Expert: Ted Barnhill (Principal, Design45 LLC) - 12/1/2007

Question
QUESTION: Hello Ted
I am having an addition added to my house. The existing exterior load bearing wall of the house will be removed in the future. The contractor i hired is putting a new roof truss at the point where the existing ceiling (2x4's)and the existing site built roof truss rest on the exterior wall. He then nailed a double joist hanger to support them and nailed the hangers to the new truss. My question is will this be strong enough to support the existing roof when the exterior support wall is removed? I did pull a permit (because i will be doing all the interior work), but the hand drawn plans the contractor gave me for permit application do not say anything about the exterior wall being removed.

ANSWER: Terry,

It depend on the size of your new truss, and its supporting columns in the new or existing walls.  This is called a girder truss, and in your case, depending on the span, should probably be at least a two-ply member (so 3" or more wide).  The total load on this truss will be large, so you need to make sure that it is safely transferred to the ground with adequate columns and footing.  

You should check with your contractor to ensure that he/she had the girder truss, hangers, posts, and footings properly engineered.  Also, this would ordinarily be on your permit set so that the inspector can verify that the truss drawings account for the extra loads.  You are correct that it is a major structural change that needs to be handled carefully.

Good luck with your project.  Feel free to follow up with other questions if you'd like.

---------- FOLLOW-UP ----------

QUESTION: The addition is 21'x16' with the 16' perpendicular out from the house. He did nail 2 trusses together for the 3". The trusses span 21' and rest on the new walls perpendicular to the house. The new walls are sitting on 2x12's that are nailed to hangers attached to a Rim joist (i think that's what its called) bolted to the house. The city is calling this a 4 season porch because there is no foundation, and it is supported off the ground by 16" cement footings drilled down 43". 4 footings along the 21' span and 1 on each side at 8' out from the house. By the way this work is being done in the northern part of the twin cities (Minnesota). As i stated earlier the plans that were given to the city were hand drawn with the addition size and footing size and locations along with a sentence saying that the roof is made with engineered trusses. I am writing to you for your opinion on this to confirm if it is being done correctly and to ease my tension about knocking out 21' of support wall.

Answer
Terry,

The details you provided in your follow-up don't put me at ease about the structural adequacy of your contractor's design.  

Depending on the span of your existing (site-built, you mentioned) trusses, and a couple of other factors, the load on your 21' girder truss may be rather tremendous.  Granted I don't know all the details, but I would have assumed that the point loads at the two bearing points would require engineered posts (rather than built-up from 2Xs) and new post footings.  Depending on the size and condition on your existing footings, you may experience foundation cracking and/or settling by concentrating such large new and existing loads on two points.

I should just repeat that I feel your concern is sensible, and that the contractor should be able to ease your mind about engineering calculations.  The truss manufacturer should have provided truss drawings indicating the design loads, which in your case must include roof loads from the existing house.  Assuming the truss manufacturer designed for the correct loads, there would still be important engineering to carry those loads down to the ground.  This would ordinarily show up on the permit set; some cities are less rigorous than others, but omitting that the wall was to be removed is, I agree, a red flag.  In particular, the city framing inspection should follow all major structural work, including existing wall demo.

If you or your contractor need to hire a third party engineer, I can recommend some good ones in the Twin Cities (where I am also located).

Again, good luck, and feel free to follow up with me.

Ted