Page 87 Chapter 9: Truss Preparation Most Common Mistakes: 1. Expecting ganged (multi-ply) trusses to arrive nailed together. 2. Placing fully recessed purlins at a spacing other than what is shown on building plans. 3. Following a discontinuous purlin layout (keep spacing same, unless noted on building plans). 4. Failure to use a framing square to mark fully recessed purlin locations. 5. Placing fully recessed purlin joist hangers on wrong side of layout mark. 6. Neglecting to place a nail through every joist hanger hole with wood behind. 7. Ridge fully recessed purlins installed too close to ridge (truss peak). 8. Purlin and truss tops not flush. 9. With ENDWALL OVERHANGS failure to read appropriate Chapter (52, 53, 54 or 55) prior to moving forward with marking end trusses. IMPORTANT: End trusses are often designed with a different interior truss web configuration than interior trusses. This allows for end trusses to bear upon some (or all) endwall columns. Often, these bearing points will be labeled on truss with a tag, stamp, or permanent marker. If any question about truss orientation, contact your Project Coordinator prior to installation. Improper installation, or end truss location (have on wrong end of building, flipped end for end, or using an end truss in building interior), could result in failure to pass building inspections, costly and time consuming repairs, building failure, property damage, injury, or death. BEFORE GOING FURTHER: Look carefully on building plans at fully recessed purlin spacing. Any deviation and chances are there will be either too many or a shortage of fully recessed purlins. Neither one is correct answer! Why might purlin spacing or size vary along slope of roof? In snow country – drift loads at roof peak increase snow loads on each side of ridge. Wind exerts greater forces in edge zones (within 10% of least building width at eave, ridge and building ends). Without turning end-for-end stack trusses one on top of another, carefully aligning them. While doing so, inspect both sides of each truss for missing or poorly placed steel connector plates. It does not occur often however is much easier to rectify before going further. Truss eave (or heel) end, will be beginning measuring point for fully recessed purlin locations, and work progresses upwards toward building peak. Avoid temptation to work from peak down.
Transcript
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Chapter 9: Truss Preparation
Most Common Mistakes:
1. Expecting ganged (multi-ply) trusses to arrive nailed together. 2. Placing fully recessed purlins at a spacing other than what is shown on building plans. 3. Following a discontinuous purlin layout (keep spacing same, unless noted on building plans). 4. Failure to use a framing square to mark fully recessed purlin locations. 5. Placing fully recessed purlin joist hangers on wrong side of layout mark. 6. Neglecting to place a nail through every joist hanger hole with wood behind. 7. Ridge fully recessed purlins installed too close to ridge (truss peak). 8. Purlin and truss tops not flush. 9. With ENDWALL OVERHANGS failure to read appropriate Chapter (52, 53, 54 or 55) prior to moving forward with marking end trusses.
IMPORTANT: End trusses are often designed with a different interior truss web configuration than interior trusses. This allows for end trusses to bear upon some (or all) endwall columns. Often, these bearing points will be labeled on truss with a tag, stamp, or permanent marker.
If any question about truss orientation, contact your Project Coordinator prior to installation. Improper installation, or end truss location (have on wrong end of building, flipped end for end, or using an end truss in building interior), could result in failure to pass building inspections, costly and time consuming repairs, building failure, property damage, injury, or death.
BEFORE GOING FURTHER: Look carefully on building plans at fully recessed purlin spacing. Any deviation and chances are there will be either too many or a shortage of fully recessed purlins. Neither one is correct answer! Why might purlin spacing or size vary along slope of roof? In snow country – drift loads at roof peak increase snow loads on each side of ridge. Wind exerts greater forces in edge zones (within 10% of least building width at eave, ridge and building ends). Without turning end-for-end stack trusses one on top of another, carefully aligning them. While doing so, inspect both sides of each truss for missing or poorly placed steel connector plates. It does not occur often however is much easier to rectify before going further.
Truss eave (or heel) end, will be beginning measuring point for fully recessed purlin locations, and work progresses upwards toward building peak. Avoid temptation to work from peak down.
