Could use a 2nd set of eyes on my framing plans

[tinybackyardhouse]

I could use a second set of eyes on my plans for framing.
This is for a 12x10 tiny house we're building (thread here: http://countryplans.com/smf/index.php?topic=10664.0 )

The building has a 3/12 shed roof and loft for a mattress. I originally drew the plans for top plates on top of 9' studs with bird mouth cut rafters but recently switched to variably sized full length studs mitered at their end.



I'm no expert of course but it seems like this would pose no problem, right?

I'm also toying with the idea of using Simpson HH4 header hangers to eliminate the need for jack studs but Im not certain if this is feasible or worth it.

[bayview]

   Probably be ok on such a small building but . . .
   
   I had been told to notch rafters and not to slant the studs/top plate.   Rafters could possibly "slide" down the plates.   Notching the rafters would send the load directly down the studs to the foundation. . .

   Or not . . .

// 

[tinybackyardhouse]

Hm

Since I live in earthquake country, I had planned to use some metal ties on the rafters to secure them to the walls anyway. Something like these Simpson ties



Sufficient to keep the rafters in place?

[JRR]

I would suggest you go ahead and draw in the roofing.  I would suggest something like 1x6 or 1x4 purlins covered by metal ... just for analysis.  Once the purlins are in place you will see an opportunity to remove the rafter "hanging in space" and also the rafter that is on the wall line.  This would make necessary increasing the height of the end walls ... up to the purlin plane ... but would make sense to me.  Just for appearance, you can always hang a short piece of rafter stock off the purlins .... at the front and rear corners.

[tinybackyardhouse]

I'm not sure I understand the idea of purlins here

The 2x8 rafters I've chosen are more than sufficient to span the 10' on their own without support of purlins. I would like space to be open and unobstructed by purlins

I could remove the rafters on the wall line and replace the 2x4 let ins for the lookout rafter with 2x8 (rafter size) blocking on top of the end wall. For aesthetics I'd then need to attach a fake 2x8 rafter. I figured just going with another full length rafter was best since I'm using almost the same amount of lumber (12' 2x8 vs six 2' 2x8 blocks) and I don't have to mess with making a fake rafter tail for aesthetics

Maybe I'm not understanding what you mean though

[firefox]

Simpson makes a bracket so you don't have to do the birdsmouth
thing. Used it on a garage and it works well if you follow the instructions and use the right nails for it. Inspectors loved it and they are pretty ornery in these parts.
Bruce

http://www.strongtie.com/products/connectors/VPA.asp

[bayview]

I'm not sure I understand the idea of purlins here

Purlins are placed above the rafters and necessary "if" you were to roof with metal . . .

//

[tinybackyardhouse]

Sorry, I was thinking purlin in terms of structural support with a post under it


I'm doing a vegetated roof. On top of the rafters I was planning to put 1x decking, then 4" foam board, then EPDM liner and etc.

[JRR]

Yes, as Bayview suggests the purlins would go atop the rafters.  And I'm suggesting you draw them in merely as a visual aide ... not to change the final design or the roof plan.  (I would consider using them ... even under sheathing.  Stiffens things up.)

You do understand my suggestion of removing the two rafters that line-up with the end-wall.  If you remove this rafter and extend the wall upward to compensate ... (I think) there will be a stronger structure.  As the rafter will tend to roll over or "rack" ... instead, the extended wall assembly (studs and sheathing) will resist roll-over very well.

[tinybackyardhouse]

Here's what it looks like with regular level top plates and birds mouth

[MountainDon]

Regarding the mention of purlins and using them with metal roofing...  I don't understand the fascination with using metal roofing over purlins. Use of purlins usually means using purlins in place of sheathing the roof with plywood or OSB.  I believe that is not a good practice.

1. Plywood and OSB sheathing provide structural strength to any roof; they provide racking strength.  Some, but definitely not all, metal roofing is rated as structural panels.

2. Using metal roofing over purlins instead of over sheathing makes it more difficult to prevent condensation on the underside of the metal roofing.

If one wants to make a roof stiffer the easiest method would be to use thicker sheet sheathing. Much simpler to install.

If you are planning a green "vegetated" roof you should be very careful with calculating the roof loads making allowances for the worst case rain soaked weight.

