Dormer

 
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Old 12-21-2017, 07:34 AM   #1
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Dormer


Hi folks, I'm new to the trade and looking for some advice...

I'm remodeling the second floor of my barn and want to put in a 15' wide gable dormer. My question is, do I need to replace the existing ridge beam with a structural ridge?

The new dormer won't reach the existing ridge. We were going to run a double header joist hung from double LVL's rafters down the sides. The gable end will sit on the outside wall.

Here's a drawing...

Dormer-screen-shot-2017-12-20-2.18.28-pm.png
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Old 12-21-2017, 08:01 AM   #2
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Re: Dormer


Your Trade is "Builder" is it?

Andy.

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Old 12-21-2017, 08:17 AM   #3
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Re: Dormer


Assuming you want to build this to code, my advice is to read the IRC on this topic.
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Old 12-21-2017, 09:04 AM   #4
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Re: Dormer


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Originally Posted by ScipioAfricanus View Post
Your Trade is "Builder" is it?

Andy.
Yes, but as I said I'm fairly new to the biz. I've built single window dog house dormers that only required doubling the rafters and adding a header. I've built smaller shed dormers that used collar ties to tie into the other side of the roof. I've done wide shed dormers that required a structural ridge. But I've never done a wide gable dormer like this. I thought someone here might have experience with it.
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Old 12-21-2017, 09:36 AM   #5
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Re: Dormer


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Originally Posted by hdavis View Post
Assuming you want to build this to code, my advice is to read the IRC on this topic.
I do want to do it to code and there is a permit on this project, although this dormer was not part of the original plan.

I have tried to find answers for this question in the code book, but am coming up empty. I like to use an online version because it lets you search for terms but it only kicks back 3 answers to the word "dormer." Two of the answers are about energy efficiency and the other one is just a definition.

Under the roof construction section (802), there's all the standard info on tables on rafters and spans. There's a section on "Roof Openings" (802.9) that I thought might apply, but it's just a paragraph and all it says is if the header joist span exceeds 4 ft, the trimmer joists and header joist have to be doubled and strong enough to support the ceiling joists and framing into the header. That's all well and good but there's nothing about how that may or may not affect the existing ridge. I feel like I must be missing something, but I can't find the answer in the book either.

Again, hoping a more experienced contractor has an easy answer...

"No, a gable dormer, no matter the size, doesn't effect the ridge beam. It's headed off and the doubled rafters at the sides carry the load to the outside wall."

"Yes, a gable dormer that big will effect your ridge beam, the doubled rafters carry the load to the outside wall but there will still be uneven pressure from snow loads on the other side so best to get an engineer to size a new ridge..."

Last edited by wladd77; 12-21-2017 at 09:39 AM.
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Old 12-21-2017, 09:38 AM   #6
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Re: Dormer


Am sure there is a few guys here that have done those. But the thing is there is a lot of home owners trying to pass as contractors coming here asking how to do things. Not saying this is the case with you. When a new guy comes here and posts a question like this with out any real introduction and involvement with the guys here. Well most guys are a little reluctant to give out information. Become part of the group and it is easier to get help or advise.
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Old 12-21-2017, 09:43 AM   #7
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Re: Dormer


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Am sure there is a few guys here that have done those. But the thing is there is a lot of home owners trying to pass as contractors coming here asking how to do things. Not saying this is the case with you. When a new guy comes here and posts a question like this with out any real introduction and involvement with the guys here. Well most guys are a little reluctant to give out information. Become part of the group and it is easier to get help or advise.
Thanks for that explanation. I was wondering why I got the responses I did, but i wanted to be respectful in my responses since I'm new. I've read posts here in the past but I could usually find the answers to my questions by searching so I never felt the need to sign up. This was a question I couldn't find an answer to online so I signed up today and posted this. I will go to the introduction thread and say hello.
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Old 12-21-2017, 09:43 AM   #8
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Re: Dormer


Nice model, think it will be okay but that is definitely not a full throated 'go for it' endorsement.
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Old 12-21-2017, 10:16 AM   #9
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Re: Dormer


The first thing to do is figure out if it already has a structural ridge. The second thing to do is review the applicable code. Searching for dormer when you have a question about the ridge isn't going to get you there.
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Old 12-21-2017, 10:46 AM   #10
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Re: Dormer


"adding dormer to existing"

https://www.google.com/search?q=addi...w=1366&bih=662
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Old 12-21-2017, 10:46 AM   #11
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Re: Dormer


With no other information, I'd just be inclined to put a matching dormer on the other side. It avoids the question of asymmetric loads, plus putting a 50ft engineered beam into the top of a barn isn't as much fun as it sounds.

The first thing my engineer would if tasked with making stamped plans is inspect the existing structure. If you don't know what you're seeing, you can't really do that yourself.
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Old 12-21-2017, 12:05 PM   #12
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Re: Dormer


Quote:
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Nice model, think it will be okay but that is definitely not a full throated 'go for it' endorsement.
Thanks. It's from a software called Home Designer Pro by Chief architect. Very easy to use for floor plans and quick mock-ups. But it would take some time to really master the program. For example, I can't figure out how to take out the little cripple rafters in front of the dormer. On the real one, I'd like the soffits to stop at the dormer so the siding can run up the face of the dormer.

