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-   -   Roof Framing: load paths? (http://www.diychatroom.com/f19/roof-framing-load-paths-136853/)

AlexHouse 03-12-2012 08:53 PM

Roof Framing: load paths?
 
I'm in the design phase of a home right now and as I'm designing I want to understand HOW the house will be built rather than discovering the problems of bad design during the actual building process. The final part of the design phase will be to have an engineer vet the design but what I want to avoid is the engineer having to do serious remedial work in order to make a design on paper buildable on the ground.

That said, I'm running into the wall of my own ignorance on the issue of how to frame this roof.

http://img818.imageshack.us/img818/4950/roofframing.jpg


Here's the problems I see:

I don't like the practicality, from a water shedding perspective, of the gable roof planes joining together in the corner, so I'll be revising this by pushing the gable over the the right, thereby expanding that corner.

The reason I placed this 8/12 pitch gable at the corner, with a 4' eave, is that by the time it intersects the left exterior wall, it's already 32" above the fascia top, thus giving me more height closer to the walls in the 2nd floor. I'm trying to maximize living space under the roof.

Now to the issue that I'm grappling with - the way I've seen roofs framed the common rafters all run perpendicular to the ridge but if that is done in this case 1.) what would be supporting the weight of the winged gable, the 4' eaves (just sheathing and a ladder frame attached to the last common rafter built over top of the exterior wall?) and 2.) where would the loads be transferred due to the eave having no load path that I can see?

Here are the particulars on the roof:

First floor ceiling height = 122 7/8"
2x12 ceiling joist + 3/4' sublfoor = 12" -------> 134 7/8"

Fascia top = 128"
Rafter intersects with wall = 128 + ((8/12)*4'=32) = 160"
2nd floor stub wall height = 160" - 134 7/8" = 25 1/8"
2nd floor ceiling height = 120" ------> 134 7/8"+ 120" = 254 7/8"

Ridge height = 339 1/2"

I can put collars to tie the two roof planes together at the 2nd floor ceiling, but isn't most of the roof thrust occurring down at the bottom of the roof? Secondly, the rafter will be pushing load down onto the short 25" 2nd floor stubb wall and is there a method to build that wall so that it can resist the outward thrust of the roof? Thirdly, is there a way to use the 2x12 floor joists as bottom collar ties?

I've still got a year or so before I start the build, so I have time to learn, and I would appreciate any feedback that people with experience and knowledge could give me on this roof design.

Thanks for your help.

titanoman 03-12-2012 10:49 PM

Too late in the day to play architect/engineer. Hire one.

WillK 03-13-2012 11:52 AM

In essence, your entire roof load is going into the valley, and that joist which is carrying that entire load is transmitting it all to the corners of the house. It doesn't sound right...

Have you tried searching around to look at designs at websites where you can browse them? An example where I found the inspiration for my concept is www.coolhouseplans.com

It seems that a gable roof with dormers where the walls can carry roof loads is better for not concentrating a large area of loads into a single structural member.

House Engineer 03-13-2012 05:59 PM

Your design is not an economical roof framing scheme, but if you need it to look that way for the "look" of it, you could frame the roof as follows.

1. Provide a load-bearing column at the location where the two roof ridges meet. Extend the column down to a footing.

2. Use ridge beams rather than ridge boards at the two roof ridges. The two ridge beams are designed to act as beams that will span between the gable end walls and the column post.

3. Design the valley rafters as a load-bearing beams to support the reaction from the rafters that frame into them.

If you frame the roof this way, the rafters become simple span rafters.

AndyGump 03-13-2012 06:07 PM

You might try defining those large dormers with cheek walls instead of the full valleys, that way not all the rain and snow will funnel completely to the corner.

Andy.

Nailbags 03-13-2012 07:14 PM

Well it looks like you ripped off my roof design! My roof looks just like that. A few things To know it going to be a 8:12 greater pitch roof. So don't plan on being on the thing once it is done unless your part Mt goat. I don't have any Valley problems with water or snow. In fact I have a few inches of snow in them right now.

AlexHouse 03-13-2012 09:27 PM

Quote:

Originally Posted by House Engineer (Post 876924)
Your design is not an economical roof framing scheme, but if you need it to look that way for the "look" of it, you could frame the roof as follows.

1. Provide a load-bearing column at the location where the two roof ridges meet. Extend the column down to a footing.

2. Use ridge beams rather than ridge boards at the two roof ridges. The two ridge beams are designed to act as beams that will span between the gable end walls and the column post.

3. Design the valley rafters as a load-bearing beams to support the reaction from the rafters that frame into them.

If you frame the roof this way, the rafters become simple span rafters.

