Can House Trusses Be Modified?

By Todd Fratzel on Design, Framing

cathedral scissor trusses 300x225 Can House Trusses Be Modified?

Common House Scissor Trusses

Can Truss Members Be Cut?

I received an email from a reader today asking me some questions about trusses in his home. The homeowner is interesting in creating a spare room in his attic. He noticed that there’s lots of room up in the attic that might have the potential for a spare room. So he asked me if I thought it was ok to cut certain truss members and reinforce others to structurally accommodate his new room.

So the burning question is: Can Truss Members Be Cut? My response to him was don’t do anything without a consultation with a licensed structural engineer. All manufactured wood trusses in this Country (this should be the rule) are designed and stamped by a licensed structural engineer. Cutting, modifying and altering a structural truss without the approval of the engineer of record is actually a very bad idea if not illegal in some states.

Can Truss Members Be Cut? The answer is maybe. Without knowing anything about the readers house framing it’s hard to say. I have seen situations where someone wants to build a room in an attic that has existing trusses. The easiest way to do this is by building a rafter system from inside the attic in the area to be modified. However, this is not as easy as it sounds and it still requires some oversight / input from an engineer. There are many issues to consider with this type of renovation; roof support, ceiling below, new floor framing, etc.

Don’t Modify / Cut Wood Trusses

My recommendation is really simple. If you live in a house with engineered, manufactured trusses then don’t mess with them. If you really want to modify the framing of your attic and roof then consult with a licensed professional structural engineer. I think you’ll find that this type of renovation will be extremely expensive and time consuming. You may find it’s much easier and cheaper to just rip off the room and install new attic trusses or rafters. Of course that raises all kinds of interesting issues such as protecting your home from the elements until the new roof is fully built.

It’s also very unlikely that a local building code official would allow you to modify trusses. However, most local code officials should have a copy of the truss plans on file for newer homes. This would certainly be helpful to a structural engineer that might be hired to evaluate the options of modifying the existing trusses.

Has anyone ever tried to modify trusses in order to build a room in an attic?

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5 Comments

  1. John C says:

    I’ve done this several times, and you’re exactly right – don’t cut anything without consulting with an architect or engineer. It’s totally possible, and it’s alot of work. I’ve done MANY 2nd floor additions and having the trusses there actually helps as far as the sequence of things when your trying to protect from the weather. I’ve actually done this while the homeowners still lived in the space below.

    Basically, you can construct a stick frame rafter system next to each truss. The first step is to remove the fascia and about 2′ of the roof sheathing right above it. Then slide floor joists in along side and in between each truss (from 24″ o.c. to 16″ o.c. usually). Since only 2′ of the roof is open at this point, it’s easy to cover for the night or for weather. Next, slide in some boards to use as planking over the new joists to temporarily walk on while you continue. Since the new joists will be taller than the bottom chords, this will allow you to leave electric wiring, insulation, and possibly some ductwork in place.

    A little more cutting and you can make room for the ridge beam, again with only a minor exposure to the weather. Cutting a bit of the trusses usually isn’t a problem given that it’s extremely temporary at this point. Remember, with the plywood sheathing already nailed to the trusses, it will act to keep the whole thing together. Temporary bracing/blocking is also important in certain areas and you can’t just cut with reckless abandon. Once you get to a point, you can go for it and start adding rafters and removing the truss webs. Depending on the new design, the existing trusses will keep a “shell” to drape tarps over as you make progress.

    Let me stress that this is not a job for an amateur and does require skill (speed in particular) and strong manpower. I hope this helps!!

    JPC
    Dagsboro, DE

    • Todd says:

      @ John – Thanks for reinforcing my point here. I certainly don’t want to make folks abandon their dreams of remodeling but it’s crucial to get expert advice on an advanced framing situation such as this.

  2. hcbertsch says:

    Good and informative article. I like this website. As a contractor myself, I am always having to tell homeowners these things.

  3. Lynn says:

    I am a remodeler, but don’t usualy modify trusses. The house I am doing now I would like to raise the cieling to 10 feet, but it has a manufactured trus with a 12/12 pitch and a 60′ span. It is Identical to the truss pictured in this artical except over the 8′ ceiling it has a center vertical support.
    _______________________
    _____[ ]______________

    This is a crud drawing of the change, but you can see I would have to cut the joist that are now the ceiling and then some how gain that structural support again. The other way would be to vault the ceiling, what ever is easier.
    The only thing good is this room is that it is dead center of the truss.
    I have also thought of modifying every other joist and leaving the remaining ones exposed in the room with a vault which may be safer: oh by the way a vegas would run down the center of the vault if I go with a vault and no exposed truss.
    I Have not at this time been able to find an engeneer that has the time to mess with it and wanted to know if you would have any ideas.
    So here is four options if any will work.

    • Todd says:

      Lynn – Modifying trusses like that really should be left to a structural engineer. Without seeing photos it’s hard to say if it’s even practical.

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