Modify Attic Truss for Attic Ladder and storage

Discussion in 'Framing and Foundation' started by Magus2727, Sep 15, 2012.

  1. Sep 15, 2012 #1

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    I have a question about adjusting a single (out of 12 total trusses spaced at 24") attic truss that is in the shape as below:

    [​IMG]

    My trusses are all 2" x 4" construction. I am not making this into a livable space just storage space. The space between lengths of support from the center metal gusset and the bottom chord are 8 Feet. My plan is to lay down 23/32" tongue and grove OSB for a floor. Should I build up the floor with 2" x 6" in the lengths I am laying floor so its on the 2" x 6" and not the 2" x 4" of the truss? Guessing I should sandwich a 2" x 6" on each side of a single 2" x 4"? so of the 12 trusses I would have installed 24 8 foot lengths of 2 x 6?

    The problem with the Attic Ladder:
    The only problem is that the only location in my home/town-home that can accommodate an attic ladder (standard 22" wide 54" long) would hit about right in the middle where the two supports of the truss web meet on the bottom chord. and the truss is centered in the hallway so I will need to do some boxing between trusses as it stands.

    how may I best modify this single truss? I was thinking of trying to make one side of it look like:

    [​IMG]

    I was thinking of something like this:
    [​IMG]

    The RED section would have 2" x 4" reinforcement on both sides
    The BLUE section would have extra 2" x 4" sandwiched between the existing truss 2" x 4"

    New 2" x 4" will be added to make up the corner using screw tie in plates.

    This would move the tie in into the bottom chord (while keeping the tie into the top chord at the same point) enough to clear the attic ladder.

    Thoughts? Thanks for any and all help!
     
  2. Sep 16, 2012 #2

    inspectorD

    inspectorD

    inspectorD

    Housebroken Staff Member Admin Moderator

    Joined:
    Dec 17, 2005
    Messages:
    4,502
    Likes Received:
    267
    This has been asked many times before, so I doubt you will get a different answer. And the answer is, you can not modify a truss, only an engineer can do this.
    I'm not trying to be a damper on your ideas,You can do whatever you want in reality. But if you ever get snowloads, high winds or pressures you could have a roof collapse. We saw many of these a few winters ago where folks did things themselves.
    My opinion would be to insulate the heck out of that attic and close it off. Find storage elswhere and save your money on the energy bills.
     
  3. Sep 16, 2012 #3

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    I was thinking with only 1 truss being modified it might be a little different. It also looked like on searching, other large projects (using Google search) if used the tipple up of 2 x 4 on either side of the truss would add the strength back in (if not make stronger). I also saw on another forum on a similar question where an engineer (claiming to be one) said as long as you kept the structure to be similar to what other common truss configurations are available you should be fine. but a lot can be said and people can claim all sort of things on the internet.

    I wanted to get a full door (47 to 54 inch variety) but could always go with a ladder like:

    http://www.homedepot.com/h_d1/N-5yc...ord=Attic+Ladder&storeId=10051#specifications
     
  4. Sep 17, 2012 #4

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
  5. Sep 18, 2012 #5

    joecaption

    joecaption

    joecaption

    Well-Known Member

    Joined:
    Jan 8, 2011
    Messages:
    2,046
    Likes Received:
    295
    Plus 2 X 4 bottom cords were never designed to support any loads, even light ones.
     
  6. Sep 18, 2012 #6

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    So if it is done as it looks like in The PDF link 2 x 6 's need to be ran on the base. And all reinforcements need to be made prior to removing the "old" sections of the truss. Has any one followed up on this forum after contacting a structural engineer or architect to see what they have commented?
     
  7. Sep 18, 2012 #7

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
    I don't think anyone here thinks any of this is a good idea.
     
  8. Sep 18, 2012 #8

    Wuzzat?

    Wuzzat?

    Wuzzat?

    Well-Known Member Sponsor

    Joined:
    Jan 20, 2010
    Messages:
    2,471
    Likes Received:
    175
    Even if you know vectors and strength of materials and anticipated loading (concentrated or distributed) a manual truss design is laborious.
    You will end up needing to know the force and thrust direction in each member, initially and after redesign.

    What you propose may be overdesigned, underdesigned or just perfect, this last case being extremely unlikely.

    I guess you could test what you already have by putting concentrated loads in different places and measuring deflections, and then rebuilding to not exceed those same deflections, but designing this test procedure may be worse than doing a redesign. And you should test your redesign in any case.

    Your first truss picture looks wrong. That downward sloping strut is supposed at right angles to the roof rafter.
    http://www.google.com/imgres?q="roo...w=189&start=0&ndsp=20&ved=1t:429,r:2,s:0,i:79
     
    Last edited: Sep 19, 2012
  9. Sep 24, 2012 #9

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    The first picture may be wrong, it was the cleaner picture I was able to find on Google images.

    OK, so after more research and spending more time up in the attic and running some load bearing simulations at work I will not be modifying the trusses in anyway. I have located a location I can install an attic ladder and have it centered between trusses. Its not the optimal location I would have liked but will not require the cutting of the truss and relocating the webbing. I would still like to place a floor up there so my thoughts are as followed...

    Install 2" x 6" x 8' (or 10') across each the bottom chord of each truss. I will have the new "beams" tie into the top 2" of the bottom 2 x 4 chord and where the truss webs meet the bottom chord (through a combination of wood glue and screws.

    This would be enough support for a storage floor ( less than approx 100 per sq ft) you should think?

