Dormer detail question

[Manitou]

Hi folks,
I'm in the middle of a 16x20 cabin build with 12/12 roof and 3/4 loft. I plan to build a shed dormer to increase loft headroom.

Does anyone see an issue with attaching my 2x8 rafters to my dormer header (above my dormer windows) instead of the usual way of resting atop the dormer top plate? I'd really like to be able to keep the roof greater than 4/12 and get my windows as high as possible at the same time. I don't want to compromise strength though! Was thinking of using some Simpson hangers and super strong screws to attach with.

Any thoughts? Also any ideas on how best to do these or if anyone has them what they think of them?
Thx,
Steve

[Don_P]

That will work, the same as a flush floor girder vs a dropped girder... the outside overhang just got interesting though.

[Manitou]

Right, was wondering that too...How would the overhang typically be done in this scenario?

Thx Don!
S

[Don_P]

I've not done this particular scenario but have run a "fake" overhang across a shed dormer wall which is similar. Cut out the profile of the overhang in plywood, so on the left a vertical back edge against the wall, the slope of the rafter on the top, a vertical right edge on the right for the subfascia and a level edge for the soffit. Cut 2x parts to nail along these inside the perimeter of the ply, the ply is your pattern and gusset. Attach this to the wall/header, attach subfascia and fascia. Run the roof sheathing on the top, over the wall and up on the common rafters, tieing it all together. Watch the overhang, draw this all out to scale, it's going to want to get into the window I suspect.

[rick91351]

If you are building where there is a lot of ice and snow be mindful of what is shedding off the roof to below.  ie Another roof no matter how small or insignificant can receive much damage from falling ice or packed snow. 

[akwoodchuck]

Definitely doable....you can build your overhang all at once, with the rafter "tails" fastened to a ledger board, then nail the whole assembly to the wall. Run some Simpson strap from the rafter tops to the tails, tying them together. Time consuming, but should work.

[akwoodchuck]

Oh and P.S: you'll come to hate the view of that overhang in your window, I suspect! 

[Manitou]

...about the snow load, I'll have a porch roof extending out a few feet down (just under dormer window) and we get a lot of snow up here so I'll keep this in mind when I build this out.

Thinking out loud but I guess I'll need to angle my header (tripled 2x10's) with the slope of the dormer roof as well so that sheathing flows down smooth from the top to my faked out rafter tails? So if my header is same as wall thickness - 5.5", the outside of header would be lower than inside of header by the same degree as the slope of roof (4/12). Make sense??

Also, was thinking of extending my edge rafters 'out' beyond the header on the sides of it to provide extra support for the 'fake' tails.

thx for the suggestions.

[Don_P]

If you actually need the strength of the full header then you cannot cut it down, raise the rafters on the inside until they plane with the downhill corner. If you don't need the strength then you can reduce the header height... but remember it is forbidden to rip a bearing member, the grade changes. If you do so do it to the clearest, densest stick you have.

[Manitou]

right on, I'll raise them up then. Thx!

[akwoodchuck]

Thinking out loud but I guess I'll need to angle my header (tripled 2x10's) with the slope of the dormer roof as well so that sheathing flows down smooth from the top to my faked out rafter tails? So if my header is same as wall thickness - 5.5", the outside of header would be lower than inside of header by the same degree as the slope of roof (4/12). Make sense??

Do this and you may have a problem with ventilation.....remember to leave a couple inches of airspace between the insulation and roof deck, with the appropriate amount of vent (Google "1/300 ventilation ratio") at the soffit and ridge.

[archimedes]

Just curious,  what's on the other end of those rafters?  A ridge beam or ridge board?  Wouldn't that matter too?

[Manitou]

archimedes, the other end is attached to a ridgeboard/ridgebeam. I'm kind of thinking of it as a hybrid as its a doubled 2x10 board that I'm hoping will act kind of like a beam as it rests on top of posts in the gables/walls that go all the way to the foundation.  I will also be attaching collar ties for good measure as it is a built up beam and was concerned about potential weakness at the joints.

