Any practical way to frame a 24' span cathedral ceiling?

[Ernest T. Bass]

'Ello fellas...

We have a friend who wants to build a simple addition w/ a gable roof off of his house. He's not totally sure of the size yet, but it could be as much as 24' wide by 18' deep, and he wants a cathedral ceiling. The existing house is just a a simple 24' wide gable with a 4/12 pitch, and he'll just be coming of the side of one end with his addition to make an "L" shape. He doesn't want the ridge of the new addition to be higher than the ridge of the main house, so we won't be able to go much steeper than a 4/12 on the addition, depending on how wide it ends up.

Soo.. I'm thinking that scissor trusses would be the easiest option by far, but he wants as much interior pitch as possible, and that won't translate to very much if you've only got 4/12 on the outside. I know you can get the trusses engineered to feature a steeper inner pitch, but cost is also an issue.

As far as I know, the only other option is framing a rafter roof. If we went with an 18' load-bearing ridge beam, it would definitely need a mid-span post support. The addition is still in the planning stage and a central post could possible be incorporated into the design, but I think it would be preferable not to have one.

The last option that I'm not sure about, would be no ridge beam and 24' tie beams spreading clear across the room. I don't even know if those would be self-supporting at that length. He would like the look of log tie beams, but I'm not sure how you would securely attach the ends of the log ties to the wall top plates? Also, having a beam every 4' would be rather visually obtrusive to the open ceiling.. Could you possibly beef up the top plate and go with fewer ties, maybe 6' apart?

Sorry for all the rambling; I know it can be hard to visualize these things without pics. Hopefully I made everything clear, but if you need further clarification just let me know.

Thanks a million,
Andrew

[davidj]

As far as I know, the only other option is framing a rafter roof. If we went with an 18' load-bearing ridge beam, it would definitely need a mid-span post support. The addition is still in the planning stage and a central post could possible be incorporated into the design, but I think it would be preferable not to have one.

Our neighbors have a 24'x32' cabin with just one center post, so their beam span is 16'.  I'm not sure what snow load it was built to, but something between 100psf and 150psf.  I think they ended up with a beam a notch or two down from a 6 3/4 x 18 glulam (which is what I needed for my 20'x20' open area @ 100psf snow load).

[Ernest T. Bass]

Oh wow, I wasn't aware that the beams could support that much. Perhaps we could make the 18' span unsupported after all..

Our snow load is around 70psf. I'll have to do some further research on those lvl beams..

[John Raabe]

Yes, your neighbor could do a ridge beam with support at either end. You do want to make sure the "post" at either end gets down to proper bearing at the foundation. On the house side you may need a new heavy header if there is an opening under the beam support. There will be a lot of weight coming down under those beam ends.

An alternative is to have rafter ties such as double 2x6 in the bottom 1/3 of the ceiling height (usually 48" o/c). This eliminates the beam and is usually less expensive.

See "timber style open ceiling detail" on page two of the PlanHelp details here: http://www.planhelp.com/public/programs/downloadsearch.cfm?StartRow=16&searchtype=simple&searchmode=cat&keywords=All%20Files%20in%20Plan%20Details%20Category&cat=Plan%20Details&sortby=name

[Ernest T. Bass]

Yes, your neighbor could do a ridge beam with support at either end. You do want to make sure the "post" at either end gets down to proper bearing at the foundation. On the house side you may need a new heavy header if there is an opening under the beam support. There will be a lot of weight coming down under those beam ends.

There would actually be no opening directly bellow the ridge, so that would work well. My biggest concern is the weight.. We'd rather not have to use a crane to get that beam up there. Could a glue-up made in place be strong enough?

He's also decided to narrow down the addition a bit and make the room an 18' square, so that should help a lot.

[rdzone]

You could probably use parallel chord trusses like we did in our cabin to give the highest catherdral ceiling we could get.  Our pitch is 7/12 however.

http://countryplans.com/smf/index.php?topic=1499.20

[Ernest T. Bass]

Awesome place! Would you mind my asking how much a truss like that costs?

[NM_Shooter]

We did timber trusses and purlins.  The trusses have an adzed texture.  Not as open as a scissor truss, but we like the way they look.  This is a 5:12 pitch.



Got them locally from Adobe Building Supply.  Their web pages are fun to look at, and they will ship anywhere:

http://www.abslumber.com/index.html

[Ernest T. Bass]

Those look absolutely incredible, but I'm sure that they're out of his price range..

[rwanders]

I have found that suppliers of large glulams usually deliver with crane trucks and will put them in place for you if you have the required framing ready to receive the beam----no extra charge. I have a 6x16x36' glulam ridge beam in my 24x34 with no intermediate supports for a 12/12 cathedral roof.

[John Raabe]

Very nice job on those trusses Shooter!

