Stregthen trusses

[jimmiedee]

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.

[gfretwell]

They use 2x4 trusses under concrete tiles too. You don't need the advice of a roofer, you need an engineer. It might be fine just the way it is.

[Michael Daly]

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?

[tbasc]

My reaction too. Have an engineer analyse the existing trusses. or 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. TB

[marson]

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

[spamTHISbrp]

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

[kevin]

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.

[jimmiedee]

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.

[BobK207]

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

[Michael Daly]

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 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.

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.

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

[jimmiedee]

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.

[BobK207]

"> > to put aside her knitting needles and design a new space shuttle. If

"found my box beam plan was way overkill"

"I did find out that just adding 3-1/2" OSB strips will more than triple the strenght of the existing 2x4 trusses. "

Really?

cheers Bob

[Michael Daly]

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 years 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.

Mike

[Banty]

EE = Electrical Engineering, right?

If you had appendicitis, would you hand a medical book and a scapel to one of your EE buddies?

Banty (designs computer chips, gets experts to deal with my house)

[Goedjn]

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.

[gfretwell]

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.

[Banty]

OK - I'll take your word for it ;)

Banty (metallurgist by training)

[kevin]

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

8' lengths.

[jimmiedee]

Yes, I should have been more clear, That is a piece of 1/2" OSB on each side 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.

[fredfighter]

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.