Trusses - which is best solution

I own a modular home with a roof pitch of 3X12.(Gables on the ends) I have an awesome plan to add a large rec room to the rear of the house and enlarge one bedroom with a small addition to the front. When complete, I will have two roof lines forming a cross with the new roof being above/on top of the old. I have consulted two different truss manufactures and have gotten two different opions. One tells me to stick build the roof addition using rafters over/attached to the existing roof and he will construct the trusses for the additions extending beyond the current structure. The other tells me he can construct trusses for the entire "new" roof which would be secured to the existing roof (smaller trusses, of coarse as we near the top of the existing peak). According to him, his design will more evenly distribute the load across the existing structure. I'm no engineer but I like this idea and want to make sure it is the right thing to do. Thanks for any responses!!

 

[JAE]

Lou,
I agree that using trusses will help spread the load better than single rafters coming down on one valley line of your existing roof. At some point, near the top, you would stop the trusses and just stick frame the last few feet.

The trusses have diagonal webs that can engage the existing roof framing across the width of the addition. You can do this, to some extent, with rafters...adding additional stick-diagonals but in the end you are simply making a truss in the field and its cheaper to order them.

The more diagonals you can get in the trusses, the better. If they provide trusses with only two diagonals, or simply a king-post truss...you're not really getting much benefit vs. the rafters.

 

JAE,

Thanks for the response! I have a much easier feeling about this decision. Thanks again!!

Lou-I

 

[billz]

The second truss manufacturer is offering to provide what is known as a "valley set." In my area (Florida), valley set members are not true trusses, inasmuch as the internal members are all verticals, and not diagonals. This is fine, because the existing roof is the "business" part of the arrangement: the valley trusses do not transfer the load to the bearings, but only straight down.

A stick-built valley set is fine also, but it should have vertical supports at approximately 4' intervals. A common abuse is to build a stick-build valley set with only a center post, or no verticals at all. This creates concentrated loads at the point where the rafters hit the underlying roof. Rafter performance can be suspect as well. If the distances are small, then this will not be a big problem, but check with an engineer or experienced truss technician.

Several nailing specifics for the stick-built valley set and the prefab also should be obtained from a qualified source.

Bill

 

[Trussdoc]

Another item to consider....if you're installing the valley set over existing roof sheathing, you may have to cut away a portion to ventilate the gable area and provide airflow through the new roof. Dont cut too much as you may leave the existing rafters without adequate lateral support.