Setting 275 lb trusses – Commercial site
New job. Next week I will be part of a crew of 4 who will set fifty tall 275lb +/-Mono trusses on top of 16′ high walls. Yikes.
The GC has confidence in me, and I don’t want to let him down. This is going to be my biggest roof job so far. I was probably hired because I’m a half-decent carp who’s good at planning, foreseeing, problem solving – all without undermining the boss’ authority.
Can you guys pitch in with some advice, or whatever else comes to mind?
This is a commercial job: new roof over existing metal fab shop plus large addition. Overall span, outside to outside, is 70′. Building is 50′ long. Bearing wall (masonry) heights are 16′. This is going to be scary, though we will scaffold inside and out to our heart’s content.
My preference would be to hire a crane – the same monster that unloaded the truss package – so as to get her done quickly and safely, without exhausting our middle-aged bodies.
These 3.5/12 trusses are BIG and HEAVY and look FLOPPY:
- 25 mono trusses have 30′-6′ long bottom chords and 8′-9″ high ends.
- 25 mono trusses have 40′ bottom chords and 11′-2″ high ends, with an overhang on the high end.
Trusses will be set in pairs, on a staggered layout, with high ends abutting over a 2×8 top-plated center bearing wall. One roof plane is higher than the other: the new roof over the existing shop will be 2′-5″ higher than the one over the addition. This stub-wall area will be protected by an 18″ overhang. All trusses are 3.5/12 pitch, with 2×6 top and bottom chords, and 2×4 webs.
An existing ‘mid-span’ masonry wall (currently the exterior wall of original shop) will carry the heel loads from the high ends of each staggered pair mono trusses. The existing flat roof remains, so we will open it up to build short bearing stemwalls (2×8 top plated), so as to attain our 16′ overall height.
- We’ll gang mark the trusses for permanent internal lateral bracing, while bundled.
- Make reference marks at heels so as to get even soffits and fascias.
- Pre-mark 24oc layout on the 2x4s used for temp purlin bracing.
- Install bomber starter temp bracing on the far gable end.
- Strike layout (staggered on middle bearing wall) on bearing plates.
We intend to set the 40 footers first (the ones with the 18″ overhang on the high end), as we want to get the overhang bits out of the way. Also, these go over the existing roof, so we are a bit safer. The 30-6 footers can swing under those without getting hung up, I believe.
- Is it feasible to safely set these trusses by hand, with a crew of 4? or a bigger crew? This would be using push poles and pulling with lines.
- My preference is to hire a crane, save on labour, minimize the chance of accident. There will be a lot to do, what with lateral and diagonal temp bracing, staying on layout, not stepping off the scaffolding, etc. With the crane, we can get to permanent bracing in a day.
- When craning such large monos, do you guys hook up to a spreader bar, or do you sling with webbing and be done with it? I want to minimize noodling and flopping and enhance control.
- If craning, would you go with a different sequence, ie, the 30-6 first, then the 40 footers that have the 18″ overhang?
- When sill-plating over a high masonry wall (sill bolts set in wall), do you guys separate the wood from the wall with tarpaper or foam gasketing? We have no termites here in the BC Rockies.
- Sill bolts are not long enough to catch the triple top plate on one wall (we need the extra 1.5″). Is it kosher to bolt down the first 2 plates, then PL and lag the third one down? Truss hold-downs will be nailed into all three plates.
- Anything else to watch out for?