Cathedral roof question

[Ignavus]

Hello,
I am looking at building a small cabin, 14' by 20', and would like to do a cathedral style ceiling, to allow for a small storage/sleeping loft at one end, and an open ceiling at the other. To do this I've gone with a 45degree roof pitch, giving a total height (from floor to highest peak in the ceiling) of around 15'. I am obviously keen to avoid trusses and collar ties, so I can keep the ceiling as 'open' as possible. I've thought of making a metal brace in a V shape, made of 1/4" steel welded at a 45 degree angle, approximately 2.5 feet long on each edge. This would be bolted to each of the rafters to prevent lateral spread.

Does anyone know if this kind of thing is suitable, whether I need to get an engineer in for the details, or of an alternative method? I don't have a ridge beam at the moment - how would a ridge beam go at such a steep pitch, with no collar ties?

Thanks,
Andre

[Erin]

I'm no engineer, but I would bet that wouldn't be enough to prevent spread...

If you don't want cables or collar ties or anything in your ceiling space, have you considered scissor trusses?  

[Ignavus]

Hi Erin,
Unfortunately, even at 45 degrees there isn't quite enough head room in the loft, so scissor trusses would (I believe) cut into that too much.

I'm also contemplating making the metal brace into an A shape, rather than an inverted-V, so there would be some element of horizontal bracing in there. However I anticipate that this wouldn't come down very far, so 1' or so.

Also, I'm building this in New Zealand, and the building dept. are pretty hard on this kind of DIY building without full engineering details, so I may be out of luck with anything this extreme.

Thanks,
Andre

[Squirl]

Ridge beam. Pitch doesn't matter with a ridge beam.

I don't know the roof line is 14' or 20'.  Either can be done. 20 ft would probably need a post some where mid span.

http://countryplans.com/smf/index.php?topic=10511.0

Use the roof and ceiling beam sizing for the ridge beam.

You can also buy a glue lam beam to size for 20 ft.

Sizing depends on ground snow load.  Where are you located or what is the ground snow load?

[Bob S.]

I looked into this same issue. I descovered that parrallel scissor trusses that were 2' thick was not a lot more money than rafters and coller ties. And you do not have to worry about the spreading thrust on the walls.

[Ignavus]

Hi Squirl,
The roof line is the 20' dimension, and again, I'd like to avoid having something in the middle of the space to support it. The snow load is essentially negligible, as we're almost at sea level in an area with no snow. However the official level is about 20psf (1kPa).
It sounds like the best solution might be to go with some kind of engineered ridge beam.

Regards,
Andre

[Squirl]

Many building departments here in the states will accept the engineering stamps for glu-lams without additional engineering.  No engineering is required if it is prescribed in the code which I posted a guide too. You calculate the load by multiplying the square footage by the snow load.  So a 14 x 20 = 280 sq. ft. For hypothetical say you have a snow load of 30 lbs. So that would be a total roof load of 8400.  The ridge beam would have to hold half of this, or 4200 lbs.

The collar tie is to prevent up lift and can be placed anywhere in the upper third of the roof line.  Don_P wrote a nice guide somewhere around here.


Just remember to carry the load directly down to the foundation.

[MountainDon]

You have indicated the building department there being hard on DIY'ers. Probably for good reason; many DIY'ers with little to no formal training do get themselves into areas beyond their skill level.  With the fact of a building department possibly being in the picture they may be the place to start.

You state you want an open overhead. There are two parts of a proper rafter built roof that get into the way. One is the collar ties you mentioned. Here in the USA those are the connections made in the upper third of the rafter triangle area; usually up fairly close to the peak. The other, that has not been mentioned, are the rafter ties. Rafter ties are located in the lower third of the typical rafter built roof. They are frequently do double duty as ceiling joists in a non cathedral ceiling. They are necessary to prevent the outward rafter thrust from spreading the wall tops. Collar ties are not a substitute for rafter ties. Sometimes beams can be placed across the wall tops in the open area to hold the walls together. Again, the building dept should be able to tell you what they would require as far as spacing and size. Depends how friendly they are. Here, they only have to pass the plans or not and are under no obligation to supply design info. But many departments have good people that will advise on what will be required to pass.

To build a proper cathedral ceiling a structural ridge beam is frequently the best solution. A manufactured beam can be sized to cover that 20 foot distance with no intermediate supports. The suppliers of such beams can provide the correct size for local conditions if supplied with the plans for the balance of the building. They may or may not be able to advise how to support or transfer that load to the foundation. Sometimes an engineer is needed to do that.

Scissors trusses could be used; they almost eliminate all outward thrust. Properly designed they will contain the outward thrust to within good design parameters. In loft area the trusses could be raised a couple of feet on short "kneewalls". In the open ceiling area the wall studs could be 10 foot instead of 8 foot, for example. That would increase the headroom in the loft. Narrow buildings are difficult to design with any amount of loft headroom, comes with the narrow width.

