Balloon wall height

[archimedes]

Can the balloon walls be made higher than 10ft ?  No snow load.

[John Raabe]

Yes, if you are talking about the 20' wide 1 1/2 story plans. The 10' studs can be increased to 12' giving 2' more height in the loft sidewall. This makes the usable space about 4' wider.

[Bob S.]

If I remember right, John previously pointed out that you can not use 12 foot walls with a cathedral ceiling room. You have to have a loft when you go to 12 feet. Or mabe you can get by with coller ties if thay are in the lower 1/3 of the rafters.

[rwanders]

I believe that a critical factor is whether you using a ridge board or a ridge beam supported properly by the gable ends---a ridge beam allows you to build a cathedral ceiling---ridge board design would require collar ties in the lower 1/3 to counter the roof thrust forces.

[archimedes]

If the building is 25 ft wide would that changes things appreciably? 

I'm not opposed to a ridge beam, but would prefer to avoid the cost if possible.

[John Raabe]

The issue Bob pointed out is correct. Unsupported wall height is limited to 10' in cathedral areas and taller walls require an engineering review if they are longer than 10' (such as a 12' high wall in a 14' wide living room). I understand this is to be a separate issue from how you support the roof load. The concern is lateral loading on tall walls.

Thus if you do a full length loft all the walls are braced by the floor system and the lower floor could be 8' or 9' or even 10' tall, but you couldn't have the full 12' tall wall open in the cathedral living room. This complication was one of the main reasons I didn't design the walls with 12' studs to begin with - I wanted the flexibility to have open cathedral spaces where the owner desired.

Here's the earlier link:
http://countryplans.com/smf/index.php?topic=8839.0

[Don_P]

A cathedral room without ties at the plate or in the lower third of the height from plate to ridge requires a ridge beam.

At 25' wide and with no other information given you aren't understanding the physics of it yet. What is either tieing or supporting your rafters?.

Table R602.3.1 in the codebook lists allowable stud heights, read the exceptions under the chart carefully.

[archimedes]

Sorry, you're right I haven't provided enough info.

No cathedral at all.  Full second story.

I like the 1 1/2 story plans, with a full second floor.  But I think the 2ft knee wall is too short for my tastes. 

If I use 12ft balloon walls, tied together with floor joists the entire length (at ~8ft high) and collar ties at the ridge, would that work with a 25ft wide building?

[John Raabe]

When you take a 20' wide building designed to have full span joists and make it 25' wide you need to redesign the floor system usually with an interior bearing beam or wall that supports both floors and carries the load to girders and posts at the interior of the foundation.

This is beyond the type of help you can get from this forum and would involve working with someone who can trace the loads and do a structural analysis on your revised design. Depending on how you layout the floors and walls you can place the support posts appropriately and then determine the spans and sizes of the beams.

At the same time you could plan for a taller sidewall at the upper level, a new truss for the wider roof, and a full sized stairway to the now larger upper level (this stair comes in the booklet with the 1-1/2 story plans).

[archimedes]

Well, I guess that rules out going 25ft wide 

What I'm trying to decide is whether to go with the "2 story Universal" plan or the "20 X 30 1.5 story"  plan. 

I could live with the 1.5 story if I could make the balloon walls 12 ft high, thereby increasing the knee wall height to ~ 4ft.  If that's not possible the I'll go with the 2 story Universal.  It seems like the 12ft balloon wall is possible with no loft (or with a loft and rafter ties).

[John Raabe]

You could do the balloon framed 12' walls on the 1-1/2 story and have the extra height in the loft area. But you would need to do the loft for the length of the building - no open cathedral area over the living room. Since the loft is now a larger space, I would suggest trading the steeper cottage stair for the full sized "U" stair.

In many ways this is what the 2-story Universal already has, except the stair is already designed in, the 2nd floor walls are full height, the roof can be trussed and both floors can be platform framed. The Universal is actually simpler construction and probably a bit less costly per usable SF than the 1-1/2 story. But, it doesn't have the "tucked under the roof" feel of the 1-1/2 story and looks taller.