[rdzone]

Ernest,

I think the the total for the trusses was about $2100 that is for the the two gable end trusses and 17 parallel chord trusses, I had an extra one due to my chimney placement.  That was engineered here in Alaska for the snow load, which can be significant.  I will say they were pretty heavy as the crane couldn't set them on our top plates do to the height of the cabin 32+ feet not including the slope of the hill. 

[Don_P]

Just  for something to stick under your cap, a kingpost truss like NM Shooter's can act as a midspan support "post" for a ridge beam if sized to do so. There is alot of design flexibility once you step outside of the box. Another way, the proposed 18' span could be done with just log purlins bearing on the framing at each end. Doing a quick check, if the purlins were set on 4' centers spanning 18' it looks like they would need to be about 12" diameter midspan for "average" species and grade. If that design incorporated a kingpost truss at midpoint the purlins could drop to ~ 7-3/4" midspan diameter, just more options.

[NM_Shooter]

Very nice job on those trusses Shooter!

Thanks... but not my handiwork.  I bought those trusses turnkey.  I think they were $900 each if I remember right.  I suspect that given some time we could have fabbed them ourselves, but I wanted them done quick.  Quick I ain't.  Anyway, we like the look.  We also like the way the room looks with the cable lights that run under the bottom chords.  We have some of the spots aimed upwards, and the shadows and highlights caused by the trusses and ceiling are interesting too.  

It's awful to dust though.  Good thing we don't bother with that.  

BTW... the dimension on that room is 25 X 40, and behind the cabinets is a half bath.  The wet bar / cabinet island helps to break the room up into two areas. 

[Ernest T. Bass]

There is alot of design flexibility once you step outside of the box.

That's for sure! I knew you guys would tell me all the different ways to skin the cat once the thread was started. If it were me, I might opt for the log work, but we would want to get this done as quickly as possible and we aren't the most experienced of carpenters.. Also, while one exposed log might look nice, a whole timber-framed ceiling probably wouldn't match the existing house too well.

It sounds like a glulam or lvl ridge beam might be the way to go. I've got an idea of the difference between the two, but what are the pros and cons of either? I'm seeing a lot of lvls that are 1.5'' thick, so does that mean would could get the beam in two pieces and sister them together? My other concern is cost and availability.. Can you buy those things through a lumberyard chain like Doit Best?

Sorry for the stupid questions--yes, I know there is no such thing. I'm showing my naitivity (sp?) here..

[John Raabe]

Thanks... but not my handiwork.

Buying counts too! I'm curious, did a local truss company do those or was a woodworker/cabinet maker the craftsman?

I tend to think of truss companies as being good at stapling 2x4s together with metal plates.

ETB: I think you will likely be into a glulam - lvl's are normally used for headers and such and not left exposed. They are just higher rated 2x's that can be doubled or tripled. Glulams are single built-up beams that are bigger, longer and wider... and meant to be exposed (ie under and supporting the roof rafters). They are not doubled and can be designed with a camber so that when loaded they are flat.

[Don_P]

LVL's are often built up and used for ridges and girders though. I have cased a few in finish boards that were exposed. The beauty of that is they can often be hefted ito place by hand. They also are available in depths and lengths that you can't find dimensional lumber in, for example 18" deep is pretty common. LVL's are very easily supplied by the local lumber company. Glulams are nicer appearancewise and are almost always left exposed as is. They are a bit more difficult to source but generally available everywhere. Above about a 4x12 you need to have a small crane or boom truck handy.

[Ernest T. Bass]

LVL's are often built up and used for ridges and girders though. I have cased a few in finish boards that were exposed. The beauty of that is they can often be hefted ito place by hand.

That's kinda what I was thinking... The ceiling will be finished in wood anyway, so you could easily box the beam in.

Are there span tables for the lvls, or does the lumber company calculate the size for a given load depending on the brand?

[NM_Shooter]

Buying counts too! I'm curious, did a local truss company do those or was a woodworker/cabinet maker the craftsman?

These were done by a local timber company.  They do lots of heavy timber work, and sawmill ponderosa to size.  Carvers and other craftsmen employed.  They have a machine that they feed the material through to give it the adzed finish. 

Engineering documents are available for their timber trusses upon request (extra fee).  And they ship their stuff anywhere.

Check out this website:

http://www.abslumber.com/index.html

[Don_P]

There are LVL guides and calcs on the web, the local building supply has the tables and software to size the brands they carry. All the manufacturers back up the building supply with their in house engineers. I've had situations where the inspector wanted an engineer's okey dokey on a change of size and span and the local building supply faxed the proposed change to the manufacturer who sent back a stamped letter for the beam. That's a nice service and saved the customer a coupla hundred and the time it would have taken for me to get a local engineer out. Glulams are serviced the same way. Engineered lumber has much better product support than sawn lumber, go figure. I've sent a request in to the folks that made the joist and rafter calcs asking for beam, girder and header calcs but in this downturn they said they are strapped right now.