...your right, akwoodchuk. So seems like I'll need to have my rafters extending ~3" above the header on the inside to give me a 1"+ space on the outside of header, room enough for my baffles. This leaves ~4.25" of my 2x8 rafter ends attached to my dormer header.

Is this enough??!

Maybe I should sister some blocks between the rafter ends and attached to my header in addition to my simpson hangers??

[archimedes]

the other end is attached to a ridgeboard/ridgebeam. I'm kind of thinking of it as a hybrid as its a doubled 2x10 board that I'm hoping will act kind of like a beam as it rests on top of posts in the gables/walls that go all the way to the foundation.  I will also be attaching collar ties for good measure as it is a built up beam and was concerned about potential weakness at the joints.

^^^^This could be a problem.    From what I've gleaned from your description you've got a 20' long building with a double 2x10 ridgeboard acting like a ridge beam.  "ridgeboard/ridgebeam" are completely different things,  just like "carton" and "cartoon" are very similar words they have totally different meaning.   

Also, I'm guessing that that ridgeboard is built up (not one continuous 20' long piece) right?  Twenty foot long dimensional lumber is available,  but at least around here,  usually only by special order.   Is it built up or continuous?

Your profile doesn't say where you are.  Do you have a snow load?

I'm not an engineer,  and I may be completely off base with my concerns,  but I think you're worrying about the wrong end of those rafters.  And collar ties don't resolve the problem. 

Others will hopefully chime in.

[Manitou]

I'm aware they are quite different which is why I'm treating my ridge as if it is non-structural (it is built up) and adding in collar ties every 32 to 48" on my 2x8/16 center rafters. To my way of thinking i just have an extra beefy rafter board in this application, but I'm also hoping with the post supports on either side of the 20' length that some 'structural' benefits may be achieved as well.

I threw this topic around on another msg board, the advice I got was to just make sure I had the collar ties as well.

thx.

[Don_P]

Umm  , I think archmedes has found the weak tree in this forest. Often when we think one part is helping another what happens when the dust settles is one part failed and then the other.

[Manitou]

The doubled rafter board as the weak tree?   
This would be weaker than a single 20' rafter board built up?

[Don_P]

We were absorbed in looking at a detail when archimedes backed up and pointed out the first order of business.
To keep out of engineer land it should probably be an lvl ridgebeam capable of supporting half the roof width (8') x the 20' roof length, so 160 square feet supported by the ridgebeam. 4'x20', or half of each 12/12 rafter is supported on the lower wall, 4'x20', or half of each dormer rafter is supported on the dormer wall. The center half, 4' of each opposing rafter on each side of the ridge, or 8', is carried by the ridge... let's back up and see what that would take, what is your snow load?

[Manitou]

Lots of snow ya, bout 70lbs. Was hoping a 12/12 with metal would shed a lot of it though.

I haven't gone too far, just the ridge is up with only a few rafters in place and would consider backing this out if you think it's really unsafe. Only have bout 8 weeks b4 snow tho and as weekend warrior, would need to act fast. Thx.

[Don_P]

The 4/12 will hold snow and unbalanced. I'm just looking quickly, the supplier would do the engineering on this as a free service. 70Live load +10 Dead Load=80x160= potentially 12,800 lbs up top on a 20' span. Looking that up in the LVL manual that's a double 16" deep ridgebeam with posts and footings capable of supporting it at the ends.

[Manitou]

hmm. Maybe I'll scrap the dormer, keep the 12/12 ... the question is would it be ok then?!
We have a 12/12 cottage roof (same place) w/shingles and it never holds more than 6-8" of snow on it so I'm thinking it would be fine.

Or, could pull down what I have and order an LVL, would a double LVL be true dimensions, 4"? (or is it 3", like doubled 2x10s).
Then do the dormer.

Gotta say I'm a bit surprised by all this as the locals suggested I'd be fine w/doubled 2x10s sandwiched, that this is commonly done on smaller outbuildings. 

Appreciate this though guys and I'll be backing this out if there's any doubt...