[SkagitDrifter]

What about...
12' walls with timbers on 4' centers at 8' high and 2x6 car decking on top.
Could you run the 20' long timbers all the way- the full 30 feet length of the building, but hold the car decking back to create the cathedral ceiling?  It would be the timber that was acting as the collar tie, right?  You would be left with at least one timber hanging out in space but the loft would be open to the main floor- somewhat.  Any thoughts?   

[Don_P]

Prescriptively (no engineering) rafter ties are required on every pair of rafters in the lower third of roof height, these prevent the roof from splaying. Collar ties are also required in the upper third or across the top at a maximum of 4' on center. These prevent the roof from pulling apart at the ridge in a storm. So both are required, both do different things. Kneewalls do not change the codebook prescriptive requirement of a tie across each rafter pair in the lower third of roof height unless checked by an engineer. I drew it one time and IIRC made it work with a 5' kneewall on a 20'wide with rafter ties at 8'

As the building gets wider the kneewalls can be in the range you desire and the rafter ties will be in the lower third @ 8' off the second floor. There are clearspan floor trusses that can span this width without center support.

With engineering confirmation what you propose will work in many locations SD.

Then there is the structural ridge option.

Stairs and heights start working well in 24' wide buildings IMO... why are we limiting ourselves?

[John_M]

I went with the 12' walls (ballooned framed) with 4x12 timbers set at 8 feet (the bottom of the timber).  It gave me about 3.5 feet upstairs until the roof started.  My cabin is only 18 feet wide.

[John Raabe]

Here is an informative piece from an international association of home inspectors:

http://www.nachi.org/collar-rafter-ties.htm



Note that they support using rafter ties on every other rafter pair not every one. This has been my understanding and what I show in the open beam cathedral ceiling details on my plans. These have been successfully built in many code enforced areas.

[rwanders]

Now I'm a little confused-----I built a 24x24 1 1/2 story with a 10x24 loft at the rear with a 10x24 double-decker porch on the front. The roof extends over the decks. There is a 14' open area at the  front with a 4x14 "bridge" crossing to the upper covered deck. My side walls are 12' 2x6 studs with a glulam supporting the loft. This results in a 3' knee wall in the loft area and a 10x19 usable loft area. Roof is 12/12 pitch with a 6x16x36' ridge beam supported at gable ends. Rafters are 2x12x22' which gave me very wide eaves (5' on the run) to dump snow 4" from the cabin walls----wind loads are fairly light and no tornadoes here so I thought I could get away with the big overhangs.  Completely clear cathedral ceiling  with no collar ties-----should I be worried about stresses on the walls? I did run by my plans with a structural engineer friend but no formal analysis was done or required. 

[John Raabe]

While we can't give specific engineering advice here, having a ridge beam support 1/2 the roof load does away with the need for collar ties. You deep rafters should be fine for the wide overhangs. If you have had a trusted engineer look at the project you are probably on solid ground.

You might ask him about the open area at the front. If I'm visualizing it right you could, if needed, tie the walls together with a tension member through the bridge if there is a concern.

[Don_P]

Be very careful with the terms used to describe these parts, try to use the correct terms to avoid confusion.

A collar tie is always required. It is usually under the ridge as in John M's pic, but can be anywhere in the upper third as in John's pic, or can be a metal strap running from one rafter's top edge, over the ridge and nailed along the top of the opposite rafter. A very common failure, especially with improperly installed gable trusses, the gable blows in and the roof zippers open along the ridge. Two roof diaphragms are seen leaving the scene. Garage doors blowing in and lifting the roof from the inside is another way you see this failure. Collar ties are required at least every 4'.  A collar tie does not resist rafter horizontal thrust, it is tieing the rafter tops together, it is too high up to effectively keep the rafter feet from spreading.

