Common House Scissor Trusses
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.
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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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
@ 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.
Good and informative article. I like this website. As a contractor myself, I am always having to tell homeowners these things.
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.
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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.
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.
Hello, my name is Lucas, I'm a home inspector in Virginia. We always write up cut trusses as in need of evaluation by a structural contractor even if we see other trusses with repairs/reinforcement.
I did have a question about cutting the vertical studs at a gable end truss. Someone notched 4 of the studs (2 on each gable truss) 1" deep and about 10" long to fit a HVAC supply duct.
Are the studs considered "webbing"?
Lucas - In my opinion my cases of gable trusses the verticals are framing infill only. Especially if the gable truss is properly sheathed.
In the UK, our houses are small, so modifying roof trusses, after confirming the modifications with an engineer first, is very common for us to gaining an additional room.
I'm glad that you talked about how we shouldn't cut or modify our trusses after we get them. I've been wanting to change the framing of our home a little bit, and I think that being able to have some help for when we get started would be good. I'm going to have to see if we can get some help when it comes to the trusses from professionals so that I don't do more harm than good to my home during our project!
I'm currently about to undertake this kind of project. My builder is very confident, and the engineering was quite reasonably priced (about $800). The current home has a 24x28' attached garage with a truss built roof, pitched at 14-12. The plan is to tear out the drywall ceiling in the garage, place 16" 2x4 floor trusses 19" O.C. and build the decking on top of that. Then sister 2x12's to the existing trusses and use plywood gussets with an engineered nail pattern in place of a ridge beam. Throw in some collar ties and your done. Cut away the truss webbing and finish out the space. It will create about 600 additional square feet (some of that space is not above the garage) at a cost of about $52,000. I live in an area where costs are about $175 a foot to build, some I'm way ahead of the curve.
Nice...basically you're creating a raftered roof so you can remove the trusses. But I love that you got the proper help and now you'll have some extra space at a reasonable cost.
If a truss is designed for 40 PSF ground snow load, with no bottom chord live load, could a reduced e.g. 30 PSF ground snow load rating and larger bottom chord gussets allow for a 5 PSF bottom chord live load?
Ken - You'd really have to check that with the truss manufacturer. Depending on the age of the home it may not be hard to contact them.
I am wanting to simply add some small live/dead load to my over-garage attic space. The local truss mfg would have built my exact truss with a 2x6 bottom chord to allow for this storage. These are single fink with 90" clear in the center creating plenty of space for light storage. It seems like I should be able to sister a 2x6 to the bottom chords, not cut anything and be fine for light storage. The truss design software indicates this to be true even with 2x4 chords as long as they were 1.8M SS grade 2x4s. Seems like glue-screw sistering would easily accomplish the needed bolster. The top chords may also need it. I only have 14 of them...so this is not much effort.
You are likely correct, without seeing them or the calcs myself I can't say for sure. Sounds like you're on the right path though.
Todd, i am a homeowner doing a renovation on a 200 year old converted church. I would like to create some usable space in the attic which has a king post truss that spans 42 ft. i would like to modify one of the trusses to allow for headroom. my initial idea was modify the truss my leaving the vertical post in place removing the two diagonal struts and replacing those with collar ties from the post to the rafter and add to additional vertical posts. can this be safely done? the existing material is approximately 10x10 timbers