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.

On 10-Apr-2006, " wrote:

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

If the stuff you build ends up stiffer than the existing trusses, they will take most
fo the load. If it is less stiff, it will not take load until the existing trusses have
deflected somewhat. If your upgrade isn't matched to the existing trusses, you may
end up not improving the situation at all.

When the roofer told you to sister the existing trusses, he should have said to make
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.

Mike

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?

yeah, call a truss company. they have engineers who are used to roof
trusses.

Of course, this is an opportune time to consider pulling the whole
roof, trusses included, and putting in a cathedral ceiling, or another
room, etc.

Dave

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.

"kevin" wrote in message
oups.com...
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.

wrote:
"kevin" wrote in message
oups.com...
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.

cheers
Bob

On 11-Apr-2006, " wrote:

Actually we have made I beams with OSB and 2X4s for a boat house.

And how do you know it is anywhere near the spec for a real engineered beam?

a pretty savy group with a couple of EEs in the group

Asking an electrical engineer to solve a structural problem is like asking my mother
to put aside her knitting needles and design a new space shuttle. If you want a
structural problem solved, you get a structural engineer.

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.

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 existing
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.

Any other way of doing except by
my plan would probably be too expensive to meet my budget.

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 will
cost. Then decide on whether you should do this at all. You and your buddies
don't sound like a good solution.

Mike

"Michael Daly" wrote in message
...

On 11-Apr-2006, "
wrote:

Actually we have made I beams with OSB and 2X4s for a boat house.

And how do you know it is anywhere near the spec for a real engineered
beam?

a pretty savy group with a couple of EEs in the group

Asking an electrical engineer to solve a structural problem is like asking
my mother
to put aside her knitting needles and design a new space shuttle. If
you want a
structural problem solved, you get a structural engineer.

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.

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 existing
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.

Any other way of doing except by
my plan would probably be too expensive to meet my budget.

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 will
cost. Then decide on whether you should do this at all. You and your
buddies
don't sound like a good solution.

Mike

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.

kevin wrote:
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.

IF you understand the principles around which beams are designed
it is trivial to make a homemade beam that is better than
a pre-manufactured I-beam. Instead of an OSB web, join
the flanges with plywood sides instead of a 2x3 flange, use a
2x4, instead of #2 SPF, use #1 hem-fir. Viola, a better beam.
It will cost you more in both materials and labor, but it will be
better.

To be cost-competative the manufacturers
have to make their product as cheaply as possible while still (barely)
meeting code. They are so good at this, that the material cost of
homemade beams will almost certainly exceed the cost of pre-
manufactured beams of the same stiffness.

OTOH if you do not understand the relevant principles it is easy to
design beams that are heavier and not as stiff as the pre-manufactured
ones.

For the existing roof, it would be trivial to double the strength and
stiffness by sistering a new truss next to each old one, exactly
duplicating the dimensions and materials of the old. This is
where there may be an advantage to the do-it-yourself
approach as it will not be necessary to take the whole roof
down and rebuild it from scratch.

If that is not good enough, then probably the thing to do is to replace
the old trusses with new, designed to carry the new load. Here the
problem will not be with the roof, which OP is going to redo anyhow,
but the interior ceiling which is likely to be slung from joists that
are integral to the trusses.

Either way, the local building codes will probably require a
PE to sign off on the design. Pre-fab trusses will usually be
pre-approved having already been designed by an engineer, as
noted above.

--

FF