changes?

Started by lee5267, February 10, 2008, 06:42:24 PM

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lee5267

After more thinking and pondering over the little house plans, I have come up with yet another idea to suit my needs.  The idea is to build a cabin very similar to Rob Lemay being 16x 24 and having 10ft walls incorporating a 6/12 roof.  I still plan to put the loft in both sides as the plans show, by installing the ledger or putting a Jack stud beneath the loft joists.  I'm planning to build the walls at 16" OC and space the rafters at 16" as well. I'll also keep the 4' OC 2x collar ties b/w the lofts.  The foundation and all the rafters, joists etc will be increased to incorporate the increased width according to another set of plans I have for a 16ft wide cabin and the Wagner framing book.  Is there anything I'm missing?  I was thinking of making 4x8 beams and placing them every 4ft similar to Lemay's cabin to increase the sturdiness of the lofts and being stronger collar ties than 2x4's and more pleasing to the eye.  If I decide not to use the beams will the 2x4 collar ties be strong enough or should they be beefed up a bit? 

I'm also wondering if I can simply extend the rafters out about a foot past the walls for my overhang.  I was suprised to see this wasnt an option in the plans.  Is there no option for this to make things more simple by having no birdsmouth cut on the rafters?  Thanks for the help

Lee

MountainDon

About the rafters and extending them for overhang. Yes that's commonly done, Little House jusy happens to be drawn without them.

The birdsmouth gives the rafters a nice solid flat (horizontal plane) to sit on the wall top plate. That allows it to be securely nailed down into place. Use of hurricane ties will make it a very sturdy roof. So no there's really to way around them other than to go with manufactured trusses at extra expense, and nor really needed and use of them will mess up the loft plans.

If cutting the birdsmouths is giving you cause for worry, you're not the first. But it can be done. Even a pro, the good ones at least will cut one pair to be sure they fit, then test them at points along the wall. Then those will be used as a pattern to cut the rest.

Once your floor is sheathed you can draw out a full size rafter cross section if you desire.

Re the loft: What pitch roof do you plan on or want?

John's Victoria's Cottage is 16' wide and includes a couple variations on doing the roof and upper floor.
Just because something has been done and has not failed, doesn't mean it is good design.


lee5267

I'm planning on a 6/12 roof cause it will give me 4 ft above the top of the walls.  With the loft floor being 2ft below the bottom of the wall this will give me a total of 6ft of head room at the peak which is more than enough for a simple sleeping loft.  The loft loads will also be very minimal, basically there will be a bed and maybe one smallish dresser and two people max.  I chose the 10ft walls and 6/12 roof so that the roof slope will be more manageable for me sheathing it and installing the metal roof while still giving me the same head room as the 12/12 roof would. 

I was also wondering if the birds mouth cut with extending the rafters as the over hang changes the how the load is transfered down through the walls.  Seems as though if you cut the rafters off flush having them sit flush on top of the walls it will produce more of an outward force on the walls compared to extending the rafters using the birdsmouth.  Basically is there a way to install the rafters that produces less outward force on the walls?

Thanks again,

Lee

Willy

Quote from: lee5267 on February 10, 2008, 08:13:16 PM
I'm planning on a 6/12 roof cause it will give me 4 ft above the top of the walls.  With the loft floor being 2ft below the bottom of the wall this will give me a total of 6ft of head room at the peak which is more than enough for a simple sleeping loft.  The loft loads will also be very minimal, basically there will be a bed and maybe one smallish dresser and two people max.  I chose the 10ft walls and 6/12 roof so that the roof slope will be more manageable for me sheathing it and installing the metal roof while still giving me the same head room as the 12/12 roof would. 

I was also wondering if the birds mouth cut with extending the rafters as the over hang changes the how the load is transfered down through the walls.  Seems as though if you cut the rafters off flush having them sit flush on top of the walls it will produce more of an outward force on the walls compared to extending the rafters using the birdsmouth.  Basically is there a way to install the rafters that produces less outward force on the walls?

Thanks again,

Lee
By beefing up the ridge board it takes load off the spread but then you need to support the ridge with posts or make it real strong size wise. As the load pushes down on the rafters they tend to push twords the ridge which in turn takes the weight of the roof. But this now trys to spread the rafters and push out on the outside walls as the ridge goes down from the weight. If the ridge cannot go down the outside walls won't spread and the only way you can stop this is with collar ties or a beam for the ridge board and sitting on posts. If it is stronge you can do it on the ends only, if not you need more posts under it. Mark

MountainDon

Quote from: lee5267 on February 10, 2008, 08:13:16 PM
I was also wondering if the birds mouth cut with extending the rafters as the over hang changes the how the load is transfered down through the walls.  Seems as though if you cut the rafters off flush having them sit flush on top of the walls it will produce more of an outward force on the walls compared to extending the rafters using the birdsmouth.  Basically is there a way to install the rafters that produces less outward force on the walls?

The weight of the roof (all the materials, snow, water, etc.) acting on the walls can be translated into 2 directional forces. There is the vertical downward force and the horizontal outward force.

The downward force is resisted by the foundation. The outward force is countered by the rafter collar ties, or the ceiling joists (in a simple structure such as the Little House), or a combination of the two. When tracing the load path to the walls the rafters sit on, it does not make any difference if the rafters have tails (then part that creates the overhang) or not.

With walls that extend above the level of the upper floor as you describe (knee walls) the outward forces will try to bend the walls at the "joint" at the ceiling/upper floor joists. These forces must be resisted by the collar ties OR by using a structural ridge beam. It helps to have 2x6 wall studs instead of 2x4.

If using a structural ridge beam the beam must be strong enough to support the roof weight. The beam is supported at the gable ends with the gable ends then becoming load bearing. The gable ends must then be strengthened. (Gable ends generally only support their own weight, none of the roof weight.) The ridge beam must be strong enough to resist the weight without sagging, or intermediate posts will be required.

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


lee5267

Ok, so as I understand it the rafter tails make no difference on the proportion of forces the roof places on the walls.  As for the knee walls, are there any rules of thumb that are generally used?  2x6 walls would be a benefit for obvious reasons as you said although I have seen others do it with 2x4's.  Aside from that, do the  collar ties need to be at the top of the walls where there is no loft or would it be ok to put them at 8ft just like the loft floor? Seems like putting them at the top of the wall and also attaching them to the rafters would be the best option for negating the outward force on the walls as well as the bending force at the loft floors on the walls.  This might look a little odd though with the loft floors at 8ft and collar ties at 10.  What are the thoughts on this?

Lee

John Raabe

Yes, the collar ties work best where they are at the plate line. Walls can be upgraded to 2x6, which what I've done with the Enchilada upgrade. Roof rafters are also designed for better insulation. The Little House is designed for the lowest cost easiest to build cottage. The rafter tails can be extended either with longer rafters or extension 2x6.
None of us are as smart as all of us.