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Raising the roof on a gambrel .

checked | Posted in Construction Techniques on May 8, 2007 02:55am

I have a house that is 18′ by 30′ and is a gambrel style .  I only want to raise the roof on one side and it will be a shed dormer. However, plans are to have this dormer the full width and rest on the outside walls of the existing structure.

The gambrel construction takes place like this : 2 by 6 rafters from the ridge to a 2 by 6 plate for the first angle and the second slope takes place from underside of this plate similar to a stud wall but in this case flared in order to produce the gambrel look. The ceiling in this setup is constructed from 2 by 6 collar beams that are approximately 6 to 8 inches up from the plate separating the two roof angles.

My concern about all this is that I want the new shed dormer’s ceiling to be level with these collar beams. In order to do so, I must remove not only the lower roof’s rafters but the plate between the two angles up to and as far as the collar beams.

The questions are : will a properly sized header attached to the ends of both the upper rafters and collar beams not compromise the stability of the whole roof. Also, do I have to install ceiling joists and tie them back into the untampered rafter side of the house, or will the the sloped leanto of the  shed dormer’s roof rafters be sufficient in maintaining integrity of the roof ?

By-the-way, this proposed header will be supported along it’s run, ten feet in, from both exterior walls by two nonload bearing partitions. Plumbing down from this header to the floor, the exterior wall plate will be 2′ away from the load. Any problems there that I should be aware of or attempt to remedy ?

  Thanks !

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Replies

  1. rez | May 09, 2007 03:39pm | #1

    Greetings checked,

    This post, in response to your question, will bump the thread through the 'recent discussion' listing again which will increase it's viewing.

    Perhaps it will catch someone's attention that can help you with advice.

    Cheers

     

    I would rather sit on a pumpkin and have it all to myself than be crowded on a velvet cushion. I would rather ride on earth in an ox cart, with a free circulation, than go to heaven in the fancy car of an excursion train and breathe a malaria all the way.
    -Thoreau's Walden
    1. checked | May 09, 2007 11:12pm | #3

      Glad to meet you Rez ! Thanks for coming to my aid.

  2. Piffin | May 09, 2007 08:46pm | #2

    I see lot of problems.

    First - you say the proposed header will be supported by two non bearing partitions.

    When you oad them with the roof header, they become load bearing, so they would have to be built to handle it and load path transferr to foundation.

    one major factor you leave out is whether you have a ridge beam or just a ridge board.

    some of the other details I am less clear on without elevation or cross section drawing.

     

     

    Welcome to the
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    1. checked | May 09, 2007 11:34pm | #4

      How you doin ?

      The gambrel has just a ridge board....1 by 8.  Yes, the two partitions that run perpendicular to the proposed header are nonload bearing, however, the point of bearing at the second story floor would be 2' from the exterior wall. Could I possibly tie back the load from the header to this exterior wall with perhaps an angled brace from within the the partitions ?  

      I apoligize for not producing a drawing of what I'm talking about...I'm new and have not figured out how to do just that....I shall look into it.

      Appreciative  ! 

      1. Piffin | May 10, 2007 12:02am | #5

        I am still trying to visualize this. With the new wall two feet in from the exterior one that rafters land on, what do you gain by doing this change? 

         

        Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        1. User avater
          dieselpig | May 10, 2007 03:49am | #6

          I'm having the same problem swallowing this one.  Seems to me he's just swapping out one wall for another..... must not have my head around it right.

          Either way.... dude needs an architect I'd say.View Image

          1. Piffin | May 10, 2007 04:23am | #7

            and/or engineerProbably a ridge beam.Lot of work for what I concieve as little gain - maybe for a couple windows - they could be dormered into existing easier, I would think. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          2. User avater
            Dinosaur | May 10, 2007 06:37am | #8

            I'm having the same trouble visualising his whole proposal, but I think what he's talking about is what we used to call a Dutch dormer.

            But he's planning on running it full width, gable wall to gable wall, so it's not really a dormer, but is in effect a whole new roof on the back (front?) of the house. So I don't understand this bit about NLB partitions 2 feet in from (somewhere?).

            It sounds like he's taking off from the upper roof (maybe at the same slope?) at the point where it meets the current lower roof.

            Whatever, I think the real problem he has is going to be getting that ridge beam appropriately sized. My code tables don't get anywhere near a 30-foot clear span; a 5-ply built-up beam of 2x12s is only rated to span 16.5 feet (and that, for a snow load of 1.0, which isn't much). Unless he wants to use steel or a flitch-plate or a railroad truss or some humongous glue-lam job, he's gonna have to find or build some support about midway on that span, and it's gonna have to transfer the loads straight down through the house to a footing.

