Replies

1. 
   Guest_ | May 29, 2000 01:42pm | #15

   *
   A few years back I built a 1&1/2 story cape cod (12/12 pitch) for a customer with attic trusses. It has 1 dormer @ 4' wide where we doubled the trusses to allow for the opening. Now the customer wants 3or4 more dormers installed. My question is how do you create the req'd openings without killing the structural integrity of the truss system ? The trusses are 24" OC and the new dormers will be 3' and 6'.
   PS.Just so you don't think I am insane the house is "T"D by a garage which will have an office above thus the 6' dormer and the 3'ers wil be in the bedrooms. TIA

1. 
   Guest_ | May 03, 2000 03:48am | #1

   *you have to replace the one structure with another one..one thing comes to mind is going to a truss company and having them design a new sister truss set..or you have to drop some posts under teh ridge at the span ends and put in a short structural ridge for each dormer to replace the load carrying capacity of the missing trusses..find a good structural guy.. and get some stamped prints.. your building inspector is gonna want everyone's butt covered on this one...b but hey, whadda i know?

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   Guest_ | May 03, 2000 04:23am | #2

   *Thanks for the response Mike . That is pretty much the way I saw it but thought i would throw it out there. I think sistering the trusses in situ is about the only workable solution other than making the existing trusses a design feature.(JUST KIDDING)

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   Guest_ | May 03, 2000 12:45pm | #3

   *Novy, there's only one other solution that comes to mind, but it's not pretty. You could move 2 of the existing trusses 2' over, and nail them next to their neighbors to create the 6'dormers. Obviously, that would require removing all the shingles, plywood, and subfloor in that area. This assumes that your trusses are on 2' centers, you start and stop the dormer where there is an existing truss, and it doesn't violate codes where you live.Sistering new trusses next to the old sounds good, but would also be difficult. Matching new trusses to old ones isn't easy. I generally tell people they have about a 50/50 chance of having the new trusses line up with the old ones. The shop never gets things perfect - the peak may be a little off center, or a crown built into one of the chords. Come to think of it, sistering in new trusses would also require that you remove shingles, plywood, and subfloor. So maybe my 1st suggestion isn't all that bad.............

1. 
   Guest_ | May 03, 2000 02:10pm | #4

   *ron.... i forgot to write about the part that thenew would not line up with the existing..if these were a conventioanl truss.. i think you're suggestion would be the cat's whiskers..but these are attic trusses.. i don't think he can remove enough to get them free to slide over..i was thinking of sister truss... but really i meant cousin truss.. they could carry the roof load but none of the floor load..any misalignment would be hidden in the dormer wall...but... the more i think about this one, the more i think you've got to get some engineering to transfer the loads so you can remove one truss and two trusses.....don't be shy about getting your rate for this one...

2. 
   Guest_ | May 03, 2000 10:59pm | #5

   *Mike - I'm not sure I explained myself well enough. Doubling up an attic truss is the same as doubling up a conventional truss - You effectively double the load carrying capacity. With dormer framing, the loads aren't distributed perfectly evenly. But that's a minor detail I generally don't sweat much. You mentioned removing a truss, but that's not what I was suggesting. Let me try it this way - Your trusses sit on 2' centers, so you have one 2' from the end of the building, then again at 4', 6', 8', and so on. You take the truss from the 4' position, and slide it over against the truss at 2'. You then take the truss from the 6' position, and slide it over against the truss at 8'. This leaves you a 6' hole, going from the 2' position to the 8' position. You then nail the trusses together, and"ladder frame" between them. Hopefully, this will provide a clearer picture of what I'm suggesting. I'm not always good at explaining things unless I can draw pictures, or wave my hands around a lot....................(-:

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   Guest_ | May 04, 2000 12:11am | #6

   *Mike/RonThanks for the responses guys. Seeing as the upper floor is a finished and lived in space, I need a somewhat less invasive strategy to make this feasable IE: Moving trusses would not be an option. The approach I am working on right now is still very much in it's developemental stage but may be a good seed to get started. I am thinking of a welded steel frame engineered to transfer the loads "around" the R.O.. I will try to get some drawings together over the next couple of days and post them if I can negotiate a deal with my "Puter". In the meantime, does anyone know of any site where I can confirm the engineering IE:tension,compression & Etc. loading for an attic truss ? The way I see it so far:1. the rafter is in compression2. the collar tie is in tension3. the stud is in tension4. the joist is in tension All being subject to deflection except the stud. Thanks again

4. 
   Guest_ | May 04, 2000 03:20am | #7

   *>1. the rafter is in compression In a truss, it's called a top chord. It's definitely in compression, but also has a lot of bending forces in it. >2. the collar tie is in tension Nope, it's in compression>3. the stud is in tension I assume you mean the "wall" of the attic truss, and it is in tension. >4. the joist is in tension Yup - tension and quite a bit of bending stress.

5. 
   Guest_ | May 04, 2000 03:40am | #8

   *Ron, I will never ever call it a rafter again as long as I live I promise ;-) But I completely fail to understand how the collar tie is in compression ?PS. I guess deflection is what happens after bending ?

6. 
   Guest_ | May 04, 2000 05:18am | #9

   *Ron... i thought that the attic trusses also form the floor.... so sliding them is not a practicle solution... that's all i meant....if there was no floor to contend with.. it might be feasible to cut it loose and slide it..but in the situationj described ... i couldn't see it..basically.. what i wud be looking for is a scissor truss only rated to carry all of teh addtional load..this could be slipped into a slot in the roof and secured to each of teh trusses on the sides of teh dormers..it would bear on the plates of the walls..

7. 
   Guest_ | May 04, 2000 12:50pm | #10

   *> I completely fail to understand how the collar tie is in compression Don't know if I could really explain it, but I'll give it a shot. You already know the top chords are in compression - That compression tries to force the peak upwards. If the peak went upwards, that would force the ends of the collar tie closer together. Does that make sense ?

8. 
   Guest_ | May 04, 2000 12:54pm | #11

   *Mike - Attic trusses do certainly frame the floor. And sliding them over definitely wouldn't be a picnic. (I said in my 1st post that it wouldn't be pretty) But he's already going to have to do quite a bit of work on the floor to add floor area in the dormers. He would also have to cut into the floor system if he had new attic trusses built. A scissor truss would support the roof loads from the dormer, but would hang down into the room, and would not help support the floor.

9. 
   Guest_ | May 04, 2000 01:50pm | #12

   *ok...back to my original...due to this being originaly framed with ((attic))style trusses. the most practical i cud cum up with is transferring the loads to the ground .. thru one path or another...my guess.. sawzall city for the trusses.. to be replaced by conventional or post and beam frame..

10. 
    gator2121_99 | May 25, 2000 12:09pm | #13

    *why cant u cut out the intended trusses and sister normal 2x stock to the trusses add a ridge and head off as usual. just tying everything together.

11. 
    Guest_ | May 29, 2000 03:47am | #14

    *Mike,Transfering the loads here is key to the equation.I just got really busy with a project and I am going to have to get back to this one. I have been thinking about it now for about 3 yrs. I may have mentioned earlier that I felt a steel or wood frame that would transfer the compression/deflection loads in the rafter would be the engineered solution. Thnx All I will be backPS Sorry Ron I meant top chord in compression/deflection

12. 
    Guest_ | May 29, 2000 01:42pm | #16

    *> Sorry Ron I meant top chord in compression/deflection I'll let it slide this time........................(-: