Sunday, January 10, 2016

Trusses and Roof Structure

Trusses and Roof Structure
Truss design plays an important role in the passive house envelope. Mainly to allow the high levels of insulation required, a very tall "heal" is required. We will need 24 inches of insulation in our entire attic - outside edge of outside wall to outside edge of outside wall. 


So here is our list of design characteristics: 
  • 24" heal height at outer wall
  • 24" over hang from outer wall
  • interior wall is load bearing
  • simple design (cost savings)
  • Flat ceiling

In order to meet the heal height requirement of 24" (and still leave space for attic ventilation) we need the roof pitch to be greater than 34 degrees. This led us to a 10/12 pitch on our roof - and the Grater Architects recommended it to us for aesthetics. And I must agree, it fits the house very well!

The trouble with such a large truss, is shipping limitations. The manufacturer (Morse Lumber) can only ship a maximum of 12 feet, so we had to do a "piggy back" truss on the top to finish off the triangle.

Truss Installation
Here are some photo's of truss day...
Let those trusses fly!
9 of the 22 trusses are in place!

Hungry? Grab a ....

This shows we will have 24" at the outside edge of the outside wall plus an inch or so of airspace for soffit ventilation.

We did have quite a fortuitous event occur with the truss installation. Due to scheduling and labor availability we decided to install the outer wall of the wall assembly before installing trusses. Our original plan was to fly trusses and then install the outer wall. This would have been a nightmare on the gable end walls.
If you look closely the completely sheathed  truss on the outside, is temporarily sitting on the outer (non load bearing) wall. If this wall was not there we would have had to cantilever it off of the load bearing truss OR paid to have a crane come back to lift 2 trusses another day. Building the outer wall 1st worked out much better!

So the load path once everything was nailed down and installed shows the inner truss over the interior wall is pushing down. The outer gable end truss (the will support the siding) is being pulled up by the over hang lumber (called the Rake Edge). The rake edge is nailed to the next most inner truss. I should say that this picture was taken before shimming and sheathing, and turned out very straight at the end.
Here you can see a hurricane tie attached to the outer truss to help "pull it up". This puts more load on the interior walls and reduced the weight on the outer wall.  

Next we installed our hurricane screws. Normally the strip seen in the picture above this is used to attached wall top plate to trusses. We opted to go with this screw for 2 reasons...
  • The screw doesn't create issues that directly compromise our continuous air barrier
  • It took a quarter of the time to install 1 screw than it did to install 10 nails and the strap

 When I asked our builder what he thought about the screw he said he'll never use the strap again, the savings in labor more than off sets the extra cost of the screw.

The south and north side of the house also has an over hang to shade the 2nd floor windows. Those were installed by hand and sheathed.
 We had to cut some of the gable end truss members to allow us to pass the south and north over hang "mini trusses" through the wall.
View of the south side
View of the north side


One of the most common questions I have received is "what type of roofing are you going to have?" Usually the expected answer is some kind of reflection shingle, solar electric shingles, or some other new technology. Fact is we are going with a plain black shingle. Nothing new here, just a black roof.


Here is what it looks like as of Jan 10th

Once the roof is finished we will be installing the zip system on the ceiling of the 2nd floor. After that we will be prepping for our 1st blower door test! Stay tuned!


13 comments:

  1. Great post about roofing I thinks very effective solution thanks a lot for shearing.
    Commercial Roofing Houston

    ReplyDelete
  2. thanx, but very little about roof truss design, check here http://myrooff.com/roof-truss-design/

    ReplyDelete
  3. Daniel,
    Thanks for the note, I apologize if you expected me to go into the actual truss design. Basically, the design was based on simplicity, heal height, pitch and cost. Our lumber supplier did the actual truss design - Morse Lumber http://www.morselumber.com/

    ReplyDelete
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  10. Hi Mathew,

    I keep coming back to this blog again and again for insight. Thank you and congrats on the recent, well deserved awards.

    I had a couple of questions if you don't mind:

    1.) I noticed your roof trusses are different than the trusses in your detail. Is there any energy penalty from thermal bridging by having the bottom chord extend outside to form the soffit?

    2.) Also, how difficult/expensive was it to add the ZIP sheathing as your ceiling air barrier rather than using a membrane like Intello Plus? Those ZIP panels are heavy!

    Thanks again!

    ReplyDelete
  11. Hey Rick,
    Thanks for the notes.
    1) I am still working through the thermal bridge detail with the certifier regarding the bottom chord. I'll let you know what I find out.
    2) Zip was about double the cost of the fabric (and OSB for that matter). We really had never considered using a fabric as the ceiling air barrier because we were going to have 2' of cellulose ontop of it. Cellulose is relatively heavy. Also we were able to screw our electrical j boxes to the Zip in the field which made the electrician happy. As for the difficulty to install: We had built the interior wall before we hung the zip. The interior walls are built 2" low, so we lifted the zip and fed it on top of the interior walls so we were working with full panels. We then only had to lift it 2" to nail it. We went with Zip over OSB because we were still unsure of the "airtightness" of OSB. We probably would have been fine with just OSB, but much like the walls we wanted that piece of mind.

    ReplyDelete
  12. Thanks so much for the insight Mathew!

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