A rafter tie is required unless a structural ridge (or ridge beam is the term in the code) is used. The rafter tie is prescriptively required to be in the lower third of height and on every rafter pair. It ties the rafter feet together across the building to prevent them from wanting to slide outward. Sure one can step outside of prescriptive if they know what they are doing, with an engineers approval, and with good reason.

If we go every 4' with the rafter ties, the connection stress doubles, raise the ties and it about doubles again. Raised ties also put an additional bending load into the rafter. That is the reason for the reduced spans or increased depths in the code tables for raised rafter ties. By skipping every other rafter this bending load is also doubled on the tied rafters.

Go higher than the code limit and the stress begins multiplying those numbers fast. Much like the working load vs the ultimate strength on a rope the allowable stresses are not the ultimate stresses wood can withstand. When we step outside of codebook prescriptions without a full understanding of why those things are the way they are, we are into our safety an unknown amount and are using a highly variable material.

The code reference and the inspections reference describe codes that are outdated. The current building code is readily available online and is the minimum standard.

I had this discussion with a local designer less than a month ago. The ties were spaced and in the upper third. I suggested a structural ridge and suggested a size... I didn't know what I was talking about but he humored me since I wasn't going to do it his way. His engineer came up with the same ridge beam I did. The "correct" way to do this is to tie it as per code, use a structural ridge (the supplier will engineer it for free), or have it checked by an engineer.

[John Raabe]

Well, Don is right! The code wording has changed between the 2003 IRC and the 2006. Rafter ties in the bottom 1/3 of the rafter were allowed at 4' o/c in the 2003 code but in areas where the 2006 code is adopted they must now be on every rafter pair.


click to go to the PDF article.

If the 2006 code has been adopted in your area you may not be able to use the simple framed cathedral detail I have shown in my plans where a double rafter tie is exposed at 4' o/c and ties the walls together. If you have to tie every rafter pair then you cannot effectively have a cathedral ceiling using standard framing. You will need to go to the much more expensive and difficult job of installing and supporting a ridge beam. Note that raising the rafter tie off the plate and as high as 1/3 the rafter length is allowed and will produce a coffered style ceiling that exposes part of the roof slope.

Please understand that if you have built a roof in a manner that would have been approved in 2003 (or 1973!) it does not mean that this roof will be flying off the walls the next time the wind blows! What is happening is that codes are continually being revised to include ways to build buildings that address more and more unlikely fringe events. This is good and results in continually but marginally safer buildings. Are there older homes in your area? Perhaps you are living in one. Those houses have hundreds, perhaps thousands of details that are in violation of the most current codes. Wood frame construction using sticks and panels attached with wire nails is time proven to be strong, flexible and forgiving and yet it is constantly being improved. Be assured, whatever you are building now it will soon be out of date - that is inevitable.

[Don_P]

Thanks John,
One way to look at the cost of the ridgebeam is to deduct the cost of the rafter ties that it replaces. And possibly the cost of an engineer. They can be a pain to lift, however the design is driving the need. We've used machines when available but have also set them using pump jacks, towers of scaffold, gin poles, and the armstrong method.

A ridgebeam makes the kneewall thrust issue disappear as well. If the ridge cannot drop the rafter feet don't try to spread. It is an easy structural solution that satisfies code.

[John Raabe]

Don:

Your points are well taken. However every time a code revision disallows a traditional framing method (in this case one that has been used for over 100 years) it pushes the home building process further out of reach of an owner builder and the small crew builder, especially those in remote areas.

Traditional small dimension wood framing was what turned every farmer into a builder and made housing affordable for millions of families that settled the West. Going to ridge beams for roof support (as I do with the Victoria cottage) locks the design of the supports and freezes the owner from making free interior layout changes. Such a design has little flexibility and changes will now require redesign and likely re-engineering all the way down to the foundation where new footings will be needed.