GP (Georgia Pacific) is our most common supplier here. I have a copy of their "Residential Floor and Roof Systems Product Guide" that has span tables and detail drawings, etc in pdf. It looks like this is the current link, I'm on dialup and its a 17 mb file so I didn't check beyond the title, my old file was 2.25 mb, they are crammed full of good info, I'll probably download this one overnight;
http://bluelinxco.com/Portals/0/docs/LiteratureLibrary/Engineered%20Lumber/GP%20Product%20Guide%2010-31-08.pdf

BC (Boise Cascade) is another large supplier that may be more common there.
http://www.bc.com/wood/ewp/software/bccalc.html

The main difference in LVL's is the stiffness rating, the modulus of elasticity. You'll see them called 2.0E, etc basically the higher numbered ones are stronger and stiffer so can span greater distances with less deep members.

If you get into glulams there is a lot of info on the AITC website (I think they're glulam.org) and links from there to manufacturers.

You are looking at around 13,000 lbs on each end, that is going to take a serious column and footing.

To answer an earlier question, no you cannot site build a glulam of horizontal laminations. It requires freshly surfaced material of controlled moisture content using structural glues and correct uniform clamping pressure. One glueline failure is very likely a beam failure. We can make built up beams on site of vertical members side by side but don't try to make a stacked up beam, its too risky.

[Ernest T. Bass]

Thanks so much for all the info, Don. You're quite the engineer; I've been trying to wrap my head around other posts you've made in different threads and forums.

You are looking at around 13,000 lbs on each end, that is going to take a serious column and footing.

You think that much? I was figuring 70 pounds for snow and 20 pounds dead load; for a section of roof 18x10 that's 16,200--only 8,100 each end. I was wondering if that might be asking too much for the existing foundation to hold on one side.. The house is on a block basement, but I don't know the width of the footer. The soil is pretty lousy clay around here, though.

We can make built up beams on site of vertical members side by side but don't try to make a stacked up beam, its too risky.

If two lvls were going to be paired to make a properly sized beam, would they have to be fastened together in any special way?

Once again, thank you guys so much for the help.. I have to go post on the "I love this site" thread now.. I knew as soon as I saw that thread title that my conscience would bug me 'till I did.

[Don_P]

 I'm just a nail banger, although I did get to play with my cousin's train set one Christmas. I have found people tend to rise or sink to the level of our expectations of them. Most of what I've shown on the web was handled by carpenters in days gone by. Most people do not realize that my old codebooks had pages of tables giving the design values of many species and grades and stated that I could submit my own calculations for beams, joists and rafters if my needs were off the tables. This is now pretty much strictly interpreted as an engineer's territory now although the codebook does not use that language. We are not heading in a good direction by prohibiting laymen from doing simple design and engineering. Its also good if everyone is checking each other. You just proved that 

Half the roof load will be carried by the ridge beam, half of that will be on each end post.  Lets try it again, I muffed it.
If the room is 18x18 then a swath down the middle of the room 9'x18' is supported by the ridge... 9x18=162 square feet. Typically 10 psf covers dead load and we have a 70 psf live load (snow) so 162 sf X 80 psf= 12960 lbs total resting on the ridge. Half at each end= ~6500 lbs under each end. That's getting much better. Lets assume its 1500 psf soil for the moment, that's generally pretty lousy but buildable. 6500lbs/1500psf= 4.33 sf minimum footing size square root=2.08' on a side, make it 1' thick minimum so the post doesn't punch through. You can always make it bigger, just make it thicker as you do.

12960/18'=720 lbs/ft on the beam total
My table shows a double 16" or a triple 14 passes cleanly
If you can drop the clear span to 17' a double 14 or triple 11-7/8 squeeks by.

There is a page of connection details in the guide. If you are hanging the rafters on the sides of the beam the connection is in the side loaded chart. It is more demanding than the top loaded connection since they need to share the load from one side to the other. For that load it's calling for 3 rows of 16 commons at 12" centers along the beam.

Thanks for the high praise, boy am I gonna be embarrassed if I blew it again 

[Ernest T. Bass]

Thanks a bunch! I forgot to mention that he was thinking of going 20' wide with the addition now--that's why I said the ridge would be supporting a 10' swath. Would a double 16'' lvl still pass according to your table?

EDIT: I just downloaded the GP pdf you posted and it appears that two 16''s would still be in the clear. Thanks for the link!

This is a pretty exciting project for me, as I've never built anything the conventional way before! I'm trying to find all of PEG's framing advice from the past several years..

[jr1318]

Double 2x14" lvls 22 ft long no ceter post. Tese are supported on ends to headers, joist hanges on top huricane clips on bottom and will handle northern Wisconsin snow loads

[Ernest T. Bass]

How wide is your building, jr1318? We went 20x20, and the lumberyard calculated we needed a double 2x16'' lvl beam, so that's what we used. Joists are notched over the top, beam will be boxed in w/ pine from below. We got the shingles on last Thursday--managed to insulate the floor from above and didn't even have to tarp it!