Thank you so much for taking the time to detail a way to bring this about.

I have a 3 story (basement to 2nd) elevator enclosure resting on footings smack dab in the middle of the junction between the two ridge lines. Would the bearing point have to be directly under the junction of the ridge lines or would an offset of 16" towards the dormer still suffice as a load bearing column?

I want to be clear that I'm understanding your point - instead of a roof distributing its load along a path of exterior wall, the ridge and valley beams would concentrate the entire load down to the point of contact between the beam connection and the load bearing member. Is that correct? Is so, is there anything I should pay attention to with respect to the design of the wall or columns which accept those loads?

Is there any concern about the short stub walls which form the exterior 2nd floor walls under the roof plane upon which the dormer is built and the issue of roof thrust?

Mainly I'm reassured by your comment that this can be done, even if inefficiently. Knowing this I'll leave it up to the engineer to work out the specs - I was principally concerned about designing something which would be a nightmare and expensive to build.

Nailbags,

Glad to read that "great minds think alike" and that you're not having problems with water. I was concerned about all that roof surface area being funneled down into one corner. I went so far as to slide the roof over a bit to the right to give me a bit more gutter area to handle the downflow. The trade-off is that I lose a bit of height against the exterior wall in the 2nd floor room on the left side of the house, but isn't design always a trading off of conflicting goals.

http://img220.imageshack.us/img220/3...offraming1.jpg

AlexHouse 03-13-2012 09:31 PM

Quote:

Originally Posted by AndyGump (Post 876927)
You might try defining those large dormers with cheek walls instead of the full valleys, that way not all the rain and snow will funnel completely to the corner.

Andy.

Thanks Andy. I'm giving this some thought. It's a trade-off between aesthetic and practicality and I'm not 100% sure which way I'm going to go. Your suggestion has the merits of practicality but I'm trying to avoid cheek walls where I can and go for the "low slung" look - wide dormer with lots of roof area but that poses the problem of concentrating water runoff from a large roof surface down into a small gutter area.

woodworkbykirk 03-13-2012 09:52 PM

thats actually a pretty simple roofline compared to some of the ones ive dealt with...
your asking about stick framing it though.. it would be much quicker to just get trusses. in todays age theres maybe 1 framing crew for every 100 that can stick frame a roof to those that can only do trusses.. . we stick frame many of our roofs but its do to how we mostly do additions.. on some of our jobs its easier to stick frame than it is to design trusses to tie into the existing as old houses arent square or level. we can make the new portion square and level by custom fitting every peice of wood

AlexHouse 03-13-2012 10:38 PM

Quote:

Originally Posted by woodworkbykirk (Post 877091)
thats actually a pretty simple roofline compared to some of the ones ive dealt with...
your asking about stick framing it though.. it would be much quicker to just get trusses. in todays age theres maybe 1 framing crew for every 100 that can stick frame a roof to those that can only do trusses.. . we stick frame many of our roofs but its do to how we mostly do additions.. on some of our jobs its easier to stick frame than it is to design trusses to tie into the existing as old houses arent square or level. we can make the new portion square and level by custom fitting every peice of wood


Yes, the whole roof is to be stick framed because trusses would sacrifice too much space where stick framing gives me a whole 2nd floor that is usable space and with the pitch of the roof there is even attic space above some parts of the home.

It's too bad that stick framing is becoming somewhat of a lost art.

woodworkbykirk 03-14-2012 02:56 PM

talk to the roof truss plants engineer.. they can make trusses which allow for a living space in them. ive installed them many a times

titanoman 03-14-2012 04:17 PM

Quote:

Originally Posted by woodworkbykirk
talk to the roof truss plants engineer.. they can make trusses which allow for a living space in them. ive installed them many a times

They're called attic trusses.

Nailbags 03-14-2012 04:59 PM

Quote:

Originally Posted by AlexHouse (Post 877058)


Mainly I'm reassured by your comment that this can be done, even if inefficiently. Knowing this I'll leave it up to the engineer to work out the specs - I was principally concerned about designing something which would be a nightmare and expensive to build.

Nailbags,

Glad to read that "great minds think alike" and that you're not having problems with water. I was concerned about all that roof surface area being funneled down into one corner. I went so far as to slide the roof over a bit to the right to give me a bit more gutter area to handle the downflow. The trade-off is that I lose a bit of height against the exterior wall in the 2nd floor room on the left side of the house, but isn't design always a trading off of conflicting goals.

http://img220.imageshack.us/img220/3...offraming1.jpg

It was a night mare to do and a butt load of money! But I have some real cool man cave attic space!


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