Quote:
Originally Posted by hdavis View Post
The first thing to do is figure out if it already has a structural ridge. The second thing to do is review the applicable code. Searching for dormer when you have a question about the ridge isn't going to get you there.
It's not a structural dormer. Just a 1x8. I'll search the code book for ridge beam and see if that helps.

Quote:
Originally Posted by SmallTownGuy View Post
I spent about 4 hours looking up variations of "Dormer addition" on Google last night, though I didn't try that specific wording. I took a quick look at the results before posting this and it looks like more of the same. Anything wider than 6' is a shed dormer which I've done but don't want to do on this project. I'll give those results a longer look at lunch. Thanks.
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Old 12-21-2017, 12:08 PM   #13
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Re: Dormer


Quote:
Originally Posted by hdavis View Post
With no other information, I'd just be inclined to put a matching dormer on the other side. It avoids the question of asymmetric loads, plus putting a 50ft engineered beam into the top of a barn isn't as much fun as it sounds.

The first thing my engineer would if tasked with making stamped plans is inspect the existing structure. If you don't know what you're seeing, you can't really do that yourself.
Yeah, that would definitely work but we were hoping to keep the other side clear for solar panels at some point. The other side faces south and is completely clear of obstructions so it's a good spot for it. I still have to do a lot of research on that. I've never done solar panels, but I did ask the electrician to run a pipe up there so if we do decide to add it we'll be able to pull a wire through without having to open up walls or have a conduit on the outside.
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Old 12-21-2017, 12:37 PM   #14
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Re: Dormer


I have plenty of gaps in my knowledge. I've tried to do it all, and the trade-off is that, while I'm never bored, I scratch my head a little more than I wish.

And this is why I have an engineer. I draw something up, and run it by him. Sometimes it's really simple, but I just want that second opinion. Had to call him the day before yesterday about a little mishap, and an exceptable way to fix it. 5 minutes on the phone.

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Old 12-21-2017, 01:06 PM   #15
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Re: Dormer


OK, You're going to have trouble if you don't have the terms right. You can either be talking about a ridge board (which a 240 yo barn may not even have that) or a beam for a structural ridge. A board is 1x material. Search for ridge, and it will cover both. I think It's somewhere around 8.07, but don't quote me.

You also didn't mention what type of framing this has. If It's 240 yo, I'd expect post and beam framing.

The short story on this is you compare the roof system to current code. Almost all are underbuilt by today's standards. You now are responsible for making changes to a substandard roof system. That takes judgement in evaluating.

If you really want to just have the one dormer, have an engineer stamp a solution. $300-500 well spent.

JMO.
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Old 12-21-2017, 02:12 PM   #16
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Re: Dormer


Quote:
Originally Posted by hdavis View Post
OK, You're going to have trouble if you don't have the terms right. You can either be talking about a ridge board (which a 240 yo barn may not even have that) or a beam for a structural ridge. A board is 1x material. Search for ridge, and it will cover both. I think It's somewhere around 8.07, but don't quote me.

You also didn't mention what type of framing this has. If It's 240 yo, I'd expect post and beam framing.

The short story on this is you compare the roof system to current code. Almost all are underbuilt by today's standards. You now are responsible for making changes to a substandard roof system. That takes judgement in evaluating.

If you really want to just have the one dormer, have an engineer stamp a solution. $300-500 well spent.

JMO.
Yes thank you, it's a ridge board, and the barn is post and beam framing. We dug under the posts to put in new footings and poured a curb around the perimeter. The roof is made of 2x6's that are 24" o.c. which is undersized for the span. We planned on splitting the bays with new 2x6's to make the roof framing 12" o.c. which pushes it well into the green as far as carrying the load over that span.

I'll take a look in the book for ridge boards/beams. 807 is attic access and 806 is roof ventilation, but I'll see what I can dig up. If I come up empty again, the engineer is my next move.
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Old 12-21-2017, 04:35 PM   #17
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Re: Dormer


New construction, a structural ridge is required for slopes less than 3:12.

Section 802.2

Last edited by hdavis; 12-21-2017 at 05:26 PM.
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Old 12-21-2017, 05:36 PM   #18
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Re: Dormer


A 15' wide span for the dormer would put it beyond the prescriptive designs in the IRC. I think that getting an engineer involved in this is the prudent thing to do.
Being as you are in MA. you have snow loads and wind loads to consider, well the engineer has to consider those loads.

Andy.
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Old 12-21-2017, 05:55 PM   #19
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Re: Dormer


I'm not an engineer....

I say as long as your doubles are strong enough to carry the load you should be fine. Personally I would triple them. Obviously your floor is going to keep the rafters from spreading and if it can't spread it can't come down. You will also need to support the dormer ridge at the gable wall. If you want to post a link to your .plan file I'll see if I can fix the overhang in front of the dormer for you. I'm certainly no expert, but you might need to change the wall type directly below the dormer to high shed wall type or you should be able to edit that section of the roof directly.
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Old 12-21-2017, 05:59 PM   #20
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Re: Dormer


Typically you would double the ridge due to unequal loading. Give the proximity of the header and lateral support of the gable I think it's fine.

Or I should say, I think the SI would agree.

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