    I already have blown in insulation in the attic and was going to add additional insulation by means of 2" thick insulation board to nail along the top chord of the trusses (so I have a good and unobstructed air flow from the eaves to the air vents along the center of the roof.). I was going to do a similar thing to "enclose" the area I have the floor for the upside down V in the center of the truss. I also was going to add an attic fan with the temp control switch to provide better airflow and cooler attic in the summer.
     
  10. Sep 25, 2012 #10

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
    I am happy that you no longer plan on changing any trusses.
    Trusses are designed to carry the weight of 5/8" drywall and the roof and sheeting and what ever snow load you have in your area. As they are not designed to set any load on interior walls you will still be taking chances on saging. That sagging will show up on the ceiling the floor below the interior wall and the roof.
    You should call your local truss company and see if they can make any suggestions.
     
  11. Sep 25, 2012 #11

    Wuzzat?

    Wuzzat?

    Wuzzat?

    Well-Known Member Sponsor

    Joined:
    Jan 20, 2010
    Messages:
    2,471
    Likes Received:
    175
    Figure out how much deflection [floor stiffness] you want: L/240, L/360, etc.. For this application you can probably stand a springy floor.

    The engineering toolbox website will help but it's a bit tricky converting lbs per sq. ft to lbs per linear inch distributed load on a joist.
     
  12. Sep 25, 2012 #12

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    I do have a couple main walls that run perpendicular to the trusses. I believed it is a load bearing wall as it carried from the second floor to the first with a large heavy I-Beam on the ceiling of the first floor to provide the open space on the first floor. So that is a center support that runs perpendicular along the entire length of where I would have floor. Would it be better to have the 2 x 6 "floating" above the drywall that is the ceiling? I figured that, that would remove the load from only being on the bottom chord and would reinforce the bottom chord and also put weight into the webbing. I would most likely still ad in some 2 x 4 reinforcements tying the 2 x 6 into the top chord by additional supports. But since in do have the wall in the middle that would provide a center support if I put the 2 x 6 flush with the ceiling or in other words the center wall support.
     
  13. Sep 25, 2012 #13

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
    How far is it from the outside wall to the bearing wall, if you could run joists all the way out, you would not effect the truss.
     
  14. Sep 26, 2012 #14

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    I am not sure... I think if I got 12' length 2 x 6 it might go edge to edge with a "joint" in the middle wall. I will have to get back up in the attic and measure my room walls. Appraisal documents show that I have 30' between front and rear exterior walls. So it may fit... I have bays and outcrops. If not including those than its 26'. So I would need to go out to the ceiling wall correct so some locations I would only need 26' of new beam and in other spots 30'. They don't make 15' or longer 2x 6 do they?

    Edit: looks like they make 2x6 in lengths 8', 10', 12', 14', 16' and 20'... so should be able to span the whole room while having a "joint" in the middle.
     
    Last edited: Sep 26, 2012
  15. Sep 26, 2012 #15

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
    If you going from the exterior wall to a center bearing wall, you could use a 2x10 up to 16 ft for more than that you would want to go to 2x12s. Either way yould would want to this at 16" on center not the 24" that the trusses are set at. I'm not sure how you would get lumber like this in the attic but lumber is available more than twenty feet long.
     
  16. Sep 26, 2012 #16

    Wuzzat?

    Wuzzat?

    Wuzzat?

    Well-Known Member Sponsor

    Joined:
    Jan 20, 2010
    Messages:
    2,471
    Likes Received:
    175
    The 16" will give the OP 1.5x the load bearing capacity of the 24".
     
  17. Sep 27, 2012 #17

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    What is the typical attic storage / loft storage dead weight (PSF) rated for? Dead weight would be higher than live weight right since your going to be storing more than living. I don't think I understand the PSF thing right being 180lbs and my "foot print" is close to 1 squr. foot.

    Worst case I have runs that are about 13 feet from load bearing to load bearing. Looks like it would be best to run 2 x 10 every 24" for that? Calculator is telling me I could go 12' 11" close enough to the 13' for me. That's with a live load of 40psf and dead load of 10psf...

    The PSF thing still leaves me clueless. I have dumbbells that apply more pressure statically then that.
     
  18. Sep 27, 2012 #18

    Wuzzat?

    Wuzzat?

    Wuzzat?

    Well-Known Member Sponsor

    Joined:
    Jan 20, 2010
    Messages:
    2,471
    Likes Received:
    175
    You mean
    distributed load
    http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html
    vs.
    concentrated load
    http://www.engineeringtoolbox.com/beams-support-forces-d_1311.html
    ?

    And
    "Comment: In addition to a 30 psf live load requirement for attics where development of future
    rooms is possible and a 20 psf live load for limited attic storage where development of future
    rooms is not possible, "

    I suppose a distributed load spans more than one joist and the load footprint is a significant portion of the beam span.

    And if you press an ice pick tip into the floor it is probably thousands of PSI.
     
    Last edited: Sep 27, 2012
  19. Sep 27, 2012 #19

    nealtw

    nealtw

    nealtw

    Contractor retired

    Joined:
    Nov 4, 2010
    Messages:
    23,896
    Likes Received:
    3,117
    There is no standard for attic storage in trusses unless they were designed for that. If you are going from bearing wall to bearing wall you are building a floor that just happens to be in the attic. If we use 20 lbs psf and your floor is 10ft x 13ft your load limit would be 13x10x20 for a total load.
     
  20. Sep 29, 2012 #20

    Magus2727

    Magus2727

    Magus2727

    Member

    Joined:
    Sep 15, 2012
    Messages:
    8
    Likes Received:
    0
    Think the 2 x 8's every 16" will be sufficient. What about lateral anchoring? Should I just use "X" 2x4 braces at the ends of the 2x8's to maintain the 16" spacing ? I am sure there is a more "standard" way to do it?
     

Share This Page