[Don_P]

What you are trying to design is a moment resisting connection. With bolts I fear that you'll get into splitting around the bolts as the connection tries to rotate. This can be done but you are into an engineer's territory. I'd go with a structural ridgebeam myself. The supplier will do the calculations and size the beam, you'll need to check the supports at each end and safely deliver those loads to adequate footings.

I do the math a little differently than Squirl but they get you to the same place... if we include everything. Assume the self weight of the materials, the dead load, is 15 psf + the live load of 20psf= 35 psf total load. Half the width of the roof is 7' x the span of 20' = 140sf x 35 psf= 4900 lbs. Half of that will be supported by each end column, don't forget to add any overhang to that load. You can also install metal straps from rafter, over the ridge, to the opposite rafter to prevent uplift at the ridge as opposed to a collar tie in the upper third, either is holding that top connection together in a storm.
I'm coming up with a 2 ply 14" LVL as being adequate from your description.

[Ignavus]

Thanks for the responses. It seems like you're saying that with a properly engineered ridge beam, I could do away with collar ties and rafter ties. Is that correct?
There is some element of rafter ties, in that the loft floor joists will connect the opposite walls, and reach 8' down the length of the building. These joists are supported by a ledger, so the ceiling height underneath the loft is only 7', although the external wall is 8'.

I may not be describing the layout particularly well - my preliminary drawings are at http://www.ignavus.net/1200B%20Okains%20Bay%20Plans.pdf for anyone interested. Hopefully the metric measurements won't be too off putting.

[MountainDon]

Yes, the loft floor joists do the job where they are in place. But that leaves the other end of the cabin open to rafter thrust spreading the walls. That where beams across would do the job and add visual detail. Or the ridge beam. Ridge beams work as then the rafters just sit on the ridge beam and the walls; the only force is down; in theory at least, winds might cause some lateral loads. But the ridge beam removes the constant outward thrust that will develop with a standard ridge board and rafters.

Nice drawings. You do them?


Remember the ridge beam needs proper supports to take the load down to the foundation/ground. That requires some sturdy posts in the gable end walls plus a means to get that to the ground.

[MountainDon]

Pier bracing is not illustrated in the plans. longitudinal and lateral bracing is required for a sucessful pier foundation and is often lacking or inadequate in many builds. The larger the cross section of the piers and the greater the depth in the ground, the better everything will be.

NZ can experience some good winds as far as I know. Do you have wind charts there like we do here in the USA?

[Ignavus]

Thanks for the point on the pier bracing, I had thought of that at one point, but obviously missed putting it into the drawings. I'll rectify that shortly.

For the wind loads, there are tables available, but I haven't done the calculations yet. In general the specifics of the calculations seem a bit tricky, and I've been told that the building inspectors can be friendly enough to help you out with those things. I'll see how that works out when I eventually submit the plans.

Regarding the ridge beam vs ridge board, I may be missing something there. Is the difference simply that a ridge beam is supported at the end by the end walls all the way to the foundation, while a ridge board is held in place by the rafters?

I did do the drawing myself - I spent ages hunting around trying to find a decent vector drawing program that understood dimension lines etc... In the end I found Cadstd - http://www.cadstd.com/. There is a freeware version (which I used), and is very good with dimensions etc... Can be a bit fiddly though, but overall I found it quite quick. Probably spent about 4-6 hours overall to do those drawings. I think I spent longer looking for the software than I spent using it.

[Ernest T. Bass]

Regarding the ridge beam vs ridge board, I may be missing something there. Is the difference simply that a ridge beam is supported at the end by the end walls all the way to the foundation, while a ridge board is held in place by the rafters.

Yes indeed. I thought someone should point out--you asked earlier if a ridge beam would do away with a collar ties and rafter ties. While it may do away with rafter ties to some extent, collar ties or an equivalent are still required to prevent the rafters from pulling apart due to high wind and/or seasonal movement and such. You could secure them very un-obtrusively with a metal strap over the top of the rafter pairs.  

Really good drawings! You know things are going to work out well when you plan ahead so carefully.

[MountainDon]

Regarding the ridge beam vs ridge board, I may be missing something there. Is the difference simply that a ridge beam is supported at the end by the end walls all the way to the foundation, while a ridge board is held in place by the rafters?

A ridge beam is designed to be large enough to support one half the total roof load while being supported at the ends, or in some cases with intermediate supports as well. With only end supports one half the total roof load is divided between the two beam ends and on down to the ground. Each side wall then only supports 1/4 the weight of the total roof loads.

The typical ridge board is simply a spacer between the rafter pair and helps locate each rafter pair ib relation to the adjacent rafter pairs. The load on the roof is is divided between the two side walls; half on each wall. The load on the wall is resolved into a downward force, down to the foundation, and an outward force, trying to spread the wall tops. The steeper the roof the less the outward force. If you have ever seen a building with a roof ridge that sags in the middle of the run you have seen what can happen when the rafter outward thrust has not been properly restrained.


Thanks for the collar tie replacement reminder ETB