            He's gonna need an engineer for this one. (Didn't you say that already? <;o)> )

            Dinosaur

            DON'T MISS THE FEST!

             

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          3. checked | May 10, 2007 03:10pm | #11

            Good day to you !

            The best way I can explain what I have in mind is perhaps to go at it from a different direction . Visualize a gambrel roof and what you would have to do in order to construct a lean-to type of dormer with a single window ( as they say ...a doghouse  dormer ) on the roof . Now , I would think that most gambrels are constructed similarly enough that raising the roof on one style should be the same work more-or-less on any gambrel .

            However, the only difference in my problem with that of a small doghouse dormer is the size or length in this case . I want to raise the entire lower roof to that of the upper roof pitch along the whole side of the house . And yes , the new raised section would take off from the end of the first roof angle ... in essence , it would be continuing the angle or run until it was over the exterior wall plate .

            Cutting into the roof now ! ... Checked

             

             

          4. Piffin | May 10, 2007 07:34pm | #13

            There are actually quite a few differences between this and a doghouse dormer. This is a shed style you arte changinb to while a doghouse is a gable style roof. Yours will seat load directly on the lower wall and floor system. A doghouse dormer bears on top of the existing roof structure with a doubled rafter on each side supporting it.This mis-use, unintentional as it is, of terminology based on your inexperience points up the reason why there is mis-understanding and why you need a real pair of eyes on this, either by hiring an engineer or top flight framer, or at the very least with pictures and drawing to this site. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          5. User avater
            Dinosaur | May 11, 2007 08:32am | #17

             I would think that most gambrels are constructed similarly enough that raising the roof on one style should be the same work more-or-less on any gambrel .

            Actually, no.

            There are at least six common ways to frame a gambrel, not counting trusses. A few years back, FHB ran an article showing some of them, including mine (which is to build the lower roof as a slanted wall and the upper roof using rafter trusses that sit on top of the top plates of those 'walls'). You might want to search the archives for <gambrel roof framing> or <framing a gambrel roof>.

            What you are trying to do here is build an entire new roof on the back half of your house; this is not the same as building a dormer on top of it. This means you have to demolish half of your existing roof structure...and once you do that, you will have changed the structural form of it so that you will need to provide support in places you didn't need it before.

            At a minimum, you're going to need a structural ridge beam at the main ridge (unless you have a longitudinal LB wall running right across the whole house directly under the main ridge). Then, if you don't re-frame with bigger rafters sized for the longer span, you will need another beam (it's not a header, so don't look in the header tables for the scantlings) to hold up the outboard end of the old rafters and the inboard end of the new ones. And that beam will have to sit on LB walls or supporting posts which transfer the load straight down to a footing under your basement floor (which you may have to pour). You cannot <step sideways> when transferring load down from above. It's got to be columnar.

            As Piffin pointed out, you really need to get a top-notch framer or engineer involved with this. This is way different than just plopping a little dormer onto your roof.

            Dinosaur

            DON'T MISS THE FEST!

             

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          6. checked | May 10, 2007 02:39pm | #10

            The new proposed wall will be built directly over the existing exterior wall where the gambrel rafters meet the wall's top plate . I will gain 2 feet along the length of the upper story of which is 30 feet . Since a gambrel roof's design on the second lower angle is a combination of a roof/wall ,well , I lose headroom . If I could cut the lower roof/wall out and replace it with a plumb wall , I think , the gain would be worth it.

             A bedroom closet could be moved back or deepened ... the stair landing would feel more spacious and would be accommodated with a proper single hung window instead of what is there now ... a skylight. And lastly, the washroom has a tub installed along side the exterior wall , but , has no shower because of the angled roof/wall which does not lend itself to a person standing upright .  

            In need of a shower....checked

          7. Piffin | May 10, 2007 07:24pm | #12

            Personally, I dn't check to see if I am in need of a shower. I just automaticly take one every day, whether I need one or not.;)So, I am not understanding the earlier comment about two feet in, but I do think I understand what you are describing now. obviously I don't have all the details, but this should be doable without any special header if you can sister to keep your ceiling joists continuing to the new wall as rafter ties. The roof load will be on the new wall and the load transferrance is to the foundation via the same exterior first floor wall you have bearing that now.But maybe I am still missing something. You really need to have somebody on site who knows his stuff to guide this or to design it. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          8. checked | May 10, 2007 08:19pm | #14

            Thank you ... thank you very much !....as Elvis would say.

            I appreciate the time you have taken in trying to get your head around my mesmerizing description of a construction problem . You are to be complimented for your patience and determination .... you must be a woodworker for sure !

