 5534 equation F.1 compared to EN 384:2016 equation 4), but this is a topic for another day.<\/p>\n We will look at the Irish code of practice, S.R. 82:2017 another day too – but for now it is worth noting that it has different visual grading rules (for knot size).\u00a0 It makes the same assumption of equivalence with GS visual grading (Annex A), but here refers to the current<\/em> sense of GS.\u00a0 It is also different in that the calculations are done as EN 1995 (limit states).<\/p>\n Addendum 27\/04\/2018<\/strong><\/p>\n Visual grading rules specific to tiling battens were added in 1997 (Annex E of BS 5534-1:1997).\u00a0 Prior to that, the visual grading standard BS 4978 was referenced.<\/p>\n BS 5534-1:1990 Clause 11.3 points to BS 4978:1988 (interestingly, referencing machine grading as an option since it was part of BS 4978 back then)<\/p>\n BS 5534-1:1978 Clause 11.3 points to BS 4978:1973<\/p>\n But even back then, BS 4987 had a minimum size requirement that was larger than the battens (\u226535 mm x \u226560 mm in 1973 and 1988)<\/p>\n The possibility of machine grading the battens (as alternative to visually grading, by eye or machine) has come and gone over the years<\/p>\n 1978\u00a0Clause 11.3 refers to a list of visual grading clauses of\u00a0BS 4978:1973 (and none of the machine stress grading clauses of that standard)<\/p>\n 1990 machine grading definitely an option since Clause 11.3 refers to Clause 9 of BS 4978:1988 (“machine stress grading”)<\/p>\n 1997\u00a0Clause 2.12.2 refers to visual grading requirements within Annex E of BS 4978:1997, no machine grading possibility<\/p>\n 2003 Clause 4.12.2 was amended in 2010 to include a note saying “When using machine graded battens, designers and specifiers are advised to consider the desirability of using products that\u00a0are supported by United Kingdom Accreditation Service (UKAS) accredited third-party assessment” which implies machine grading is possible, despite the requirement to apply Annex C visual grading rules.\u00a0 (Edit 13\/11\/2018 Possibly this refers to a machine doing visual grading rather than “machine grading” in the current parlance.)<\/p>\n 2014 machine grading only possible when the depth is >72 mm (Clause F.3)<\/p>\n","protected":false},"excerpt":{"rendered":"
\nBS 5534:2014+A2:2018 gives bending stress as 5.3 N\/mm2 and mean MOE as 9 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C16
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 : C18 (because of the MOE)<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 4.7 N\/mm2 and mean MOE as 9 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C14
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 : C18 (because of the MOE)<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 4.1 N\/mm2 and mean MOE as 6.5 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C14
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 : C14<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 4.2 N\/mm2 and mean MOE as 9 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C16
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 :\u00a0C18 (because of the MOE)<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 4.2 N\/mm2 and mean MOE as 9 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C16
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 :\u00a0C18 (because of the MOE)<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 5.3 N\/mm2 and mean MOE as 8.5 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C16
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 :\u00a0C18 (because of the MOE)<\/p>\n
\nBS 5534:2014+A2:2018 gives bending stress as 6.8 N\/mm2 and mean MOE as 10.5 kN\/mm2
\nCurrent GS for this species and origin (EN 1912:2012 and EN 338:2016) : C18
\nEN 338:2016 strength class that meets the BS 5534 values, as implied by\u00a0BS 5268-2:2002 : C24 (because of the MOE)<\/p>\n