Attic Construction Using Engineered Timber Components

This section deals with the construction of attic roofs incorporating the engineered timber components described above for the floor diaphragm, and explores these and other new developments in engineered components for the roof element of the attic. This section is therefore particularly relevant for new house construction, but is equally applicable to the replacement roof when building an attic on an existing structure, i.e. roof replacement.

Although most of the discussion above has referred to 'floors', the sloping rafter element of one side of an attic is in effect a floor tilted to the appropriate pitch. Instead of carrying the dead load of the furniture and fittings and the live load of the occupants, the rafter plane carries the dead load of the tiles (and snow), and the live loads of winds plus possible loads from any high level ceiling and water tanks installed in the space above. Using engineered timber components for attic rafters is similar to the use of these components in floors. Trimming openings in the floor for a stairwell, becomes a trimmed opening for the dormer of the roof window. Ease of access for installing services is equally necessary through rafters as it is through the floor, and the larger spans required of attic rafters are easily achievable with engineered timber components. Some of these engineered components can also be used for purlins on the smaller spans, and on longer spans engineered components can be produced to the length required without the necessity for joining soft wood timber purlins, but of course may need internal purlin supports. The depth of engineered timber components also allows excellent space to install a large thickness of insulation to give better performance without the need for battening down the sloping ceiling area of a conventional solid timber rafter or even that of an attic truss rafter, and also allows good space for ventilation. Special end detailing is of course necessary at the eaves to wall plate or floor diaphragm construction, as the floor diaphragm is being used as a tie in most instances. The engineered timber rafters form simple 'couples', as illustrated in Fig. 1.1, but are often able to carry much greater loads. Typical wall plate and eaves details for 'I' beam rafter construction can be seen in Fig. 4.6.

Reference can also be made to illustration 7.16 which shows an attic construction using Gang-Nail fabricated floor and roof beams.

Ol 00

TJI4 Joist Hole charts - Round, Square and Rectangular Holes

No field cut holes in hatched zone

Minimum distance from Chart A

2 jf p, appims to 9)1 round holes including 3$ mm hofei

Minimum distance from Chart R

No field cut holes in hatched zone

No field cut holes in hatched zone

Minimum distance from Chart A

2 jf p, appims to 9)1 round holes including 3$ mm hofei

Minimum distance from Chart R

No field cut holes in hatched zone

150 mn A 38 mm hole can be cut J _ anywhere in web outside hatched area

D-i 2.x diameter D2 of largest hole (minimum)

Do not cut holes larger than 38 mm in cantilever

150 mn A 38 mm hole can be cut J _ anywhere in web outside hatched area

D-i 2.x diameter D2 of largest hole (minimum)

-2_ 2.x diameter Li of largest hole (minimum)

Do not cut holes larger than 38 mm in cantilever u o f

DO MOT Cut or notch flange

Fig. 4.5 Typical cutting restrictions in timber I beam. Technical Guide - United Kingdom/Ireland; Silent Floor® and Roof Framing Systems; Reorder #GB-1001. Copyright (2002) Roise, Idaho: Trus Joist, a Weyerhaeuser business. Reproduced with permission. All rights reserved.

Bevelled plate

Bevelled plate

Bevelled bearing plate required when pitch exceeds 1.4°

TJI® blocking panel or 32 mm TimberStrand™ LSL shear blocking

Bevelled bearing plate required when pitch exceeds 1.4°

- adjacent span maximum 600 mm maximum when modified

Web stiffeners Additional nailing or twist required each strap and backer block may be side at R7W required

Bevelled bearing plate required when pitch exceeds 1.4°

Pitch limitations: 14°-45°

Variable slope seat connector Verify capacity

32 mm TimberStrand™ LSL V-cut shear blocking or TJI® locking panel

Variable slope seat connector Verify capacity

Birdsmouth cut

Allowed at low end of joist only

Bevelled web stiffeners on both sides

38 x 89 mm solid timber block for soffit support

3 adjacent span maximum 600 mm maximum when modified

Bevelled web stiffeners on both sides

38 x 89 mm solid timber block for soffit support

Cl hjHIKY.

Fig. 4.6 Typical I beam eaves details. Technical Guide - United Kingdom/Ireland; Silent Floor® and Roof Framing Systems; Reorder #GB-1001. Copyright (2002) Boise, Idaho: Trus Joist, a Weyerhaeuser business. Reproduced with permission. All rights reserved.

Was this article helpful?

0 0

Post a comment