House was built in '74 using trusses made of 2x4s. It needs a neew roof and
the design of the house lends itself to replacing the asphalt shingles with
Spanish terra cotta tiles. In this case probably the light weight concrete
ones. Roofer said the existing trusses will never support the extra load and
I agree.His suggestion was to build new trusses sistered next to the old one
out of enginner lumber(wood I beams) not that difficult to do in this attic
but probably very labor intensive. I need something me and some of my poker
buddies can do without hiring a crew.
I have at least couple of years to do this project before thes roof has to
be replaced, I figure since wood I-beams are so strong. box beams would be
even stronger. My plan is to use use 9" strips of OSB one each side of the
joist and rafters and completing the box beam with another 2x4. This is
something I could do myself over the course of a couple of years or less.
Attic is tall and has a 6 ft wide walkway down the center. Insulation is
sheets of rigid foam.
Also could build I beams in place using exsiting trusses as part of the I.
Any thoughts on my plan would be appreciated.
My reaction too.
Have an engineer analyse the existing trusses.
Ask a truss builder to price new trusses for the new load.
In addition, the clips and ties holding the trusses to the building
probably don't meet code.
An engineer can tell you that too.
If the stuff you build ends up stiffer than the existing trusses, they will take
fo the load. If it is less stiff, it will not take load until the existing
deflected somewhat. If your upgrade isn't matched to the existing trusses, you
end up not improving the situation at all.
When the roofer told you to sister the existing trusses, he should have said to
them the same as the existing ones or sister with components that will move with
the existing ones _exactly_.
Before you go on with this, get the advice of an expert.
PS a box beam is only as strong as the shear strength of the join between the
webs and the flanges. Do you know what it takes to make that right?
I doubt your plan to make your own box beams would give you anything
near the strength of a manufactured "wood I-beam". The companies that
make the I-beams hire engineers and specialists in materials to design
these things, and for that you end up something that is known to work.
Your poker buddies are likely to just slap together scraps of whatever
OSB and nails and glue they have lying around, hoping to get something
that sort of looks like the real thing, at least in appearance.
Actually we have made I beams with OSB and 2X4s for a boat house. Local
supplier didnt have them and we didnt meet the minimum order. My poker
buddies are a pretty savy group with a couple of EEs in the group. There is
a stuctural engineer in the group but he say he hasnt done anything with
wood in 30 years and any real engineering at all in 15 years. I havent been
able to get them around to the house yet but in a couple of weeks we will be
down at the house and I can get them together for poker and steaks. Then I
can see what they know.
Some of the comments did get me thinking. Plan on building a test section of
the box beam mades of OSB and 2x4 and see what the failure strength is and
how much a given lenght deflects under load. I figure this will give whoever
takes a look at it a good starting point. Any other way of doing except by
my plan would probably be too expensive to meet my budget. Would be nice if
I found out I didn't have to do anything at all to it.
Stop guessing & get an engineer to take a look at the roof system,
There is no need to build a test specimen, that's what calcs are for
Shop around for an engineer who would be willling to design a fix (if
any fix is needed) that could be built onsite by your group.
And how do you know it is anywhere near the spec for a real engineered beam?
Asking an electrical engineer to solve a structural problem is like asking my
to put aside her knitting needles and design a new space shuttle. If you want
structural problem solved, you get a structural engineer.
A good starting point for what? You don't know what the existing roof can do so
what will knowing about box beams tell you? It is entirely possible that the
trusses have sufficient reserve capacity to support the planned roof upgrade. If
they cannot, the box beam doesn't sound like the right solution anyway.
Then sell the house. Either do it right or walk away.
Better: Start with an expert and find out what you _need_, then find out what it
cost. Then decide on whether you should do this at all. You and your buddies
don't sound like a good solution.
Obviously you dont know the capabilites of my friends or me. If we dont
know we learn. We really hate spending money for not knowing. That is the
one thing we have in common. Im sure my EE friends will let me know if they
cant handle it. They are at the point in their lives where they can learn
something new for themselves without taking a class. My dad said this point
in your life should come when you are about 12.LOL
I really thought I might get some practical info here, Something on the
order of "I've done that before" or the info I got from the local college
ths morning. Just wishful thinking I guess.
