Trusses

Bearings

Unless the trusses are specifically designed, the truss must be supported directly on the ceiling chord immediately below the intersection of joist and rafter. Approval from the truss manufacturer must be sought where special design is necessary.

Diagram D2 TRUSS BEARING Diagram D2 - Truss bearing

Overhangs

The overhangs illustrated are acceptable if the manufacturer’s recommendations are followed when bracing the rafters and joists.

DIAGRAM D2A Truss Overhang Diagram D2(a) - Diagram illustrating overhang of a truss

Fixing trusses to walls

The manufacturer’s instructions should be followed when erecting trussed rafters. Usually, the instructions will include fixing the trusses to the wall plate using proprietary truss clips or strapping to both the wall plate and the masonry wall. In some cases, fixing using nails may be permissible.

At least 2 wire-round nails more than 100 mm in length and 4.5 mm diameter, and in accordance with I.S. 105-1: 1977 Wire and cut nails for building purposes, staggered and fixed at either side of the truss, should be used to fix each truss to the wall plate such that no damage to the punched metal plate fixings occurs. Trusses should not be notched to seat on wall plate.

Straps

Twisted galvanised vertical restraint straps

If a roof has a pitch of less than 15 degrees and/or the site wind speed is greater than 26 m/s and/or the building is located in an exposed location (and there is reason to believe high wind speeds may occur), additional vertical restraint straps may be required. A separate design to calculate uplift should be carried out based on Irish Standard I.S. EN 1991-1-4: 2005 + A1: 2010 Eurocode 1 (Eurocode 1: Actions on structures - Part 1-4: General actions -Wind actions ).

Twisted galvanised vertical restraint straps are usually 30mm x 2.5 mm in cross section and should be included at the truss nearest the gable, and at 2 m maximum centres along the roof.

Proprietary truss clips

Trusses can be held down by fixing them directly to the wall plate using proprietary truss clips. Galvanised straps should be used to fix the wall plate to the masonry; these straps should be at no more than 2 m centres or alternatively, the wall plate can be fixed using bolts at 1.2m centres. Where wall plates are butt-joined, the trusses should be strapped (hold down straps) at no more than 400 mm centres on each side of the joint. Please note: wall plate should have a width of at least 100 mm and a depth of at least 75 mm.

Diagram D3 TRUSS BEARING STRAPPING TO WALLS Diagram D3 - Truss bearing strapping to walls

Strapping trusses to gable walls

Gable walls should be tied to roof timbers as illustrated in Diagram D4 below for stability.

Straps at ceiling level

Straps to gable walls at rafter and ceiling level should be at centres less than 2m. At party wall, straps are not required. Please note you must build the straps into blockwork as courses are being built. Technical Guidance Document A of the Building Regulations specifies that the requirement for strapping at ceiling level depends on the height of the apex and the thickness of the wall. Ceiling strapping is required in all standard pitched roof houses.

Straps at rafter level

For holding down trusses 30mm x 5 mm thick galvanized steel straps fixed to at least 2 trusses and a noggin with 50 mm long x 3.35 mm diameter wire nails should be used for connections to rafters. These straps should be spaced at less than 2m centres at ceiling and rafter level. The turn-down of the strap must be greater than 100 mm; it should be tight against the block and the straps must bear on to a full block.

Diagram D4 - Strapping trusses to gable ends at ceiling and rafter level Diagram D4 - Strapping trusses to gable ends at ceiling and rafter level

Bracing

Adequate bracing and tying is very important in prefabricated trussed roofs to ensure satisfactory performance and ensure the roof structure acts as a unit. This section contains the minimum requirements that apply to most roofs. Additional bracing designed by an appropriately qualified person is required for irregular or unusual designs or roofs with large spans.

Typical examples of additional bracing are illustrated below. Bracing details must be in line with the recommendations set out by the truss manufacturer and the roof designer.

Minimum bracing requirements

Diagram D5 - Standard minimum bracing Diagram D5 - Standard minimum bracing

At a minimum bracing is to be provided as follows:

  1. At the intersection of ceiling joist, tie, and strut known as the node point. These braces are known as longitudinal binders.

  2. At ridge level, also known as longitudinal binders.

  3. At the intersection of the rafter and the strut, also known as longitudinal binders.

  4. Rafter diagonal bracing that runs from the eaves to ridge on both sides of the roof.

Table D1 - Maximum roof truss spans for standard bracing Table D1 - Maximum roof truss spans for standard bracing

To ensure additional bracing is not required and the standard minimum bracing requirements are applicable the following must be met:

  1. The greatest length of an unsupported external masonry wall,s i.e. between buttressing walls, piers, and chimneys is to be less than 9 m.

