Stick frame rafters vs site-built truss for 20x30 1.5 story

[DutchMo]

Hi all.  I recently got the 20x30 1.5 story plans and I'm still trying to wrap my head around some of the details.  I'm a novice at this so please excuse me if I am missing something obvious.

Just curious - is there any structural benefit to the site built trusses described in the 20x30 1.5 story plans, or is it just ease of assembly?  From the way the truss looks, it doesn't appear to prevent wall thrust the way an engineered truss would (hence why the rafter ties are still used across the open portion of the cathedral ceiling, right)?

Would there be any structural difference if I switched to a traditional ridge board and rafter build?  I'm thinking it might be easier to install  traditional rafters instead of trusses - especially if I made a temporary floor across the open section (from the loft area to the opposite wall). 

My reasoning is the Working Alone book recommends against trusses because they are unwieldy for a single person.  I'm not sure if that would require additional structural members, though.

[John Raabe]

To fully take the outward thrust off the sidewalls you would need a structural ridge beam that would need one or two bearing post(s) down to a foundation footing. Then all the loads will be downward. That should be sized locally for your snow loads.

You might explain the situation to the truss supplier as their trusses would likely be lighter than the site built design. They can probably deliver to the loft floor and may have a way to site assemble two sides.

[Don_P]

A traditional ridgeboard and rafter roof system requires a tie across the lower third of the rafters to restrain outward spread. What John is proposing is a ridge beam that the rafters hang from. With that support there is no tie required.

It looks like you are in 20 psf snow country, if so and if you can locate support posts at each end and in the center of the 30' ridge length then a double 11-1/4" or a single 14" deep LVL would fill the bill. The supplier can check the size.

[DutchMo]

Thanks Don and John.  It looks like raising a ridge beam would also be a challenge without help.  I may just hire the whole roof out, but its good to know my options.

It looks like there is a truss company within range of my place, so that may be the way to go.  I really like the trusses astidham used - scissor trusses with 12/12 outside and 10/12 inside, I believe.  I'll have to contact the truss company and see what they recommend.

I'm also considering adding a shed or gable dormer over the loft portion.  Any idea on how that would work in conjunction with trusses?

[Don_P]

Typically they double or triple the trusses on each side of the dormer and you field frame in between.

[MountainDon]

If you are shopping for factory made trusses ask about having a raised heel truss. They are built to help get sufficient insulation in the space above and near the side walls. Traditionally designed trusses are often deficient in that area.  If asking for a scissors truss that area above and near the side wall gets pretty small without some special effort in design. Poor insulation in that area leadfs to heat loss through the roof causing ice dams on the roof. Of course that partly depends on the winter climate.

[UK4X4]

heres some scissor trusses on my build....I have high snow load so the are overbuilt..shows the insulation space at the wall
mines 26ft wide and 90# snow load


V's another design where it tapers right down to the wall plate

[DutchMo]

Thanks all.  Mountain Don, good tip on the raised heel scissor truss.  I think that will work well. 

UK4x4, thanks for the pictures.  Those are useful to see what I might be looking at.  Fortunately our snow load isn't that high - we're about 20 lb/sqft. 

[scody311]

Hey Dutchmo, how is this build going?

[DutchMo]

Hey scody,

Thanks for asking.  The build is unfortunately not so much a build as an adventure in financial savings right now.  I've got the land, and I got as far as having a driveway put in and some trees cleared, but now I'm stuck waiting for my bank account to refill.

I decided I don't want to start until I have all the money together to at least get dried in without stopping.  Also, I got a new job, which will help to fill the bank account, but has cut down on my free time dramatically.

I should probably start my own owner/builder thread, but I haven't yet because I haven't built anything yet (except for the land clearing, which I hired out).  Oh well, it's a marathon not a sprint!