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Starting at eave ends, place a mark on truss top at fully recessed purlin spacing. This mark will be each fully recessed purlin joist hanger’s “uphill” side. See Figure 9-1
Figure 9-1: Fully Recessed Purlin Spacing
Paying attention to truss faces becoming each future truss pair’s “outside”, use a framing square to place marks for all fully recessed purlin joist hanger locations on interior truss outside faces. Mark locations on end truss inside faces. See Figure 9-2
Figure 9-2: Marking End Truss Faces
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Ridge Fully Recessed Purlins
Ridge fully recessed purlin “uphill” side can be no closer to peak than five inches. This may result in space between ridge and next fully recessed purlin “downhill” being decreased. Ridge fully recessed purlins can be further from truss peak than minimum distances, without negatively affecting building. Roof ridge caps attach to roof steel with metal-to-metal screws, rather than to ridge fully recessed purlins, so ridge fully recessed purlins need not align with future ridge cap fastener locations. See Figure 9-3
If ridge fully recessed purlins are “uphill” further than this, roof steel will have an inadequate overhang at eave and challenging situations will result! Pay careful attention. Ridge fully recessed purlins can be a different grade or size than other fully recessed purlins. If two fully recessed purlins appear to be placed close together, double check building plans and material take-off list to verify more materials than anticipated are not used.
Interior Trusses
These have to be nailed into pairs. Refer to building plans and engineered truss drawing(s) for nailing specifics. While ganged truss orders are placed with specific instructions to deliver un-nailed, occasionally trusses will arrive nailed together. Carefully check quantity and spacing of nails for adequacy. Planning on using closed cell spray foam insulation for condensation control?
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If answer is yes, apply a good urethane or acoustic caulk between truss top chords before nailing or screwing them together, as an air seal. Failure to do so can result in warm moist air getting in contact with cooler roof steel between trusses, resulting in premature decay, mold and mildew.
Standard Hansen Buildings feature a minimum 2-ply truss for interior clearspans. If attempting to use as single trusses, catastrophic building failure chances will be high. This could result in property damage, injury or death.
Before nailing, verify no truss has been turned end-for-end. While this usually has no structural effect, prefabricated trusses are built in a “jig” and can sometimes end up slightly asymmetrical. Nailing together in same direction they were manufactured insures a greater interior (or “web”) member alignment probability.
While “out of alignment” web members do not normally create structural problems, they can be less attractive. MINIMUM nailing requirement (more may be required, verify from truss drawing provided by manufacturer) will be for (2) 10d commons at 12” on center (o.c.), staggered through all members. As an example engineered truss drawings may have nailing language such as “3 rows staggered at 0-9-0 oc.” The 0-9-0 is Feet-inches-sixteenths or in this case every nine inches. By staggered, they mean do not place adjacent nails along member length same distance from board edge (to avoid splits along wood grain).
Next, stand trusses up at a level point inside building with a truss pair on each truss-bearing column’s notched side.
Install fully recessed purlin joist hangers on each truss pair side using 10d common nails through every hole with wood behind it. These nails can be counted towards nailing into pairs requirement above.
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See Figure 9-4 or (Figure 9-5 when fully recessed purlins are larger dimensions than truss top chord.) Bottom hanger edge is to be located below truss top chord top, by fully recessed purlin depth:
While these are presumed dimensions of purlins, please field measure yours to insure top of purlins and top of trusses will be flush.
Figure 9-5: Joist Hangers on Fully recessed purlins Taller Than Top Chords
End Trusses
On end truss outside, starting 6” from heel, install 2x4 siding backing flush to truss top. Use two 10d common nails every 12”. If end truss “rise” is greater than five feet (e.g. 4/12 slope and span over 30’) a 2x4 ½ height horizontal siding backing will also be required. These horizontals are integral to end truss web bracing and must be fastened to each web member being crossed and truss top chord with four 10d common nails. Bottom chord siding backing will be installed only after end truss is securely fastened to corner and endwall columns AND all endwall door framing is in place. See Figure 9-6
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Figure 9-6: 2x4 Siding Backing to End Truss Install joist hangers to end truss inside face, except where their location conflicts with an end column. In this case hanger can later be installed on column, spaced appropriately.
Permanent Web Bracing
While great effort is made to include web bracing material with all building kits, each truss design program uses different parameters. Hansen Buildings rarely sees final engineered truss drawings prior to shipment so it is impossible, in advance, to verify all web bracing requirements. Therefore, providing adequate web bracing material for every possible case, becomes impossible. Any excess materials for web bracing required, beyond what is originally shipped with building kit, will be up to building owner to provide. Install truss web bracing as necessary. This bracing, when called for, is typically 1x4, and is usually installed as specified on building plan DTL 15/S-2. See Figure 9-7 and Figure 9-8
