You will not only need larger rafters but larger attic floor joists. If those elements met the minimum requirements of the building code you would not need an engineer but it would be a difficult job.

Anytime you cut trusses you need an engineer to sign off on how it's done. Ceiling joist are not floor joist, those will have to be dealt with also as noted by above post.

Al

There is a point in the modification of a truss that it is no longer a truss and I believe your program is way beyond that point so it is not accurate to refer to this as a "truss modification".

I would say that this program requires the rebuilding of the roof and attic floor with any existing members that might remain not contributing to the structure in a meaningful way.

Therefore, if the new rafters and floor joists meet the minimum requirements of the building code span tables and are connected at the wall top plates, an engineer is not needed.

This approach probably requires removal of the ceiling and the truss intermediate chords and sistering of the existing rafter and bottom chords with larger members. Of course, you could also do it from above by removing the roof.

The hardest part is finding room for the stair.

There will still be bracing requirements all dependent on the span, wether it's simple cross ties, or involving verticals. A lot stems from location as well concerning loads such as snow, that's why the advise for an engineer to spec it all out. Even death valley has a snow load requirement of a laughable 20 psf!

In short...no, you probably can't do this, and it isn't likely to be cost effective.

For a new simple span rafter system with new joists acting as rafter ties at the top plate level, everything you need to know is spelled out in the building code tables which take into account the snow and dead loads. It's not rocket science. Only designs not covered by the building code (like trusses) require an engineer.

The question of cost effectiveness has to do with the cost of adding an equivalent floor area adjacent to the house and how that would be integrated into the house and how it would fit on the site. Assuming there is enough space under the rafters I would think it would be cost effective.

As you can see from the responses opinions on this are going to vary greatly. Trusses are engineered to be as light and strong as needed to be for the particular application. Think of a spider web and what will happen if one or two of the strands are cut, you lose the integrity of the whole system. It may or may not fail. While what you want to do probably can be done, DO NOT do anything without consulting a structural engineer.

You should be able to save yourself the cost of an engineer unless the structure is unusually large or complex. It would help if you told us the span and slope of the roof and if there are any intermediate bearing walls, hips, dormers and the snow load.

A roof truss relies on rafter chords, a bottom chord and intermediate chords arranged in triangular patterns so that all chords are either in tension or compression. Because wood is stronger in compression & tension than in bending the size of the chords can be reduced which saves money. Of course, the strength of the connections become critical but that is solved by building the truss in a factory under strict quality control.

The removal or relocation of even one chord and/or the addition of almost any unanticipated load on the bottom chord will induce BENDING moments in the other chords that they are not large enough to take.

An engineer would have to look at the roof structure not as a truss but as a simple-span rafter system which would involve adding new rafters and floor joists sized for the loads. Such a structure does not not require an engineer because the framing sizes and configurations are clearly and simply shown in any building code. Any licensed builder can design it and get it accepted by the local building department.

If the spans are larger than shown in the building code you will need to use LVL's or I-Joists which would require an engineer. You could hire one or use one provided by a manufacturer through a local lumberyard.

The only issue not addressed by the building code is bouncyness of floors. If you don;t know how to avoid this problem just use the next larger joist size if you are close to the span limit in the code table.

I have a related question.I have a cottage with a drop ceiling that I want to get rid of.It is roughly 30ft by 30 ft with a 4/12 pitch.I am removing the drop ceiling and would like to lose some of the rafters but only to open it up, not as a loft but purely for the open feel.How can I achieve the openness I want but maintain integrity.

The only practical way to remove all the truss elements in an attic is to replace all the trusses.
As in remove the roof & trusses and replace them.

You are unlikely to be able to fit conventional framing in the attic since the eave height is likely lower than the required floor joist height.

Modifying trusses can range from 'not bad' (you know the manufacturer and they are still around and willing to detail changes for a fee) to 'not going to happen' (unknown manufacturer, older house, etc.).

Without the actual truss construction drawings there is often no way to tell what grades of lumber have been used for the different members allowing any alterations to be made.
It is pretty common to use the minimal grade for each member in a truss based on the loads in that member.