[Don_P]

The rake wall studs should be uninterrupted full length studs, remove the plate, it's just a hinge. I'd slope the upper wall plate and eliminate the birdsmouth up top, the unsupported notch is downsizing the rafter. It appears the loft is hanging from a rod, if so make sure that rafter is up to the point load.

[bayview]

   I agree the side wall studs should reach completely to the rafters. . . .    But no top plate?

  You may want to have the loft window large enough for a quick exit in case of an emergency.

  I see the point that Don_P makes about the downsizing of the rafter on the upper wall . . .    Should the lower wall still be notched?

   I agree with Mt.Don about the sheathing of a metal roof.   Personally I prefer an asphalt roof.   After having lived in a trailer for a couple of years.   Half of the night in the summer the roof would make all sort of noises when contracting from the heat/cool down cycle.   And the sound of rain . . .   

//

[Don_P]

      I see the point that Don_P makes about the downsizing of the rafter on the upper wall . . .    Should the lower wall still be notched?

Yes. The level, or seatcut, of the birdsmouth on the lower end of the rafter does not extend inboard of the wall, the size of the rafter within the span is undiminished. When you notch the upper end of the rafter, that notch is within span and unsupported. It does diminish the size of the spanning rafter.

On the top plate, I mainly meant that you should not create a hinge within the wall. There can be a raked top plate, or, the studs can simply be notched to, or nailed to, an end rafter.

[tinybackyardhouse]

Greatly appreciate all the replies


So it seems like I should go back to the design in the first post (mitered studs) for the tall rear wall to maximize rafter strength but have a level top plate for the front wall with birds mouth
Then the side walls should have full length studs with either a top plate (a bit fiddly to attach to the other plates, no?) or attach directly to the rafter



Its a bit difficult to get the rafter sizing for a vegetated roof since dead load is around 25psf and tables often list 10 or 15 only. From my homework though if I went with a very lightweight soil media of 3-4in and used either lightweight drainage material or no drainage plane, I could probably get away with 2x6 rafters (although its very borderline). I feel comfortable though spending the extra money for something more stout so I went with 2x8@16

[MountainDon]

Its a bit difficult to get the rafter sizing for a vegetated roof since dead load is around 25psf and tables often list 10 or 15 only.

Try the AWC calc. The user selects the vaules for DL and LL. If the DL selections do not provide high enough values you can usually add the missing amount by selecting a higher LL; same difference.

There are also selectable fields for species, grade, spacing, etc.

[tinybackyardhouse]

Thanks, a handy calculator.
According to it, with L/240 and 20psf dead 40psf live, a 2x6@16 DougFir#2 would span 10ft4 and a 2x8@16 would span 13ft
My span is 9ft3

So...I could get away with 2x6 apparently but could also play it safe with 2x8. Opinions?



Forgot to also reply earlier, yes, the loft is supported by a chain or rod or the like from the rafters. I've doubled up the rafter at that location

[MountainDon]

 40 psf LL?  IIRC, average moisture content soils can weigh in at 10 lbs per square foot per inch of depth (120 lbs per cu.ft.)  A good rain could raise that amount but not sure by how much.

Conservative MtnDon


Not sure on the point load on that rafter off hand. How much will the loft weigh in at, with people and "stuff"?  A simple way of looking at that could place 1/4 of the loft load on that point (I think... no engineer here)

[tinybackyardhouse]

We're planning to use a lightweight soil medium of coconut coir, expanded clay and compost

I have a book in front of me on green roofs and it has a table with the following data for saturated weight:
Sand 9.4 - 11.4 psf per inch
Topsoil 8.9 - 10.4
Perlite 2.54
Light expanded clay - 1.5-2

With 3-4 inches of lightweight media we should be well below 40psf. Calculator actually even says 50psf live is good for 2x6@16 for 9ft6



Loft will have approximately 400lb worth of people and stuff
Was planning to lag bolt it to the studs and blocking as shown then support the rest with the cable/chain/rod

Haven't done any calcs on it though...

[UK4X4]

so 40 live load 15dl gives you 55#psf

I added 400lbs at mid point plus the above with a span of 10ft and a safety factor of 1.15( ie I put in as snow)

Using Ilevel software I get a pass for 2(2x8) hem fir no 2

fail on a single 2x8

pass on a single 2x10

This does not include the hole you'll be drilling for the bed chain !

Do your own investigation and calcs - do not take my word for it

For the cost diference just use 2x12's and don't worry !