What are some options for converting a trussed roof into living space?

My understanding (please correct me if I am wrong) is that the following needs to be taken into consideration:

  • proper floor joists need to be installed/sistered.
  • Knee wall supports need to sufficiently replace the webbings of the truss to support the roof
  • space needs to be considered to accommodate proper insulation between the living space and the roof where it contacts

I've seen a lot of systems/contractors that seem to specialize in this in the UK, but I can't seem to find much information for North America. Not sure why that is.

Has anyone done this? Is it at all a DIY proposition (providing proper engineering documents are approved)?


My purely hypothetical method at this point (how far off base am I?):

  • install 12" floor joists 16" OC
  • install kneewall between current 2x4 truss 'rafters' and new floor joists.
  • install top cross-bracing/collar tie (not sure how much/spacing)
  • cut joist webbing (!?)
  • between knee wall peak, install 12" 'rafters'...these wouldn't be load bearing but act as spacers for insulation and interior ceiling.
  • leave gap between original 2x4 rafters and sheathing for ventilation
  • insulate remainder 10" of rafters and knee wall

It seems that the above plan would basically be converting the current truss system into a box storage truss, albeit with larger floor joists.

Am I in the ballpark? Again, I'd definitely be consulting with an engineer to do it right. Just trying to get an idea of what it would take.

With the above, I think I'd need to pull off the first layer of sheathing on one side of the roof to bring in the joists. If I had a load-bearing wall mid-span, could one bring up half-joists into the attic directly (since these joists are purely for floor support while the original 2x4's are acting as the ties?)


Some more details of this particular example:

The attic space is 22' on the side parallel to the trusses (28' on the other side). It's a 12/12 roof, so that's a good 11' at the ridge, which even with new floor joists and a 5' knee wall should be plenty of room for a nice space.

The trusses are double cantilever style:

enter image description here

I also came across this in Fine Homebuilding on using welded steel as part of the rafter structure. More applicable for creating cathedral ceilings but thought it was interesting:


  • 2
    I'm not sure how to make a "real" answer out of this question, but yes I've done it and yes you have the major components down. Another consideration is an egress point depending on the purpose of the living space. Commented Mar 26, 2012 at 15:55
  • Ah, yes, egress point. Good point. And yea, if it's not really 'answerable' I understand if it gets closed. I guess I was hoping for perhaps some links to guides or case studies on how it's done. I worry/wonder if North American trusses are different than those in the UK (The UK tends to build things better it seems...)
    – DA01
    Commented Mar 26, 2012 at 16:21
  • how tall of a kneewall are you looking to install? How high is the peak of the roof now? need a lot more info to really give an answer Commented Mar 26, 2012 at 22:26
  • 8
    As a contractor, I have been asked to do something similar to this in the past. I refused the job. Roofing trusses like the one you picture below, have had a LOT of hours of engineering research put into them. They are extremely reliant on their design and construction, and mucking around with them increases the probability of a catastrophic failure. Why do you wish to extend the living space in this way, is there another direction you could go (basement, garage, etc...)?
    – ShoeMaker
    Commented Apr 29, 2012 at 22:56
  • 1
    What is supporting the roof trusses? Floor loads are significantly higher than ceiling loads. Can the supporting structure carry the additional load? Beams, headers, columns, foundation?
    – bcworkz
    Commented Jul 7, 2012 at 0:26

3 Answers 3


Yes. Very possible. It has to be engineered though. We have plans to do just this. The drawings and engineering cost around $1000. I wouldn't try it unless an engineer has looked at it, drawn a plan and signed off on it.

For us we are going over a 3 car garage and the roof pitch is 12-12. So a lot of space there. The plan is to remove the drywall ceiling in the garage and install 24" engineered joists (2x4's). We lose a little head room in the garage and the space above, but not much.

Then once the floor system is in place it's much easier to move around. 2x6 rafters installed with collar ties at about 8.5 feet, and plywood gussets with engineered nailing patterns at the peaks. Then cut away the trusses. Doing it for about $80 a foot, and it will add about 500 square feet.


It can definitely be converted, but you should be asking yourself if it's worth the investment. Maybe an extension or conservatory could cost in the same region, but you will have much more real space.

It seems that truss roofs could be quite difficult to walk around with cross members. I think you could remove the cross members if you go to a roof designer. Remove all tiles, redesign, and strengthen it to maximize space.

But it sounds like a lot of work for a small amount of extra space.


It is definitely possible. My uncle has had this done in his loft. They had to reinforce the trusses with large pieces of wood, but they were single rather than double cantilever so it was easier.

Perhaps it's worth getting some quotes from professionals or hiring an advisor, as they'll be able to give you an idea of what would be involved and the complexity of the task even if you don't use their services.

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