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godin 07-04-2006 10:21 AM

rafter thrust/cathedral ceilings
Last August, SSeeley stated that an inspector has disapproved a framing inspection due to inadequate lateral support. SSeeley was building a wood framed 14x16 addition with cathedral ceiling, with 8 in 12 rafters on 24" centers. Collar ties were placed about 10'9" above the floor. The inspector expected the walls to bow.

I have a 60 year old masonry house (12" walls: 8" clay tiles with 4" face brick) about 22' wide (interior) and 40' deep. 8' ceilings and 9 in 12 rafters forming a gable roof. The living room spans the front of the house. The ceiling joists run front to back, so only the plaster ties the side walls. So why doesn't the brick crack (or worse) from the roof thrust? Apparently, the masonry is thick enough that the mortar joints can withstand the shearing force for a room only 12' deep.

I'm asking, besides just curious, because I've considered removing the wall dividing the kitchen and rear bedroom and replacing the ceiling with exposed 2x8 collar ties on 16" centers at 10'8" height. That certainly seems stronger than the living room with respect to rafter thrust (the rear room would also be about 12' deep.)

Any thoughts? Thanks

Bonus 07-04-2006 10:25 PM

Collar ties are not there to counteract rafter thrust, they merely shorten the span of the rafter so that you can use smaller material. I'm not sure I got it but it seems like you want to put ceiling joists in? (Effectively lowering the collar ties to the top of the wall?). If so, that will definitely keep the inspector happy. Another possibility is to put in a ridge beam with posts in the wall underneath (and all the way to the foundation). Once you get into it, you are likely to have to get an engineer involved and once you do the inspector will back way off.

godin 07-05-2006 06:23 AM

I thought it would be interesting to remove the ceiling (and wall between the rooms) and 'replace' it with 2x8s that were, say, 2' higher than the original ceiling, spanning the rafters and acting as collar ties (there would be no drywall attached to them, thus giving some of the feel of a cathedral ceiling.

manhattan42 07-05-2006 07:54 AM


Originally Posted by godin
I thought it would be interesting to remove the ceiling (and wall between the rooms) and 'replace' it with 2x8s that were, say, 2' higher than the original ceiling, spanning the rafters and acting as collar ties (there would be no drywall attached to them, thus giving some of the feel of a cathedral ceiling.

Whether you can do this depends upon the size of the existing rafters.

The ability of rafters to span a given distance is reduced when the lateral bracing is removed from the top of the bearing wall and raised vertically as a 'collar tie'.

What size rafters and what is their current spacing and the spieces of lumber if you can tell?

godin 07-06-2006 02:39 PM

The rafters are 2x6, probably southern yellow pine, on 16" centers. Thanks for everyone's response.

manhattan42 07-07-2006 07:36 AM

Won't Work
What you propose will probably not work without major structural changes.

Here's why:

Because clay tiles add about 20lbs per square foot of load to the roof, even with no other loads from wind or snow, the maximum distance #2 SYP 2x6s can span is 13' 5". This is fine for your current roof span of 11' as long as the cross laterals are ceiling joists on top of the bearing walls.

But once you will raise the lateral bracing vertically off of the bearing walls by 24", the spanning ability of those rafters is reduced significantly.

In your case, the spanning ability is reduced by 24%. This means that the existing rafters will now only be able to span 10' 2" and will be undersized for the existing roof loads.

If you need to consider additional wind loads or snow loads in your location, then the spans are reduced further and the threat of a major structural failure becomes very real.

For example, if you need to consider an additional but modest snow or wind load of say 20psf in addition to the 20psf clay tile load, then the maximum spanning ability of your 2x6s is reduced to 7'5" or less...nearly 4' too short for your roof span of 11 feet.

Even if a snow or wind load is merely 10psf, the rafter spans can still only reach 9' 1".

So it appears the only way you can accomplish what you wish is to either reduce the height you plan to raise the cross laterals above the bearing walls by about 12" or less, or to raise the cross laterals to your intended 24" but increase the size of the rafters to at least 2x8s.

Bottom line is, that by raising cross lateral bracing vertically off the top of the bearing walls, you decrease the stability of the roof system and the ability of the rafters to span the distance under the imposed loads.

The higher you raise the cross laterals, the weaker the roof system and rafters become.

If you want a definitive answer to your problem, you will need to have an architect, engineer or your local code official determine additional loads on your roof apart from the clay tiles and come up with a proper rafter and roof system design for the loads. Anything less is simply guessing....and dangerous.


godin 07-08-2006 09:42 AM

The roof is made from ordinary 3-tab shingles. The walls are made from clay blocks or tiles.

manhattan42 07-09-2006 06:44 PM

Ok, Ok...
3 tab shingle not clay tiles.

For Southern Yellow Pine #2 grade at 16" on center, your 2x6 rafters can span 12' 6" presuming a 30psf live load and no drywall.

Your current rafters can handle this span.

Raising the cross laterals by 24" above the top bearing wall plates to form a cathedral ceiling will reduce the span ability of your existing rafters by 24%, or allow them to span a maximum distance of only 9' 6".

Your span for the room is 11'.

The existing rafters will still be undersized if you raise the cross laterals 24".

In order to get the effect you want, you will either have to increase the size of the rafters to at least 2x10s or you will need to limit the amount you raise the cross laterals above the bearing walls to about 16" or less.

Local wind and snow loads can greatly reduce the amount you can raise the cross laterals or prohibit you from being able to raise the cross laterals at all.

You still need a design professional and/or your local code office to assess your existing structure and loads to determine if what you wish to do is possible or not.

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