[Don_P]

If you form a triangle of opposed rafters and tie across the rafter feet with a floor or ceiling joist then a ridgeboard works fine. When you raise the tie above 1/3 of the roof height or are unbalanced as in the 12/12 on one side and 4/12 on the other then the easy way is to use a ridgebeam and basically hang the rafters from it.

I applied the full ground snow load to the roof when I sized the lvl. The supplier can use a reference from the Am Soc of Civil Engineers, ASCE7 to probably reduce the beam size due to the slick surface and slope up top. I tend to go conservative there, got a call from a homeowner we built for who had a simple gable 12/12 with metal and he had 3' of ice and snow stuck up there. We had joked that nothing was going to stick to that roof and the engineer had specced 2x10's on 12" centers. That pretty much woke me up, the perfect storm is probably going to happen sometime in the life of the building.

LVL's are 1-3/4" thick so a double is 3-1/2" thick, the height is as called, a 16" is 16" tall, a 14 is 14, there are 2 sizes available the next step down, an 11-7/8" or an 11-1/4". On my table at the loading I gave you there was another option, a triple 14" which would be 5-1/4" thick. Costwise that is not the way to go.

The locals are not neccessarily wrong but they cannot provide the needed details to do it right. A tied dormer rafter pair with a ridgeboard can be designed by an engineer. What comes into play there is that the tie is up in the span on the 12/12 side and as the roof loads up the tie connections are under heavy tensile stress, the connections need to be specified, and there is a high bending load in the 12/12 rafter at the tie connection point, the rafter often needs to be upsized to resist that bending load. I've been through the math with an engineer and have my notes but I wasn't comfortable with my understanding then and sure am not now with fewer and fewer brain cells. When you go that route I'd hire one to check your assumptions, it could be fine. This is actually what the locals are using to resist the forces whether they understand that or not. Their undersized beam is not supporting the roof, it could just as easily be a single or a strip of cardboard, or non existant with the rafters simply gusseted together with no ridge. They are relying on that homemade stifflegged truss. It can work if done well, it can sure fail if they guess wrong and it loads up. Conventional wisdom often doesn't know just how deep they are into their safety belt until it snaps. Generally though if a ridgebeam will make it work that is the cheaper safe route. If you went the balloon framed kneewall route upstairs it needs a ridgebeam either way you go, whether dormer or simple gable.

[archimedes]

Seems like pretty cheap insurance  IMHO.
http://www.menards.com/main/building-materials/lumber-boards/lumber/engineered/1-3-4-x-16-x-20-lvl-1-9e/p-1520039-c-5663.htm

You probably (as Don says) need two of these^^^^
The manufacturer will run the numbers for you,  usually for free.

[Manitou]

ya, kinda sucks, but I think i'll be burning this weekend undoing what I'd done,lol. Worth it in the long run.
I called the lumber shop here in town and got them to spec out what I needed - they came back with exactly what you said, 2x16's doubled.

I'm working alone this weekend -- are these things super heavy??
I'm hauling everything from our cottage to the camp by quad.

Also, for putting them together, just glue and lots of nails like anything else??
I'll need to add an extra half inch board to my supporting posts to get them up to 3.5" width (from the 3" before) .... any other tips??

wish I'd ran my plan by you guys a few weeks ago!

S

[Don_P]

The difference between the two is that a good carpenter will fix his mistakes
A carpenter gets to burn his mistakes and stay warm, I'd never be a plumber 

They aren't super heavy but certainly not light. 16's weigh 7.4 lbs/ft so 150 lbs each. If you can get them into place on the upper floor alongside of the posts and then at their midpoint build and brace a pair of temporary posts that straddle the beam, taller than the ridge by a few feet, with a crossbeam well attached on top, dangle a hoist of some type from that and haul it up. Alternatively 2 healthy guys can climb ladders with it, each is climbing with ~75 lbs, roughly the same load as carrying packs of shingles up a ladder.

3 rows of 16 commons at 12" on center for this loading, so 2" from the top and bottom edges and one row down the middle, nails in every row 12" max apart. No glue specced or needed. Those were 16 commons if you're shooting 16 box nails add 50% more, every 8".

Do make sure you look down under the posts all the way to the footings, you have to support this beam adequately or it's all for naught.