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Creating a sloped flat roof

newbuilder | Posted in Construction Techniques on July 7, 2007 11:28am

I’m about to put on the top floor roof of this structure that I’ve been building, mostly alone, over the course of the last two years.   I’m not a pro-builder and I have a question regarding the roof construction: after I put up the final four walls and bridge across two of them with the tji’s for ceiling/roof support, how do I then achieve a slight slope in the final flat roof for drainage.   I could build one of the two walls that carries the tji’s a little higher than the other .. but this seems hokey and would mean that the tji’s only touch on one point along the higher wall.

I’m wondering if there’s a tried and true way to achieve this slope on the roof construction — I need one side to be 3 1/2 inches higher than the other … the roof is only 13 feet wide. 

Any suggestions would be greatly appreciated.

Thanks –

nb

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  1. Piffin | Jul 07, 2007 02:07pm | #1

    Three different ways.

    One - you can add 3-1/2" to the top plate of one wall before setting the TJIs and then they all slope already. Tow plates would make 3" and be good enough. But then you would have sloped cieling and other trim issues.

    Two - You can rip shims with the degree of taper you want and then use constructionadhesive and screws or nails tofasten to the tops of the TJIs before sheathing.

    Three - You can go ahead and sheath directly onto the TJIs and then have the roofer use a taper insulation system substrate nder the roof membrane. This may be more expensive, but is the more elgant solution. You can control which direction it slopes or even have it slope equally all four diretions to keep the fascia equal around.

    I would advise that you be preparred to get it all dry once you sheaathe this. The fact that you are two years just getting to this point tells that you are going very slow. If you lkeep up that rate with a flat roof, you will create problems. You do not want to trap water in this roof system. Are you using tarps?

     

     

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    1. newbuilder | Jul 07, 2007 07:49pm | #2

      I began this project in Sept of '05 .. so coming up on two years.  But it is highly unusual and I am working alone ... that's why it's taking so long.  I've mentioned it here before; it's a four-story, 44 ft. high 'tower' on one of the highest hills in Seattle.  It's a huge job for someonw without a background in building .. I'm learning as I go.  But it's passed every inspection .. floor by floor .. and the good news is that I'm now putting the final walls up on top.  Next, the roof. 

      Yes, I've been working with tarps floor by floor.  Here, every winter I have to be insane about keeping the water out.   There've been some windy nights through each of the two winters where I was literally up and down all night long re-adjusting and re-setting tarps.  And the higher I go the windier it is. 

      Thanks for the suggestions.  I 'think'  I may go with your #2.  I just wish there were pre-ripped 13 ft. shims available!  I guess a 2X4 ripped perfectly from one corner of one end to the opposite corner of the other would be near perfect.  Just wondering how to do a long cut like that as perfectly as that would have to be.

      Thanks again ... I really appreciate the feedback!

      nb

      1. Piffin | Jul 07, 2007 07:56pm | #3

        OK< are you the guy who was asking a lot of questions about used doors and threasholds etc last fall?Another option would be for you to buy trusses made for the pitch you want already made up. Set'em, sheathe, and roof it done.EPDM roofing? 

         

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

        1. newbuilder | Jul 07, 2007 08:20pm | #4

          Yeah, I've asked a LOT of questions here over the last near two years.

          I already have the tji's sitting in my ground-floor room.  I've used big-beefy mothers all the way up.  2 1/2" wide - 9 1/2 high.   I'm liking more and more the idea of looooong 'triangles' ... 'super shims' for the top-sides of the tjis.  I've never had anything to do with manufactured trusses ..everything by hand here .. I think I'll stick with that as we're talking the very last floor (roof).

          Yes .. I'm thinking epdm but haven't ordered yet.  There are lots of other products that people are beginning to talk to me about .. I'll have to poke around soon to get it goin.

          nb

           

          1. m2akita | Jul 08, 2007 08:35pm | #5

            I would highly recommend Piffin's #3 ( roofers putting in the sloped stryofoam or whatever it is they use forms).  Its quick, its done, and (hopefully), a water tight system.  If your planing on doing everything yourself, see if one of the roofing contractors or supply houses will sell the system to you.

            If your confident enough in your skills with cutting with a circular saw, I would just rip 2x4's to your desired taper by hand.  Glue/screw/nail the ripped side down onto the TJI's.  If your not confident in your circular saw skills, you could make a tapering jig to use on a table saw.