  2. Truss spacing should not exceed 600 mm.

  3. The span of the trusses is in accordance with the requirements for the height of eaves based on the angle of the roof and the appropriate wind zone.

  4. Where the ceiling below the roof truss consists of plasterboard that is either fixed directly to the bottom chords of the trusses or is fixed to battens that are fixed to the bottom chords of the trusses.

  5. The cross-sectional area of the bracing members is greater than 2130 mm2 and that the thickness of the bracing member is greater than 22 mm. 22 mm x 97 mm is the minimum bracing allowed.

  6. There should be no strength-reducing defects in bracing members.

  7. 2 nails that are at least 3.1 mm in diameter and have length greater than the depth of the brace plus 36 mm, constructed from corrosion resistant nails, should be used to nail the brace to every rafter it crosses.

  8. Braces should start at each gable or separating wall and continue uninterrupted for the entire length of the roof.

  9. A lap of at least 2 trusses should be provided where 2 lengths of bracing meet. Both should be double nailed to the rafters.

  10. At each end of the roof, at least 2 rafter diagonals are fixed to the underside of the rafter members at an angle between 35 and 55°. 45° is the ideal angle.

  11. Longitudinal binders span the entire roof.

  12. Chevron and rafter diagonal bracing extend the entire roof length. Between sets of bracing for the span between 2 trusses and single trusses adjacent to the faces of gable and party walls, this bracing is not required.

  13. Requirements from manufacturer should be adhered to at all times.

Rafter diagonal bracing

Rafter diagonal bracing should repeat continuously along the roof, fixed at an angle between 35°/55°. Longitudinal bracing should be tightly butted to the gable or separating wall. The lower ends of diagonal rafter-bracing members should be tightly butted against the end walls as closely as possible to the intersection between the wall plate and the wall.

Diagram D6 - Typical detail for rafter diagonal bracing Diagram D6 - Typical detail for rafter diagonal bracing

Chevron bracing

In addition to the bracing outlined above, the designer may specify that extra bracing be allocated. Chevron diagonal bracing may be necessary as detailed in the following diagrams. It is important to note that bracing should never cross a party wall.

DIAGRAM D7 Bracing Typical Trusses and chevron support Diagram D7 - Bracing typical trusses and chevron supports

Diagrams D8 and D9 below detail “fink” truss bracing members. Some bracing members have been omitted from the diagram for clarity.

DIAGRAM D8 Bracing to Fink Truss Queen tie Diagram D8 - Bracing to fink truss queen tie

DIAGRAM D9 Chevron Bracing to Internal Strut Diagram D9 - Chevron bracing to internal strut

Where the truss manufacturer, roof designer, or building designer specifies additional bracing, then the additional bracing must be included. Bracing details must be consistent with the truss manufacturer and roof designer’s recommendations. Please note additional chevron bracing may be required for spans greater than 11m, please consult with your structural engineer with respect to this.

The guidance provided above relates to the bracing requirements of “Fink” trusses only. In all other cases, as illustrated in D7 above, the Trussed Rafter Association (TRA) have provided standardised locations for chevron bracing which may be required alongside longitudinal and rafter diagonal bracing.

Diagram D10 below details a typical "Fan" truss arrangement with chevron bracing. For clarity, other bracing members were omitted.

Diagram D10 - Chevron bracing for spans over 8m Diagram D10 - Chevron bracing for spans over 8m

Mono Truss Bracing

Guidance on web chevron bracing for mono trusses is outlined below. This bracing may be required in addition to rafter diagonal bracing. The manufacturer and roof designer’s recommendations must be satisfied by bracing details.

DIAGRAM D11 Mono Truss Bracing Diagram D11 - Mono truss bracing

DIAGRAM D12 Mono Truss Chevron Bracing for Spans greater than 8m Diagram D12 - Mono truss chevron bracing for spans greater than 8m

The diagram above shows extra chevron bracing as may be required where a mono truss’s span exceeds 8m. For clarity, other members have been omitted.

DIAGRAM D13 Mono Truss Chevron Bracing for Spans greater than 8m Diagram D13 - Mono truss chevron bracing

Chimney and hatch openings

Where inclusion of hatches and openings cannot be achieved within the standard spacing of trusses, the guidelines illustrated opposite for increasing spacing and the recommendations from the truss manufacturer must be followed to ensure no individual trussed rafter is subject to too great a load.

