Gambrel/barrel roof trusses

Started by paulg25, November 24, 2013, 02:42:17 PM

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paulg25


Hi,  want to build a roof as per the drawing above.  Any ideas about timber sizing and joining?
Thanks.

paulg25

Sorry.... trying to include a pdf drawing.  :(


paulg25


paulg25


Don_P

Can you lose the midheight bend and have 4 planes instead of 6?.  What is your snow load?


highlandva

I built a shed with a gambrel roof a year ago and I figured out how to design and build my trusses from NavyDave, check out his project http://countryplans.com/smf/index.php?topic=12047.msg155084#msg155084. Good luck on your build.

Don_P

There are a few things that would probably be better to do a bit differently.

MountainDon

#7
Designs that amateur builders will come up with have no professional pedigree, no guarantees that the home made trusses can withstand any particular loads. In plain language, no professional engineering knowledge behind the design. Engineers, partly because of professional liability issues are mostly reluctant to dispense design advice on a free forum. We do have one or two master / professional carpenters on board here. Don_P is the only such member presently active.

Many design problems can be figured out by anyone who spends some time with the code book that covers most areas of the USA. I do not know what sort of codes or regulations you may be subject to in the UK. The code most of us use, the IRC, illustrates designs / techniques that are proven to be sound. There are tables for some things like standard rafter roofs that are easy enough to comprehend. There is a prescription for how to build to suit various spans and loads. Other things may take more time and effort to understand correctly. Some things such as self built trusses of any type or pier and beam foundations are not included in our IRC for reasons that include the inability to control the quality of materials and technique to build them.

Our member projects include those that have been built with professional engineering expertise, that follow the various applicable codes and have been inspected for compliance. Some of the projects displayed here do follow good practices and the code in general, but have been built in areas with lax or nonexistent code enforcement. Other projects in these areas of lax or zero enforcement contain poor design elements and/or poor techniques that may compromise structural integrity and personal safety. As a result some projects illustrated here may contain questionable design / technique, carry no guarantees as to suitability for any purpose and should not be used as a basis for new projects.

I am reasonably sure that if you can supply details like snow and wind loads and whether or not there will be a floor or ties between the walls where the rafter meets the wall top, Don_P or some of our other more knowledgeable members can help sort out design issues to some degree. If this roof is for a habitable building it would still likely be a good idea to seek out some professional advice.


Just because something has been done and has not failed, doesn't mean it is good design.

Don_P

That's what I was trying to say and not say for risk of offending... but you did a better job  :)
UK, does open up a whole other kettle of fish. Technically this would be a non prescriptive design here but this is a rough sketch of one way to approach something like this if you can lose a good bit of the barrel effect and simplify the design.


It is sort of a continuation of a discussion from earlier in the week about a 2 story A frame plan, this might help show what I was talking about in another way.  The rafters of the lower pitch are basically canted walls and are tied by the floor. They are topped by a series of joists. The upper rafters are tied by those joists, in this case they are ceiling joists. Everything is tied by those floor and ceiling joists to resist outward thrust. The upper set of rafters could, instead of being at the 5/12 pitch shown, continue at the same pitch as the lower rafters, and we'd have the A frame from earlier in the week.

I drew this at 18' wide, 8' of headroom under the ceiling joists provides ~12' of full height width upstairs. Lower rafters are 10' stock, tie is 12' material, upper rafters are 8' stock, or better, get 2 out of a 14'er. If framed strictly as shown with the members stacked I would gusset both sides of the joints.

An 8,000sf+ house/complex hit my computer tonight, architect drawn, wrong state, wrong code, wrong snow load, no engineer, and I've seen several detail problems already ... it isn't always the amatuers, although I did say architect  ;D


paulg25

Thanks for the responses.  I originally designed it with a barrel roof but the trusses would be too complicated.  I don't like the look of a standard gambrel roof, hence going for the 6 'panes'.  Don't know what snow loads need to be in UK... will look it up.