             

             Live by the sword, die by the sword....choose your sword wisely.

          2. alrightythen | Jul 09, 2007 04:31pm | #8

            "If your confident enough in your skills with cutting with a circular saw, I would just rip 2x4's to your desired taper by hand."

            if he's not confident ripping tapers by hand, now is the perfect time for him to get confident - a high flat roof that nobody can see. if he can rip them close enough it will be a good time to practice, even if he has to chuck the first one or two. but he's gotten this far...might as well go for it.   View Image                                          View Image    

      2. User avater
        nater | Jul 09, 2007 06:08pm | #10

        Do you have any pictures of the project. I remember you talking about it, but I don't remember seeing pictures, and I'd be interested to see how it is shaping up.

    2. newbuilder | Jul 13, 2007 01:10am | #16

      Hey!   I thought this thread had died!  for some reason no notification emails have come my way since those first four back-an-forths.  I'm glad to see that it went further with some more suggestions.  I really detest the thought of hand cutting 13 of these 14' giant shims ... I'm going to drop by a roofing store today to see what they have .. they've got some sort of material, as mentioned here, for building the roof up AFTER the roofing-ply has been put on and THEN putting ply on that!  (or at least this is what it sounds like to me over the phone).  I would like something that I can simply glue-screw to the tji's itself.  If they've got something for that I'll take it .. something out of wood or a wood product .. not simply an insulative foam product. 

      thanks to everyone for the further feedback! 

      I'll put up more picks eventually.

      T.

    3. newbuilder | Jul 13, 2007 07:07am | #17

      I am now leaning towards adding 3 1/2" to the top plate of one wall.

      What is a "tow plate"?

      nb

      1. Piffin | Jul 13, 2007 01:40pm | #18

        It is a typo 

         

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

  2. karp | Jul 09, 2007 04:20pm | #6

    Place your tji's, then rip 2x6 for the appro. slope. Sheath the structure asap.

    If you want a consistent elevation at the perimeter, you then must create "hips". This will allow the roof to drain on all 4 sides. With this method, you may need 2x8 material, so when you span the joists, you have about 3" of mat. left. (I've cheated this down to 2 plus inches, but don't tell antone I said so) I like this method best, because the facia detail is the same depth all the way around.

    If you didn't have time to do it right the first time, how come you've got time to do it over again?
    1. Piffin | Jul 09, 2007 04:29pm | #7

      To hip it same all around, I would rather let the roofers use tapered insulation system. That gives a stronger job and less than 2" build at edge 

       

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

      1. karp | Jul 09, 2007 04:46pm | #9

        I'd agree

         I've used "Posi-slope", a fibre board product, to create the slope before the torch-down is applied. The only problem is this is custom cut for every roof. Ordering, delivery time and as I recall, kind of pricey.

        Ripping 2x stock can be handled by the OP.

        But, again, your right. If he is uncomfortable with ripping, he now has another method.If you didn't have time to do it right the first time, how come you've got time to do it over again?

  3. tuolumne | Jul 09, 2007 07:31pm | #11

    Here's outside the box...this is a small roof and you could make your own joists using tapered osb webs and 2x3's with a dado.  Put the osb joint near center where the shear is lowest.  You could also acheive the same pitch by making the center 2" higher than each end with a radius and bend that top 2x around the curve, glue and clamp.  A much simpler solution would be to use 2x12's instead of I-joists and rip them down to 9-1/2" deep at each end 11-1/4 in the center.  That sounds easier than ripping 2x4's from 3-1/2 to nothing.  It sould also be cheaper than buying I-joists and 2x4's for that short of a span.

    1. jesse | Jul 09, 2007 07:39pm | #12

      If he's not going to use built up insulation, wouldn't the fastest solution for a DIYer be to do a double top plate on one side and then just run 2/4 ceiling joists level alongside the TJI rafters? So much faster, and there is no time wasted getting the thing dried in, because he can level the ceiling anytime in the future.

      1. alrightythen | Jul 09, 2007 08:56pm | #13

        were you going to have him cut birds mouths in the TJIs?   View Image                                          View Image    

        1. Piffin | Jul 09, 2007 09:19pm | #14

          It's customary to include a smiley when you are joking, yah know? 

           

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

          1. alrightythen | Jul 10, 2007 06:18am | #15

             

             

             

                                                         :P

            hope that makes up for it.

                View Image                                          View Image    

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