Definitions:

S: Standard spacing of truss – 600mm

C: Spacing which has been increased

B: Spacing which has been reduced

The conditions that must be adhered to when increasing the spacing of rafters have been detailed below.

NOTE: It is never permissible to cut a prefabricated truss.

Condition No. 1

Illustrated below is a method for adjusting the spacing between 2 trusses by approximately 10% in accordance with the first condition. This spacing increase causes no significant increases in stress in tiling battens or ceiling material.

DIAGRAM 14 Chimney Opening less than 660mm Diagram D14 - Hatch opening less than 660mm

The above example is applicable for a standard block chimney with a single flue serving one house only. In accordance with Condition 1 the the increased spacing, C can be up to (1.10 x S), the standard spacing, while the reduced truss spacing, B must be no more than (0.95 x S).

For trusses at 600 mm spacings, the max spacing that can be achieved by this method at C is approximately 660 mm.

Condition No. 2

Illustrated below is a method for adjusting the spacing of trusses where openings require an increase of spacing that is greater than 10% of and up to twice the standard spacing (i.e. between 660 mm and 1200 mm) in accordance with the second condition. Infill rafter(s) and ceiling joist(s) must be provided for extra support to tiling battens and ceiling material. Purlins, binders, ridge boards, and trimmers are all used to support infill timber in line with each truss.

DIAGRAM D15 Chimney Opening greater than 660mm Diagram D15 - Chimney opening greater than 660mm

For trusses at 600 mm spacings, the max spacing that can be achieved by this method at C is between 660 mm and 1200 mm.

DIAGRAM D16 Two Chimney Hatch at party wall Diagram D16 - Two chimney hatch at party wall

The maximum span of any tiling or slating batten (44 mm x 36 mm) must never be greater than 670 mm. In Diagram D16 above nailing 2 battens to the last truss on either side of the party wall would be sufficient to reduce the maximum span of the tiling or slating batten to 668 mm.

Condition No. 3

Illustrated below is a method for adjusting the spacing for openings more than twice the standard truss spacing and up to three times the standard truss spacing provided that two trimming trusses fixed with nails at 300 mm centres are provided on each side of the opening along all members. Ceiling material and tiling battens must be provided extra support. This support is provided by infill rafters and ceiling joists. Purlins, binders, ridge boards, and trimmers are all used to support infill timber in line with each truss.

Where spacing is required to be more than three times the standard truss spacing, design by an engineer who is qualified by examination, in private practice, and in possession of professional indemnity insurance is required.

DIAGRAM D17 Infill mono truss to chimney Diagram D17 - Infill mono truss to chimney

For trusses at 600 mm spacings, the max spacing that can be achieved by this method at C is between 1200 mm and 1800 mm. Double trusses nailed together at 300 mm centres are usually required on either side of an opening that exceeds 1200 mm.

Take care in nailing the purlin and purlin support to the rafter strut to avoid damage.

DIAGRAM D17(a) Chimney two flues Diagram D17(a) - Chimney two flues

The diagram above illustrates the truss spacing dimensions and infill rafters for a standard two flue block chimney where both are in the same house and chimney is not on a party wall.

The guidance of Diagrams D18 and D19 should be followed when supporting infill rafters and ceiling joists where they are required.

Trimming around chimneys

Trimming may be unavoidable in situations where truss spacing is increased to make space for a chimney. In these cases, the guidance given above must be followed.

Diagrams D18 - D21 illustrate how to correctly provide support to the infill timbers that are supporting the ceiling materials and tiling battens.

It is important to note that all timbers must be at least 40 mm from the chimney or 200 mm from the flue for fire safety reasons.

Diagram D18 - Typical support detail of loose timbers Diagram D18 - Typical support detail of loose timbers

To avoid damage, care should be exercised when nailing the purlin and purling support to the trussed rafter strut.

DIAGRAM D19 truss and Cut Timbers Diagram D19 - Truss and cut timbers

The above diagram shows the typical arrangement of truss and cut timbers and the typical trimming arrangement around a chimney stack. The load from the infill timbers is transferred to the double trusses on either side as well as the next trusses.

Rafters

The diagrams below illustrate the typical roof section through chimney trimming and specific junction details. It should be noted that this is a sketch through the infill timbers between the trusses.

