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ianmacneil 09-18-2010 12:57 PM

Hip roof garage
 
1 Attachment(s)
Hi all,
I’m new to the site and I have some structural questions on the roof of our unattached garage. I’ve attached a pic that outlines the basic features of the roof. First, it’s a square garage (20X20), stone walls with a hip roof (2X6 construction). It has two rear windows and an electric two-door garage door. It had a drop ceiling (old plasterboard) when we moved in (~15 years ago---not sure when it was put in) which had suffered lots of water damage and a couple years ago we had it re-shingled (some repair of rotted panels) and the drop ceiling removed. We recently had a new garage door installed and now looking at the roof from inside there are some obvious problems. The ceiling joists to which the drop ceiling had been nailed to are a wide variety of pieced-together lumber (mostly 2X4 spliced at various points). The ceiling joists are also attached at various points to the hip rafters with an assortment of scrap lumber. The ends of the ceiling joists sit on top of the stone walls. The garage door supports are screwed into some sketchy-looking short crosspieces spanning the ceiling joists, Okay, now the questions. I need to replace/repair the garage door supports but I’m wondering about the ceiling joists. Do they need to be replaced/strengthen? Do I need them all? Most of them are not in great shape.
Thanks.

CarrieP 06-29-2014 03:18 PM

hip roof demolition
 
It has been many years since I think you posted your question and I didn't see any replies. It looks like you broke into my project, I have the same questions as you do but my 20 x20 structure was a livable cabin before I did the exact same thing you did taking out the false ceiling and lowered roof only to find all these random pieces of wood. Did you ever take them out? I am not sure if they were just put in place to hold the ceiling or if they are holding the four walls together. Please if you are still on this site if you can let me know. Thanks, Carrie

cleveman 06-29-2014 04:24 PM

There was a thread a while back about ceilings in hip roofed buildings.

I have put ceilings in 24x24 garages, and I used 2x8's 24" on center, but there was solid wood blocking every 8' (so two sets). Also, I wasn't able to get the 2x8 in towards the ends, so I ran the ceiling joists back to the "common" ceiling joist.

I haven't had any trouble with these ceilings. They are only holding up some lightweight sheetrock.

CarrieP 06-29-2014 05:48 PM

I am doing the reverse, I did not like the old ceiling and after removing all the ceiling tiles and boards I uncovered a beautiful ceiling but I found all kinds of ceiling joists with pre electric day wiring which has now been removed also... My issue is if I remove those horizontal joists that the ceiling was attached to. I have visions of a roof sitting on the ground and four walls falling outward into the yard. This cabin has been here since at least before 1930, finding old newspaper and clothing shoved in the walls. Can the roof stand on just the four corners at 20 x 20' spans?

cleveman 06-29-2014 10:45 PM

The hip roof is amazingly strong.

If your walls are braced or sheathed, then you are very stable with the 4 "common" rafters and 4 hip rafters.

I think you are fairly stable with just the 4 "common" rafters.

I remember once I had to remove a garage with a hip roof and I believe I ct out the roof sheathing and old shingles all together. Then I removed the jack rafters. I was waiting for the roof to get unstable. I next removed either the hip rafters or the 4 "common" rafters. Either way, it was still stable and would bear weight.

I think I removed the hip rafters first, then two of the four common rafters, leaving just 2 rafters up there, then I took them down without a problem. I remember the tear-down being very non-dramatic. I was on a ladder in the end.

Daniel Holzman 06-30-2014 06:48 AM

Just a comment on hip roof design. I had occasion to investigate failures of about 180 properties after hurricanes Katrina and Rita, and my colleagues on the project investigated perhaps 800 others. Around the end of the project, we got together to discuss structural performance of roofs. The forces that destroyed the roofs were primarily wind, but in some cases the storm surge actually made it to roof level.

We found that in order of performance against wind damage, the worst was a flat roof, followed by shed, then gable ends, and the best performance by far were hip roofs. There was some statistical bias in the analysis, since many of the houses we examined had been completely destroyed by flood water, in many cases only the foundation was left. But in neighborhoods where it was wind damage only, the hip roofs did remarkably well, even poorly constructed hip roofs were better than the best flat roof.

I think the primary reason is that hip roofs act more like a unit than a single plate, so they resist wind loading by mobilizing the strength of the entire roof rather than one side. Wind loading is a bit counter intuitive, the most severe loading is uplift due to the wing effect of wind sweeping over the roof, causing low pressure on top of the roof. For flat roofs, you effectively get single panel uplift on the diaphragm, and if the roof is not adequately fastened, the entire roof peels off. About the same with a shed roof, but the pitch reduces uplift slightly. The gable roof adds some strength because the two sides are normally tied together with collar ties. The hip roof does even better, my theory is that the four sides are usually effectively tied together, offering the best resistance to upload.

cleveman 06-30-2014 10:11 PM

Everyone building or buying a house should read Holzman's post. It is difficult to imagine all the storm damage that could be avoided with hip roofs and don't get me started on siding and everything else.

hboogz 07-02-2014 11:18 AM

any good single resource/website that could help identify all the different roof types mentioned by Dholzman? In my area and my current home I've only ever deal with flat roofs. We don't have much to contend with in terms of wind or flooding, for the most part, but still very interested in learning more about other type of roof structures.

Thanks,

Harry.


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