          9. Piffin | May 10, 2007 11:16pm | #15

            No - i'm a remodelor!Which means - in translation, a problem solver. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          10. Danno | May 11, 2007 02:13pm | #19

            What you call a "doghouse" dormer would in fact be what is called by most people a "shed" dormer--the roof is a single sloped roof like on a lean-to shed. Dinosaur and Piffin are spot-on in their interpretation of what you want (as far as I can tell--I am far less experienced in construction than either of them, but have vast experience in interpreting meaning from what people write!) and their conclusion to hire a good framer is good advice.

          11. checked | May 11, 2007 06:35pm | #20

            Good day friend !

            You and the boys are certainly right about the differences between a shed dormer and a typical dog house dormer . However, my intention was to explain my plans for a full length shed dormer but obviously I was doing a poor job in painting the picture so to speak . After I received feedback initially, I could see the confusion of whether or not I was raising the roof or just building a dormer on back of my small gambrel home. I only resorted to the picture of a doghouse dormer when I understood there was difficulty in interpreting my feeble attempts at description .

            I knew for sure everyone would picture a doghouse dormer jutting out from a steeply pitched roof with hardly need of further explanation . What I did wrong was enter into the construction details of cutting into the roof too soon and not competently creating the  building construction details that was needed in order for you fellows to give me advice .

            My whole point then , was to receive feedback concerning cutting into a gambrel roof for practically its width and not jeopardize the integrity of the design after I was finished . I apologize for the complete confusion and mesmerizing details I have  put fort in this forum, and hopefully, have learned something about communication over a computer.

            I appreciate the time all of you have contributed to my posting . Thanks !

            Snapping lines ... Checked . 

             

          12. BZbuilder | May 11, 2007 10:26pm | #21

            The inside members on a gambrel roof are under tension.  A single rafter bolted to the top of the Gambrel would leave the inside tension members dangling. You need another tension member, either 2x4 or 2x6 (depending on span and loads -snow, wind) to couple the shed  rafter to the tension member, hopefully pinching this at the load bearing wall. Think of it as a bow string.  The string is weak, but under tension becomes quite strong.  

          13. Piffin | May 11, 2007 11:00pm | #22

            I understood you, but goiven the terminology problems we have seen in this thread so far...;) 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

          14. Danno | May 14, 2007 03:45am | #23

            Hope we helped! Just as long as you don't open everything up and have something bad happen! I think that's our major concern--that someone misunderstand what we mean in a message and end up having a structural failure that at best only damages a few parts of a roof, but at worst could hurt or kill someone. Better to be overly cautious, in my opinion. Hope it all works out well for you.

          15. checked | May 14, 2007 11:56am | #24

            Thanks to everyone for the contribution to my questions about the gambrel . I'm usually cautious about any move I make when it comes to changing something structural . After all , that is why I'm here !

            There are a few homes in this area that have done just what I'm proposing to do .... will check them out . Also , I have the local building inspector I can have to check my sketches for when I look for a construction permit .

            Thanks to you and all others who have given their advice .

            Checked

          16. User avater
            Dinosaur | May 11, 2007 08:08am | #16

            this should be doable without any special header if you can sister to keep your ceiling joists continuing to the new wall as rafter ties

            I don't see that happening, Paul; the ceiling joists probably hit the upper roof rafters right where the lower roof now takes its dive downwards, so if he continues the slope of the upper roof for the new 'dormer' roof, those ceiling joists would wind up above the rafters once they pass that point.

            Ooooooh, I could make myself real dizzy real fast on this one....

            Take a look at this quick sketch. White is the original roof; yellow is demolition (except for the extension of the ceiling joists to show where they would wind up if simply extended), red is new framing with two options considered.

            View Image

            The only way I see to frame this without a new beam would be to tear off the whole back (front?) half of the roofs--both upper and lower--and then frame from the main ridge out to the building wall on a single pitch with appropriately sized rafters...which implies the need for a ridge beam at the main ridge. Or he could possibly slope the ceiling to meet the new wall, creating a pseudo-truss form--but it'd need support at the midpoint of the original run of the ceiling joists.

            Or...he could ask Boss to design him an asymetrical truss.... (;o)>~

            Dinosaur

            DON'T MISS THE FEST!

             

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

            Edited 5/11/2007 1:09 am ET by Dinosaur

          17. Piffin | May 11, 2007 01:55pm | #18

            I did use the giant word IF...Another way is to change the pitch of the rafters IF that doesn't get too low. 

             

            Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!

        2. checked | May 10, 2007 02:03pm | #9

          These two NLB walls run across the house's second story width. They are parallel to the gable or rake end of the building. Because of this, they run in the same direction as the floor joists across the width ,which is 18 feet. The two partitions are 10 feet from their respective gable ends and 10 feet from each other, so, the upstairs is divided up into three equal rectangles.

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