Quality Control

Before proceeding with the construction of a trussed rafter roof, it would be helpful to understand the quality control imposed upon the manufacturer of trussed rafters. BS 5268: Part 3 section 8 'Fabrication', sets the standards for trussed rafter production, and this standard is incorporated in the Building Regulations 1991. All trussed rafters used for dwellings should be manufactured to this standard, whether fabricated using punched metal plate fasteners, nailable metal plates or plywood gusset joints. The illustrations in Figs. 6.3a-j attempt a graphical interpretation of part of section 8, but the reader is directed to the British Standard text itself for full information.

Figure 6.3a shows that moisture content of the timber used in fabrication should not exceed 22%.

Figure 6.3b covers the maximum gap allowed between two adjoining members under the punched metal plates. The average gap width should not exceed 1.5 mm unless specifically allowed for in the design.

22% max

Fig. 6.3a Moisture content.

Fig. 6.3a Moisture content.

Figure 6.3c illustrates that wane, the term used to describe the occasionally occurring rounded corners of the timber caused by the timber being cut near to the outside of the log of the tree, is acceptable only at certain places on the trussed rafter. It is limited on the surfaces of the trussed rafter to which other elements of the building are attached, namely the top of the top chord and the underside of the bottom chord, and within the plate area of course no wane is tolerable.

Figure 6.3d shows that to ensure correct embedment of the plate teeth the difference in thickness between the members at a node point must not exceed 1 mm.

The permissible gap between the underside of the metal plate and the timber surface shown in Fig. 6.3e should not exceed 1 mm for nails or teeth up to 12 mm long, and 2 mm for nails or teeth over 12 mm long. This gap should not exceed 25% of the contact area of any member of the joint.

Minimum thickness =

Fig. 6.3c Trussed rafter wane in joint area.

Thickness variation = 1 mm max.

Thickness variation = 1 mm max.

Figure 6.3f illustrates that timber is a living material and not man-made, and contains features which can detract from its overall strength. Such features are splits and fissures, live knots and dead knots and, in the latter case, possibly knot holes where the dead knot has fallen out of the timber section. Hairline fissures up to 55 mm long which have apparently been caused by the tooth or nail, are not seen as having any significant effect on the joint and can be ignored.

Minimum thickness =

Fig. 6.3c Trussed rafter wane in joint area.

Gaps not permitted (except close to thickness variation as above) 1

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Fig. 6.3e Trussed rafter plate bedding.

Figure 6.3g illustrates that corners of plates projecting beyond the edges of the trussed rafter timbers are not allowed. The metal plate or plywood gussets which project beyond the outer edges of the trussed rafter should have their protruding areas removed, or in the case of metal plates should be folded over once fully embedded. Particular attention should be paid to fasteners which protrude into walk spaces or other areas to which access may be gained at a later date. Where it is not possible to avoid a projecting edge, then a timber block must be placed between the plates to protect those in the roof space, and to protect those handling the components during construction.

Figure 6.3h illustrates plate location. Taking into account possible plate misplacement during manufacture (see laser location methods in Chapter 7), the British Standard in paragraph 8.2.1 states 'fastener misplacement during assembly should be within limits assumed in the design'. Unless a greater allowance has been made, fastener misplacement should be no more than 5 mm in any direction. These tolerances are essential and incorporate a safety factor in design.

Figure 6.3g illustrates that corners of plates projecting beyond the edges of the trussed rafter timbers are not allowed. The metal plate or plywood gussets which project beyond the outer edges of the trussed rafter should have their protruding areas removed, or in the case of metal plates should be folded over once fully embedded. Particular attention should be paid to fasteners which protrude into walk spaces or other areas to which access may be gained at a later date. Where it is not possible to avoid a projecting edge, then a timber block must be placed between the plates to protect those in the roof space, and to protect those handling the components during construction.

Figure 6.3h illustrates plate location. Taking into account possible plate misplacement during manufacture (see laser location methods in Chapter 7), the British Standard in paragraph 8.2.1 states 'fastener misplacement during assembly should be within limits assumed in the design'. Unless a greater allowance has been made, fastener misplacement should be no more than 5 mm in any direction. These tolerances are essential and incorporate a safety factor in design.

Figure 6.3i illustrates dimensional tolerance. Most trussed rafters are manufactured in metal jigs, but because there is such a variety of trussed rafter spans, pitches and shapes, these jigs have to be quickly and easily adjustable. For this reason and bearing in mind the inherent natural movement of timber, it is essential that some tolerance from the design shape be allowed in production.

Figure 6.3i illustrates dimensional tolerance. Most trussed rafters are manufactured in metal jigs, but because there is such a variety of trussed rafter spans, pitches and shapes, these jigs have to be quickly and easily adjustable. For this reason and bearing in mind the inherent natural movement of timber, it is essential that some tolerance from the design shape be allowed in production.

TOLERANCE

H and S for S = 7.5 m. X and Y = 6 m or less H and S for S = 7.5 m to 12 m. X and Y = 9 mm or less H and S for S = 12 m. X and Y = 12 mm or less

H and S for S = 7.5 m. X and Y = 6 m or less H and S for S = 7.5 m to 12 m. X and Y = 9 mm or less H and S for S = 12 m. X and Y = 12 mm or less

Fig. 6.3i Trussed rafter dimension tolerance. Where a camber is specified by a designer, it should be checked with the trussed rafter lying on its side before being fixed on to the roof structure. The measurement should be taken as the distance between a string line fixed between the wall plate support points where rafter meets bottom chord, and the underside of the nearest node point to mid span.

Fig. 6.3i Trussed rafter dimension tolerance. Where a camber is specified by a designer, it should be checked with the trussed rafter lying on its side before being fixed on to the roof structure. The measurement should be taken as the distance between a string line fixed between the wall plate support points where rafter meets bottom chord, and the underside of the nearest node point to mid span.

TRUSS MANUFACTURED IN ACCORDANCE WITH BS 5268 (2 & 3) EUROPEAN RED/WHITEWOOD GRADED AS MARKED

AND MANUFACTURED

TRUSS MANUFACTURED IN ACCORDANCE WITH BS 5268 (2 & 3) EUROPEAN RED/WHITEWOOD GRADED AS MARKED

AND MANUFACTURED

BRIDGE STREET, THRAPSTON, NORTHAMPTONSHIRE NN144LR TEL: (01832)732366

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SYSTEMS

Fig. 6.3j Trussed rafter marking.

Figure 6.3j illustrates a marked trussed rafter. Until the publication of BS 5268: Part 3 it was not necessary to mark the trussed rafter with the name of the company responsible for its manufacture. This has changed, with the requirement of the 1998 edition of the standard in section 8 'fabrication' in paragraph 8.1, under the heading 'marking', which requires that every trussed rafter should be clearly marked with the identification of the producer, the materials used and the standard to which it is produced. Furthermore the marking label or stamp should be placed as near to the apex as possible thus making it clearly visible in the completed roof void. Whilst it may be easier in manufacture to mark the truss near the wall plate support junction or on the bottom chord, this area is invariably covered with insulation and should anything go wrong at a later date with the roof structure then the vital information would be difficult to find.

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