DIAGRAM D20 Section Detail A of Chimney trimming Diagram D20 - Section of Detail A of chimney trimming

DIAGRAM D21 Section Details B C and D of Chimney trimming Diagram D21 - Section of Details B C and D of chimney trimming

Specially designed and fabricated “stubbed” trusses can be used as an alternative to using loose timbers as trimming. An example of this type of truss is a bobtail truss. A bobtail truss is not adapted from an ordinary truss by cutting it on-site, it is designed and fabricated specifically for such situations. The manufacturer’s instructions must be adhered to when erecting a bobtail truss.

Image - Bobtail stub end truss Image - Bobtail stub end truss

It is important to note that all timbers must be at least 40 mm from the chimney or 200 mm from the flue for fire safety reasons.

Water cistern loads

To prevent deflection or cracking of the ceiling below the cistern, it is vital that the load is spread over a number of trusses.

To spread the load of the water cistern, the cistern should be placed on two primary bearers and with two secondary bearers that rest on spreader beams. The spreader beams should be located as close as possible to the node points of at least four trusses. The table below gives the minimum dimensions for support members.

Table D2 - Minimum size of support members for water cisterns Table D2 - Minimum size of support members for water cisterns

Diagram below details typical construction details to ensure adequate support is provided for a water cistern.

DIAGRAM D22 Water Cistern Diagram D22 - Water cistern

Cover for cisterns

Cistern should be covered with a suitable lid that is rigid but not air tight.

Water storage capacity

For a 3-bedroomed house 212 litres is the minimum actual capacity required while 340 litres is the minimum actual capacity required for other dwellings.

Insulation at rooflights and around tanks

DIAGRAM D23 ROOF LIGHTS Diagram D23 - Roof lights

DIAGRAM D24 WATER CISTERN INSULATION Diagram D24 - Water cistern insulation

Important construction notes

Ensure that the tank while covered is not air tight, and that it is accessible for cleaning and replacement. Locate an overflow in such a position as to give adequate notice of overflow while not causing nuisance or dampness in the dwelling. Provide insulation to the top and sides of the tank with the bottom of the tank left un-insulated. Continuity of insulation is achieved as shown above. In cases where tanks are raised to increase the head pressure, insulate the roof and tank separately with every side of the tank being insulated in this case while ensuring adequate ventilation to the tank.

Pipes should always be located in heated spaces to avoid freezing, such as under the attic insulation or below ceilings. Insulate all pipes located outside of the thermal envelope. It should be noted that insulating pipes only slows the onset of freezing and does not completely protect against freezing.

Values for different pipe sizes and insulation thicknesses for different thermal conductivities for 12 hours’ protection in normally occupied buildings can be found in Table 1 in the Appendix of Technical Guidance Document G – Hygiene, Building Regulations 2008, as amended and corrected July 2011.

Trap doors

Where possible, trap door openings should fit between the regular spacing of trusses. Where this is not possible or not practical, design the trusses adjacent to the opening to accommodate the opening.

DIAGRAM D25 Trap Door Openings Diagram D25 - Trap door openings

The diagram above shows a trap door accommodated within the standard spacing between trusses, framing members have been notched over battens and fixed.

Gable Ladders

In cases where a roof overhang is required at a gable end, gable ladders are used. Gable ladders must be securely nailed directly to the last truss and supported evenly by the gable blockwork. For gable overhangs greater than 300 mm from the outside face of the wall, special design may be required. Barge boards and soffits can be nailed directly to the gable ladder.

DIAGRAM D26 constructing gable ladder Diagram D26 - Constructing gable ladder

Cavity must be closed by way of a proprietary closer along the top of the gable end. It is also important to ensure that all timber in the gable ladder is treated with a suitable preservative.

Forming hips in a roof

This can be done in two ways:

1) Forming the hipped section of the roof from cut timbers.

2) Using mono trusses designed and manufactured specially for the roof.

Method 1

The diagram below shows a typical hipped roof with cut timber hips and prefabricated trusses. This method is not for use in situations where the gable is greater than 5 m. Should the gable measure more than 5m in length Method 2 detailed below should be followed.

Appropriate hip details and components can be provided by the truss manufacturers as part of the design and manufacture of the roof.

DIAGRAM D27  Hipped End on Gable less than 5m in length Diagram D27 - Hipped end on gable less than 5m in length

It is important to note that where a cut hipped end is transferring load to a load-bearing partition, the load-bearing partition must have a separate foundation.

DIAGRAM D28 Hipped end with trusses Diagram D28 - Hipped end with trusses

The diagram above shows the typical detail of the junction with trusses at the top of hips.