Went to test my box beam(built it this AM) at local junior college and found
my box beam plan was way overkill. While I didnt get to test it(forgot it
was Spring break), I did find out that just adding 3-1/2" OSB strips will
more than triple the strenght of the existing 2x4 trusses. Now to find out
the load of the shingles compared to the concrete tiles.
of the truss. I was thinking plywood would be even better but found the OSB
is stronger. Plywood would be more than sufficent for my purposes and has
the advantage of being easily glued to the existing truss, the OSB I am
familar with has a waxy coating on it. Is it all like this? We discussed the
possibility of actually building trusses like I suggested. In the real world
it would be easier and cheaper to build them of enginnered lumber but
thought my idea would be plausibe for stengthening an existing structure. He
also thought there was a very good chance that the existing trusses would
work "as is" considering the truss was designed to span the full width of
the house and there are a lot of internal walls that could be load bearing.
For hte most part ther is a load bearing wall that runs almost all of the
way through the center of the house . The exceptions are the den area where
the load walls would be about 1/3 the way in on each side and for about 4 ft
where the dining room on the back side of the house overlaps the livingroom
on the front ther is a 4 foot wide area where the trusses are only supported
by the outside walls. Several years ago there was a problem with the ceiling
cracking in this area. The problem was fixed by placing 2 beams across 6
ceiling joist to support the 2 ceiling joist that otherwise were unsupported
except on the ends.
You and your friends are obviously in your manic phase. I hope you come to your
senses before you collapse your house on someone. You _think_ you know
what you're doing or can figure it out yourselvest - I've got more than enough
as a structural engineer to know you don't.
You got advice but you haven't taken it. The advice that was repeated by several
posters was "get a professional to check it out." Just because that's not what
you want to hear, doesn't mean you didn't get good advice.
Actually, you've got to be careful even asking people to
perform within their nominal field, if you're in
a sub-genre they're not used to.
I took a drawing of my attic to a game session with
some of my friends that included PEs in three different
specialties, because I wanted them to check my math,
and it still took me about 90 minutes to convince
them that the bottom chord of a truncated queen-post
truss is a tension member.
Sad but true. On the other hand the in house engineer at a truss plant
can glance at a design and tell you what it will hold.
The OP could work the problem backward and ask what a truss looks like
that will handle the load and compare it to what he has.
I know they use pretty standard looking 2x4 trusses here under tile
roofs. The number and placement of the bracing will be the likely
difference, if there is any.
Are you talking about a full-length continuous piece of OSB? Where do
you find 3-1/2" x 20' strips of OSB? I know the engineered wood
companies use single pieces for building their beams. But all I have is
I guess we didn't get the memo telling us of you and your friends'
There's learning, and then there's learning. You may feel you've
mastered the first, but it's obvious you need to work on the second.
Every so often someone pops up on one of the construction newsgroups
and wants to do something similar to what you are contemplating.
Invariably they get the same advice. The curious thing is that the
replies come from many different people who are pros - engineers,
architects, experienced contractors, etc. - it's not always the same
people spouting the company line. Invariably, if the response isn't
quite so rosy as the OP hoped, the OP's response is, "What do you
know?!" I find that amusing.
Ummm, no, those strips won't triple the strength of the trusses. They
may strengthen the chords, but chord strength alone does not make a
truss. Wherever you got that information, don't go back there for
To the problem at hand:
If you're in an area that requires building permits it's a sure thing
that the building department will require sealed drawings. Since you
have some engineer buddies, that shouldn't be a problem. I'm sure your
buddies are up on the major code revisions that were implemented in the
last few years, and if not, are capable of coming up to speed. I'm
sure that they'll also realize that it's not the member strength but
the connections that will be the Achilles heel of your modified truss
design, and that adding intermediate supports to a truss that was not
designed for them can create localized stresses that are beyond the
capabilities of the original design. A quick way to make your attic
ground floor accessable.
Everyone approaches such remodeling in different ways. I'd have to see
the roof structure to make up my mind for sure, but from my
understanding of your description, I'd be more likely to wait until the
roof is ripped and add new trusses at that time. You'd have all of the
roof work done in a day or two, as opposed to many days or weeks, you'd
have a designed solution, and you'd have new sheathing. I'm sure your
buddies pointed out that the sheathing will almost certainly have to be
replaced as it will be undersized for the new load. Smart people keep
up with current events, and a couple of EEs and a PE will have
commented on the major changes in construction techniques with respect
to wind loads, uplift and required metal connectors due to recent
hurricanes. These things are a pain to deal with while you're lying
flat on your stomach reaching into the eaves, but a snap when the
sheathing is removed.
I think it's great that you want to do the improvement, and that you
would like to learn how to do it yourself. These are admirable traits.
Another admirable trait is to not dismiss the people you have asked to
help you. It's kind of like insulting the waiter before your food
arrives - not a good idea.
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