Of course there is no one to speak for these owner-builders when codes are being revised and they are a quickly dying out at any rate as building becomes a Pro-only specialized occupation. Who other than Pros could hope to keep up with the constant rule changes?

OK, enough of my self-serving complaining.

I would love to know how many and under what conditions roofs actually failed because rafter ties were 48" o/c rather than 24".

[John_M]

Let me add this additional photo, it is a little later on in the process.  I meant to add it first but must have clicked the other photo.  It shows my finished ceiling height of 8 feet.  So I guess I have rafter ties AND collar ties?

[archimedes]

thanks guys, I've learned a lot from this thread.

[John Raabe]

John_M

While there may be several ways an inspector might find fault with this, let's do a thought experiment.

Do you have a metal framing anchor at the rafter to wall connection? You should and they should hold the rafter against outward forces.

Do you have a metal strap or gusset or wood collar at the top of the rafters to keep them working as a unit along the ridge board? You should.

Then you have your deep rafters on either side tied together and triangulated by the ceiling rafter/tie. This is making the rafters work together a bit like a stiff legged truss.

And all of these elements are further united into a diaphragm with the nails and roof sheathing. These parts all work together to share the loads from side to side and between the rafter sets.

[nsd66]

Hello all,

Noob here.  I've been lurking for 6 months or so.  Bought the 1.5 story plans.  No land yet, just evolving plans and ideas, and a monthly deposit into the cabin fund.

First, thanks for the great forum and all the fantastic input.  I've been learning a lot.  Not much for me to post, since I can generally find answers/ideas with a search.

I've been going in circles with wall height and framing for the walls and roof, so this thread piqued my interest.  For the roof framing, I think that the codes and my desired location (NW lower Michigan) have finally overwhelmed my DIY impulse.  I don't want to use a ridge beam, so it's looking like parallel chord trusses are my current design direction (e.g. http://countryplans.com/smf/index.php?topic=8693.0).

Personally, if it wasn't for code, I'd be O.K. with the site-built trusses and 2x12 rafters with a rafter tie every 48".  This is in part because I don't think that 60 psf of snow (~10' worth of powder, ~3' packed powder or ~1' of liquid water) would ever be able to stay on a 12:12 pitch metal roof.  However, for insurance and potential resale implications, I do want to pass code.  Dear God, if I do build this, please don't let me have to sell it........

As part of this exercise, I did some frame analysis for the trusses and balloon framed walls (using this: http://www.ralfmartinhansen.de/twdframe/indexe.htm).  I'm not a structural engineer, and there are a lot of assumptions that have to be made, so I'm not sure if everything I assumed was "best practice" (based on some papers I saw, it even looks like "best practice" isn't all that well defined).  In the end, my takeaway was that IF there was 10' of powder on the roof and IF I balloon framed with a let-in ledger and IF my wall height was 10-12 feet and IF I hadn't used some _really_ good 2x6 lumber for the walls.....I sure wouldn't want to be jumping up and down in the loft.

Please don't take this as anything near a negative remark on the plans....I think they're great and, as I said, would like to build them as is (I'm conservative, so I might beef up the trusses a bit and put the rafter ties right at the top plate).  It's just that with my snow load I don't think they'll pass a thorough engineering review.  I spent some time looking at possible reinforcements at/near the knee wall, and it
definitely could work, but the added time and cost, plus the need for an engineering review were turning me off.  I finally started adding it up:

60 psf snow load --> added bracing and engineering review (plus way too much time analyzing things)
R49 --> fiberglass needs > 12" rafter/truss depth, other insulation options more expensive
24" ties --> not so attractive

So, with all that, I think I'm finally letting go of the do-it-myself thing.  I'm thinking that in the end the trusses might even save money.  Now, can I somehow shove the engineering for the >10' walls I want on to the truss manufacturer?

Sorry 'bout the long and rambling first post.  I hope that at least some of this might help someone in a similar situation......

Best,

--Nolan