Method 2

Hip is formed using a specially designed girder truss and mono trusses along with cut infill timber. The following should be noted when using this method to form the hipped end:

  • Ensure that the number of flat top plies, usually 2 or 3, as set out in the manufacturer’s instructions, are the same as the number in the girder truss. A girder truss is a truss that comprises 2 or more trusses fixed together that are designed to carry larger loads, e.g. loads from other trusses.

  • Ensure bearing details match the manufacturer’s instructions.

  • Fix braces to the top chord of the flat top trusses and the girder truss as is set out in the manufacturer’s instructions.

  • To support the mono trusses at the girder truss, use truss shoes.

  • To ensure support for the hip rafter, birdsmouth the hip rafter over the girder truss.

DIAGRAM D29 FORMING HIPPED END Diagram D29 - Forming hipped end

The diagram above shows a hipped roof created using a specially designed girder truss and mono trusses along with cut infill timber. Because much of the load from a hipped roof is transferred to the walls at the corner, care must be exercised when fixing and joining the wall plate in order to avoid kicking.

Appropriate hip details and components can be provided by the truss manufacturers as part of the design and manufacture of the roof.

DIAGRAM D30 foot of Hip

Diagram D30 - Typical detail at the foot of a hip

In hips that are heavily loaded, an angle tie and dragon tie should be used as an alternative.

Handling Trusses

Care should be taken in storing, moving, and fixing trusses as they are designed to be loaded in a particular way. Never notch or drill trusses to accommodate services. Never accept damaged or distorted trusses – handle trusses with care.

DIAGRAM D31 Do not accept Damaged trusses Diagram D31 - Do not accept damaged trusses

The diagram below illustrates the correct mechanical handling technique for trusses.

Diagram D32 - Handling trusses into place Diagram D32 - Handling trusses into place

Preventing roof spread

Due to the magnitude of the outward thrust generated at the corners under the hip, it is necessary to take measures to reduce this thrust. There are three main measures taken to ensure the roof does not spread.

First, the joining wall plates are half lapped as shown. Secondly, the corner of the wall plates are reinforced with a galvanised steel strap as shown also. Finally, the corner is reinforced with an angle tie that is securely nailed to the wall plates. Additionally, when the load is very large, a galvanised steel dragon tie could be used to join the hip rafter to the angle tie.

Diagram D33 - Tying hipped end at corner wall Diagram D33 - Tying hipped end at corner wall

Reinforce the corner using a galvanised steel strip. This is strongly recommended.

Add an angle tie; securely nail it to both wall plates.

Construction details

Diagram D34 - Foot of hip Diagram D34 - Foot of hip

Diagram D35 - Typical cutaway at corner Diagram D35 - Typical cutaway at corner

Diagram D36 - Dragon fly steel tie Diagram D36 - Dragon fly steel tie

A galvanised steel dragon tie should be used for heavily loaded hips. This tie is securely fixed to the hip rafter and also to the angle tie.

Diagram D37 - Hipped end corner gable wall Diagram D37 - Hipped end corner gable wall

The diagram shows a typical junction where a hipped roof meets the corner of a wall. Due to the outward pressure applied on the wall from the hip, the measures outlined above are required to resist this pressure.

Valley rafters

Sufficient support is required for valley rafters as they generally carry large loads. To reduce deflection and stresses, a properly supported purlin may be extended to provide support to the valley rafter. Care should be taken to ensure that all rafters bearing on the valley rafter are fixed correctly.

Where a valley rafter is birdsmouthed over wall plates or purlins, ensure that the rafter is not notched too deep. In general, valley rafters will be large, deep members due to the loads that they must bear and also to ensure they can be birdsmouthed sufficiently. Rafters bearing on the valley rafter from either side should bear at approximately the same point.

Hip rafters

Generally, intermediate support for a hip rafter is provided by a purlin. This purlin must be appropriately supported. Hip rafters should usually be large, deep members supported at purlin level and at wall plate. The reason for the size is the same as that of valley rafters: to avoid over-cutting at support points due to excessive birdsmouthing. Rafters that bear on hip rafters from either side should bear at approximately the same point.

Ridge boards

Ridge boards are lengths that the rafters from either side can bear against. It is important that rafters bearing from either side bear at approximately the same point.

Water cistern

Adequate support of water cisterns is important in cut timber roofs. Cistern(s) should bear on load-bearing partitions where possible. Where the cistern load is not supported by load-bearing partitions joists carrying the cistern should be specifically designed, by a suitable qualified competent person.