hip roof - self supportive?

[bighammer]

I am in the process of bidding a shelter house for the state goverment. the building is a 4 sided hip without a ridge board. they have in the plans hand framing the roof, without any ceiling joists or collar ties. my question being is a hip roof self-sustaining?? and good ways to cut the rafters?? what about a header acrosss the hips and jack rafter it like you would in a truss system (if that makes sense to anyone...) the plans are not stamped, you would think they would get this enginered, right??

[Richard A Hetzel]

A lot depends on the size of the building. I assume it is a square. If it is very small (like less than 10 feet on a side) you might get away with no ceiling joists/rafter ties. There is still lateral thrust from the roof, but it isn't evenly-distributed like it is in a gable roof...it varies from nothing at the sdie walls to the maximum in the center of the supporting wall. In a very small building, the horizontal beam formed by the double top plates of the wall can resist the outward thrust. In a larger building, it cannot do so, and rafter ties, usually ceiling joists, are needed.

[bighammer]

the building is only 16' x 16', and I have bid it using ceiling joist, however, If you are saying that I can get by wihtout them, I totally agree. I will know if I got the job today, how about you (Richard) come to Iowa and help me frame the roof......=)

[Richard A Hetzel]

Hahaha, I just draw the pictures. =) However, I"m worried that 16x16 might be pushing it...are the walls 2x4 or 2x6? You may not need ceiling joists on 16-inch centers, and even if you did, they only take care of the thrust in one direction, leaving two walls still unsupported at the top against lateral thrust. If the walls are 2x6 and you can make the top plates continuous (without splices), I'd worry a little less, because now you'll have horizontal "beams" of 2-2x6 to resist the thrust.

[Don_P]

If the hips are structural and the corners reinforced then the thrust goes to the corners not the walls. A professor of engineering mentioned this technique with approval in a class I took. I've got an article in pdf I can send, pm me your email if you'd like it, ditto to you also Richard.

[bighammer]

don, that would be great...please send it to marcuslumberclay@gmail.com.....I did get the project with the digging starting Thurday next week and I will have to keep you both informed......Thanks for the good advise

[Richard A Hetzel]

IF the hips ARE structural, then yes, it's the same idea as a structural ridge, but in ordinary hip roofs, the hips are sometimes considered simply as ridges, and not structural members, and in that case, horizontal thrust is produced on the supporting walls if rafter ties (often in the form of celing joists) are not provided. If no one has specifically designed the hips to act as girders, then the walls, it seems to me, need to be tied together with at least a few rafter ties, in the case of the example 16x16 building.

[Don_P]

As long as we're if'ing, I sent a couple more thoughts to bighammer but I'll post them here as well.

The ties in Richard's solution above would theoretically need to be 4' oc one set at plate level and another set just above them going the other way.

There could be a beam going wall to wall with a post to the peak.

There could be a beam going corner to corner each way with or without a post to the peak... a kingpost truss

There could be horizontal beams as or atop the plates, a plant shelf. If the plates cannot bow the roof is supported.

[Richard A Hetzel]

Good ifs. =)

[bighammer]

well, we got the hip roof framed together and it passed the inspector with flying colors (no suprise...haha)...i have attached a picture of the inside....that was really a fun project, i am glad it was not any bigger....

[Richard A Hetzel]

Passing inspection doesn't necessarily mean it's right. Inspectors do not do structural calculations to check a design. They are the responsibility of the one preparing the design that's submitted. So, of course, it's no surprise.

[bighammer]

I am aware that inspectors do not inspect on design loads, that is what architects and engineers are for, to spec out design and headers...etc, however, if my framing job passed inspection, that means that it is up to code and it the building is constructed as drawn, and passed code than I did my job as a framer.....

[Richard A Hetzel]

No, friend, it doesn't mean it's up to code...inspectors overlook major errors all the time, including code violations.