hey guys, i’m doing a house with a pretty large gable roof. it’s 12′ to the ridge and 42′ outside to outside. the rafters will be about 25′ long with the tail. no ceiling joists. the second floor is inside the roof. it will have a few 6′ wide dormers on each side that plane with the wall face.the spans of the rafters will be broken up with walls and beams so we will be using 2×6. i was wondering if anyone had some pictures of long rafters.i’ve never done a roof this large on my own and it helps me to visualize.thanks.
RTC
Replies
Knee walls.
blue
it's an icf house and the top plate is 4' above the subfloor.there are some walls that will be breaking up the span.originally the plans called for a wall running down each side about 8' in but we have altered the plans and haven't got them to the engineer yet.
RTC
You will not have room for any typical insulation in that 5.5" rafter space. Did the designer/arvchitect do any load calcs in this idea?
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i'm not exactly sure what you mean by typical insulation and load calcs.could you elaborate?
RTC
With standard fiberglass insulation, 5.5 inches will only get you R19 which is less than the R30 required or R38 recommended where I am in CT. Using certain types of spray foam (closed cell is at least one) can provide insulation values that meet code in most areas with a 2x6 rafter.
The load calc would be done by the HVAC contractor to determine the sizing of the HVAC equipment based on construction details including insulation properties. More insulation in the proper places generally means the HVAC equipment can be sized smaller.
i've never heard of required insulation values here in south texas?maybe i'm being ignorant.anyways, i wanted to use 2x12 rafters but the engineer called for 2x6 and the home owner doesn't want to spend the $$.
RTC
Mojo explained what I meant by typical insulation.
When I refered to loads, I didn't mean heat and cooling loads, but live loads on the roof, from snow and wind.
I realize that 2x6 is the common frame in Texas, having lived there, but i also know that placing a room in under the roof, Cape style, is not caommon there.
Since you get no snnow, the engineer may be right that 2x6 us acceptable from the load/structural viewpoint, buit wrond RE: insulation. You don't need to worery so much about heating, but a second story of a house with inadequate roof insulation is going to be intolerably hot for human beings, unlesss the owner wants to spend a lot of money on AC to cool it. He should be considering this in his budget conscious mind.
Another point that YOU should consider, is that a roof that is minimally OK for bearing the live laods on the roof, can still havea lot of bounce in it during high winds. That means that the Sheetrock will open the seams and show cracking at the joints from that structural flaw - excessive deflection.
So have a heart to heart with the HO and explain to him that it is in his best interest and yours to have a better structure, and better insulation, both of which will pay for themselves in a few years. Before even going to the HO with htis, ask the engineer if he considered the fact that there will be living space immediately under the roof, or if he ONLY considered the minimal structural loading. He might say, "Ooops!"
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> .... a roof that is minimally OK for bearing the live laods ....
He did say that the rafter span is broken up by walls and beams, so we don't know for sure if it's pushing the limits.
-- J.S.
neither do we know if those partition walls are load bearing, direct to foundation, or if they merely transferr loads to midspans on floor/cieling joists below,so this is of course, acedemic, but meant to hilightr areas of concern for him to look into, to avoid liability and to help him formulate a plan to get everyone on the same page, so he can do the best job instead of merely adequate
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thank you so much for your input. i am pushing for deeper rafters, hopefully the cost diiference won't be a problem. but this being an icf home may cancel out the roof insulation problem, still i woyld rather use a 2x12.
does anyone have any pictures of long span rafters ?nobody around here builds large gables.
thanks
RTC
Piffin, you were ever the diplomat, so I'll say it instead: No way should 2x6 rafters be used on a 21' run. When the engineering comes back, the HO will see it in black and white.
Looking in the Canadian code the options are either 2x12 @24" o/c or 2x10 @16" o/c (both SPF #2 & Better). That's at the very lowest load factor of 1.0 kPa which is appropriate for places like Vancouver and Victoria where snow is very rare but heavy rain is not. Those conditions would be reasonably applicable for the Texas coast area where RTC is.
You would actually be buying fewer board feet if you went with 2x12 because of the wider spacing, but I'm sure that 2x12 in those lengths is going to be a good bit more per BF than 2x10. Either way, those will be expensive, special order lengths of lumber, not to mention very heavy and tough to handle.
If traditional span-reducing options such as collar ties or knee walls are not acceptable, my personal preference would be to use TJIs, there is a 9.5" depth that would be perfect for this application. Lighter and stiffer than solid lumber, easy to handle. But if the HO is balking at the cost of 2x12 compared to 2x6 then he ain't gonna go for TJs.
RTC: if you haven't already insisted on it, get the thing engineered. That is one major call-back if you go ahead with 2x6 and then it fails because of overspanning.
Wally
Lignum est bonum.
You may have missied in your reading that he said it had already been engineered. it was my diplomatic side recommending that he review that with the engineer tactfully pointing out a few things. I have no idea where the kneewals are or whether they can be considered load bearing.I would prefer the TJIs also for my own choice, but he is in Texas.There is no way in maine, canada, or texas that a collar tie does anything to reduce load spans
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Maybe we are running into issues of differences in terminology here. I remember a while back we found that in Canada we have a different definition for cripples than you do in the states.
In my trade school framing text, collar ties are defined as "horizontal members used to strengthen the rafter and reduce the unsupported span. They are attached to opposite pairs of rafters." Lignum est bonum.
Now prove that they do anything to reduce spans. A dictionary doesn't do it. That dictionary doesn't even clarify whether these are what we here call either rafter ties or collar ties
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What is the difference between rafter tie and collar tie? Lignum est bonum.
a cileing joist functions as a rafter tie to resist outward thrust of the rafters on a wall.
A collar tie is placed in the upper third of the rafter space and resists upward lifting forces that would tear a ridge apart - required in southern building codes where hurricanes and tornadoes are common.
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I guess when I think about it a horizontal tie can do little to support snow load even tho that book says it does. Resultant force is still downward directly below ridge. Wouldn't be the first time the book has been wrong or at least inaccurate.Lignum est bonum.
I'll take part of that back. I think in terms of vertically applied snow laods on rafters.Buta true collar tie resists the upward load of wind sucking a roof surface away from itself. It would also take a horizontal wind load andtransfer that load to the opposite rafter and thence to the wall supporting it.but they do nothing to resist snow loads
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Slykarma, I was taught a ton of stuff that was wrong in my carpentry apprenticeship trade school (union school).
One of our instructors always insisted that trusses were the bane of construction and that all trussed roofs would be severly sagging in the next couple of years. The only roofs that I see sag are the conventional ones!
Don't believe everything you hear.
blue
even when ya hear it here, hear?;)
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yeah like piffin said,the plans are enginereed. there's lots of walls and beams.most of the 2x6 are in spans of 13' or so.
the engineer is a good guy and knows what he is doing. the hard part for me is installing this roof and planning what height the interior walls should be to land right on a midspan seat cut or should i even do seat cuts or just sit the rafters on the walls/beams.
basically i'm trying to find a way to simplify this roof install.i'm new at this.
oh yeah we've got collar ties
RTC
Edited 9/18/2005 7:58 pm ET by RTC
the engineer is a good guy and knows what he is doing. the hard part for me is installing this roof and planning what height the interior walls should be to land right on a midspan seat cut or should i even do seat cuts or just sit the rafters on the walls/beams
Since you're having such a hard time for such a fundamental aspect of rough frame, I'll offer this tidbit.
Snap (draw) out the roof in full scale on your first floor deck.
blue