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I have an attic space I'd like to make more user friendly. The rafters are original 2x4's, same goes for the purlins, and the kickers.

The span is approx 12 feet. This particular space is over a bedroom and bathroom, it has been used for storage over the years and the underlying floor joists have been fine. No sagging, no cracking of ceiling below.

If I were to sandwich each of the 2x4 rafters with 2x6 rafters, glue and screw, would I add enough strength to do away with the purlin and kickers?

The roof itself is shake over felt. There is no plywood sheathing.

*** editing to add more information:

The attic space is 20 feet wide by 10 deep. The roof pitch is 12-12. The very old 2 by 4 rafters are 24 inches on center. From the point the rafter meets the top plate of the wall to the center on the attic space is 10 feet. I am determining this 10 feet to be the so called rafter span.

Basically a 10 deep by 20 wide box with a 45 degree roof. The floor joists, also 2 by 4's, run parallel to the rafters thus I assume the floor joists are tying in the side walls at the eaves.

As this is not my line of work, I have been using some of the rafter calculators. If I am using the calculators correctly, 2 by 4's should not be used to span 10 feet - more like 6 feet. Thus I assume the original builder bolstered everything with the purlins and the kickers/struts to compensate for the under-sized rafters. After 80 years, the roof looks fine, no sag.

This same calculator reports that a 2 by 6 can span 10 feet 6 inches. Thus I was thinking of sistering not one, but two 2 by 6's - one on each side of the existing 2 by 4 rafter. There are six rafters on each side. Thus when all is said and done I'd have 36 rafters - 18 per side, 3 to a set, 24 inches on center, thickness of each rafter being 2 + 1.5 + 1.5, width being 5.5 inches.

The rafters are 14 feet in length.

For building codes ... this is an old home. Much of the 80 year old home is not code compliant, for example wires are spliced and taped, post/tube wiring, etc. Due to the age and condition of the home it is highly likely a new buyer will raze it, it seems to be the trend nowadays. Out with the old and put up a McMansion.

Regarding consulting outsiders: a carpenter friend suggested collar ties only should do the trick. I did not pursue this. A structural engineer took a look and suggested I beef up the rafters and I'm done. He further mentioned the floor should be OK, if the rooms below have lasted this long with people/furniture as a real world load walking and moving above them, they will be no worse off if I convert the space to something like a half bath. However, because lumber is relatively inexpensive, I'd most likely sister 2 by 6's to the floor joists as well as I think it could only help.

I will not take advice from this forum and start building with abandon, I'm just looking for friendly feedback and additional opinions. I got this idea about 6 months ago, and as you can tell, I haven't gotten very far.

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  • Modifying trusses is tricky. The dimensions of the truss are very important. You have a good start with the type of roofing, but at the very least, a sketch showing the dimensions of the trusses will be helpful.
    – hazzey
    Sep 30, 2015 at 1:07
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    It is possible to make modifications to a roof truss to achieve what you want. But as @hazzey says, it is not always simple. I would encourage you to consult an engineer or qualified builder who can come and look at the roof and recommend how to proceed. I don't think anyone here could give you suitable advice without actually seeing what you are dealing with in person.
    – mg4w
    Sep 30, 2015 at 9:26

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No, you shouldn't make structural modifications like this without them being approved by a properly licensed and certified structural engineer. Although you didn't say where this is, most likely it is against the law to do so anyway. In most jurisdictions, it will require something like a "building permit" to make modifications like this. This will trigger inspections, which will likely require drawings signed off by the right kind of engineer.

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Look at it from a liability point of view.

The roof structure on your building is located above people and the contents of the house. If an unqualified person (non engineer) tinkers with the structural integrity of the roof, he makes himself criminally liable to any accidents involving people or goods.

This means if the roof collapses and someone is seriously injured or even dies, depending on the laws, this might end up in a culpable homicide or manslaughter charge. If nobody is hurt, I am sure that 99% of all insurance companies will investigate why the structure collapsed and then see that it was no engineer. They would be very special if they would then pay out.

If the roof does not collapse, but starts sagging (more likely) you might end up with leaks. Again, the insurance will use any excuse to avoid having to pay out on a claim.

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