Engineered timber beams and trussed rafter fabrications for attic construction

Attic roof construction using conventional methods has been detailed in Chapter 3. Further conventional floor and trussed rafter attic construction is dealt with in Chapter 6; Fig. 6.15 illustrates a construction suitable for conversions, Fig. 6.17 illustrates conventional floor joists with cross wall purlins and trussed rafter assemblies, and Fig. 6.18 illustrates a conventional trussed rafter attic constructed roof. What follows is a combination of the use of engineered timber joists which have been discussed at length above, and simple trussed rafter fabrications. Again the method at the time of drafting this revision, was being promoted by one particular company, Trus Joist, but undoubtedly will be followed by others in the future.

Note: Solid blocking or herring bone strutting is not required on the Simple Framing System®

s Joist blocking or 38 x 89 mm (min.) 'squash' block on each Fsa (F8W side required when supporting a load-bearing wall above

Versa-Lem® trimming joist or a BCI® trimming Joist V1 V7

38 mm knockout holes are pre-punched at approximately 305 mm c/c along each BCI® Joist

Note: Solid blocking or herring bone strutting is not required on the Simple Framing System®

s Joist blocking or 38 x 89 mm (min.) 'squash' block on each Fsa (F8W side required when supporting a load-bearing wall above

Versa-Lem® trimming joist or a BCI® trimming Joist V1 V7

38 mm knockout holes are pre-punched at approximately 305 mm c/c along each BCI® Joist

For load bearing cantilever details, see technical manual

Masonry wall restraint strap (Fls)

Note: Minimum bearing length -45 mm is required at joist ends; 89 mm at intermediate supports

SCI" Joiits

Fig. 4.7 Typical I beam floor framing layout.

For load bearing cantilever details, see technical manual

Masonry wall restraint strap (Fls)

Note: Minimum bearing length -45 mm is required at joist ends; 89 mm at intermediate supports

SCI" Joiits

Fig. 4.7 Typical I beam floor framing layout.

Versa-Lam® LVL support beam

BCI® Joist blocking for lateral support

Note: Ventilation - The 38 mm, prestamped knock-out holes spaced at 305 mm centres along the BCI® Joist may all be knocked out and used for ventilation

Versa-Lam® LVL support beam

BCI® Joist blocking for lateral support

Note: Ventilation - The 38 mm, prestamped knock-out holes spaced at 305 mm centres along the BCI® Joist may all be knocked out and used for ventilation

Note: Unless otherwise noted, all roof details are valid for slopes of 45" or less

BCI® Joist trimmer

Multiple BCI® Joists may be required, depending upon opening size

Fig. 4.8 Typical I beam roof framing layout.

Note: Unless otherwise noted, all roof details are valid for slopes of 45" or less

BCI® Joist trimmer

Multiple BCI® Joists may be required, depending upon opening size

BCI" Joists

Fig. 4.8 Typical I beam roof framing layout.

TMTrus Joist is one of the companies manufacturing engineered timber 'I' beams and, recognising that there is a considerable market for attic construction, has developed Spatial Roof. This is a room-in-the-roof or attic construction which interestingly combines engineered timber 'I' beams and trussed rafter assemblies. Recognising that there are problems in hoisting and erecting the bulky true trussed rafter attic (as illustrated in Fig. 6.18), this method seeks to provide relatively lightweight components thus acknowledging the recommendations of the manual lifting regulations aimed at avoiding personal injury to those erecting large and heavy structures. By taking the conventional 'I' beam floor diaphragm, and covering this with a waterproof floor decking, the method quickly creates a sound and solid working platform from which to erect the roof element of the attic. The waterproof platform provides protection for the building below allowing other works to continue whilst the roof is under construction. This also lends it admirably to roof replacement projects referred to later in Chapter 12. It should again be noted that this is an individually engineered designed system for each structure, the design and products being supplied as a package as with a conventional truss rafter roof. This ensures full compliance with Building Regulation requirements, making both the floor and roof structure very easy for the building designer as this is prepared for him by the manufacturer. Figure 4.9 illustrates the construction principles of this system.

First, the first (or second) floor structure is constructed using Trus Joist 'I' beams. The 'I' beams are engineered items using Microlam flanges and Performance Plus OSB (Orientated Strand Board) webs. Just as it is not possible to make a Wolf or Gang-Nail truss rafter without the correct products and equipment, the same rationale applies to these engineered beams - they must be factory made where quality of raw material and assembly is carefully controlled. These, like all other engineered timber items, are not simply pieces of 4 x 2 glued to a piece of OSB from the timber merchant. There are a number of waterproof deckings available, one such being Weatherdek 2, which uses a specially made moisture resistant tongue and groove chipboard with the upper surface sealed with a factory applied protective film. This system uses special adhesives to bond the decking to the 'I' beams held in place by the specified nailing system. The tongue and groove joints between the panels are glued, and a final waterproof joint covered tape is applied. The decking system provides a safe waterproof working platform which (with appropriate edge protection) satisfies the Work at Height Regulations. It also provides Trus Joists' Silent Floor, designed to provide a squeak free flooring system devoid of shrinkage and unevenness so often associated with traditional timber joist and board construction.

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