The most common type renders the attic unusable for anything, and very hard to work in.

They are much less expansive than a framed roof, and given the few carpenters that can even frame any thing more complicated than the simplest roofs can be a bargain.

We have a very complicated roof line in the back of our house (the front is a simple gable) which is part of the reason the trusses are appealing. We will have a two story house with walk in storage on the second level and an unfinished basement so basically the air handler will be up there, accessible by pull down stairs in a closet.

There will be one area above master bedroom that will also have attic walk in storage . . . Not sure we really need it but it is on the plans. Our builder seemed to think they could design the trusses in that area to have more headroom?? Or something like it?

I got the feeling the savings was substantial but didn't get dollars and cents.

we have them...it does make things quick and easy for framing, but I hadn't realized how painful it would be for future, since the architectural designer also put one of the furnaces up there! If you do go with trusses, I'd definitely not put anything else in the attic. We had some major issues with the trusses and the furnace space...

the biggest and only real disadvantage is open spaces within the attic. There are attic trusses and bonus room trusses available to put livable space up there, but the rooms are usually only about half the width of the truss span. This a crane, they can have the trusses set in a day for most houses, where as framing the roof can take much more time. Material wise they will cost slightly more then 2x10-12 framing in most cases, however labor is a fraction of the time, which is where the savings is. If you do not need space up there, trusses are a great way to go.

I was curious about this after you asked, so I asked my builder this question today. He said that trusses were very strong but that they were "kind of up and down." He meant that there were slight variations that caused the roof decking and the shingles to undulate as they moved from truss to truss. He stated that the first time he walked up to a house, he could tell if it was built with trusses.

The only disadvantage I see is the possibility of Truss Uplift that can disrupt your drywall/plaster in the center of your house. It could pull your walls up as well, but I think the more likely problem is having gaping of crown mould or cracked plaster. I couldn't get agreement about whether this is likely to happen, but there are brackets that are supposed to be used to attach the top plate of your interior walls to the trusses so that the truss is able to slide up and down. As load is added to the roof, the center of the truss will rise, not fall.

We used trusses, and it is substantially cheaper. Remarkable how little the trusses can cost, and you can have tray ceilings or cathedral ceilings build into the trusses.

Storage trusses allow attic space to be used for storage.

Truss uplift only occurs in cold climates because of the heat differential between the bottom of the truss buried in insulation and the top exposed to the cold attic air.

The biggest disadvantage I have found is that the house must be built precisely as designed otherwise the trusses don't fit. (Sometimes I see improvements that can be made after construction has started. With trusses, you're stymied. That's because there is a long lead time on building the trusses.)

Buckhead - that is interesting. My builder said the company he uses is out of Atlanta and they have a higher end division and a tract home division. I don't know what the difference is. I do know that he mentioned using thicker OSB for the roof deck than is generally used - maybe to prevent that? The two advantages I have heard is that the material is protected from weather - trusses are made in a climate controlled factory so you don't have the issues with building a roof of lumber that has been exposed to moisture, etc., and also that they are designed by engineers rather than carpenters which is an advantage with complex roof lines.

I haven't heard of truss uplift - will have to figure out about that!

"Truss uplift only occurs in cold climates because of the heat differential between the bottom of the truss buried in insulation and the top exposed to the cold attic air. "

Wood does not change dimension with temperature (though it does with moisture content).

Uplift is caused by the lower cord not bending stiff enough to resist upwards bending from the loads placed on each end.

It is a really a design issue since uneven roof loading was not accounted for correctly.

It is less common in Fink trusses, but still happens.
It is rather common in truss designs with open space in the middle since the down loads are moved to the ends of the lower cord (it needs to be beefed up in anticipation of the loads and that they may not be uniform on the roof).

Wood does not change dimension with temperature (though it does with moisture content).

Technically correct, except the two are inseparably linked.

"While the bottom chord is warm and is drying out, the top chords are doing just the opposite. The cold winter air has very high relative humidity. The top chords absorb moisture from the air causing them to elongate." (emphasis added) CarsonDulop, Consulting Engineers.

designed by engineers rather than carpenters which is an advantage with complex roof lines.

A house structure should be reviewed by an engineer or Code officials.

Material wise they will cost slightly more then 2x10-12 framing in most cases, however labor is a fraction of the time, which is where the savings is.

Cost savings are the only reason I have gone with trusses. For a 3,500 sf house, the material cost was $5,000 less than stick-built. No difference in labour costs. Of course, this may be different elsewhere.

If structure were a selling point, I'd do stick-built every time.

As brickeyee points out above, the shortage of true carpenters makes trusses easier for complicated roof designs. One carpentry contractor I used apparently averaged one real carpenter for every three crews. He would go around setting them to work every day and doing complicated cuts himself. When he was off elsewhere, my job would grind to a halt with one apprentice at a snail's pace while other trades snickered behind their gloves.

Here is a link that might be useful: Truss Uplift

Our custom builder doesn't use roof trusses, they only do stick built roofs. I will ask them their reasoning and post what they say! We do have steep gables, but don't need the storage space in the attic as we have walk-in storage over the garage.

"cold winter air has very high relative humidity"

Really?

Cold air tends to have a high RH?

Better tell the weatherman.

The shrinkage is not lengthwise (parallel to the grain) with moisture content, it is across the grain.

The change in cord length is minuscule, and one of the advantages of trusses is that the members are not that wide.

Whomever wrote that does not know about wood movement with moisture content.
Some remedial reading is needed.

Here is a link that might be useful: Wood Handbook, Chapter 3

worthy it seems they built labor costs into the material quote maybe? This is the first I have ever heard of no labor cost differences between the 2. I just got a quote back on a house package where the truss package was $3843 and wood for 2x10 framing and center double ply LVL ridge beams was $3120. Labor for trusses setting was 2500 (including crane). 1 day setting time. Stick frame was 4 day framing, and $4500.

From http://www.woodweb.com/Resources/wood_eng_handbook/Ch03.pdf
Page 3-8.

"Longitudinal shrinkage of wood (shrinkage parallel to the grain) is generally quite small. Average values for shrinkage from green to ovendry are between 0.1% and 0.2% for most species of wood. However, certain types of wood exhibit excessive longitudinal shrinkage, and these should be avoided in uses where longitudinal stability is important.
Reaction wood, whether compression wood in softwoods or tension wood in hardwoods, tends to shrink excessively parallel to the grain. Wood from near the center of trees (juvenile wood) of some species also shrinks excessively lengthwise. Reaction wood and juvenile wood can shrink 2% from green to ovendry."

A 0.1 to 0.2% change in length cannot account for the multiple inches of truss uplift, and that is from green in the tree to ovendry.

The movement of kiln dried wood wood is far less since it never gets that wet..

Any explanation that is relying on changes in the length of the cords cannot be correct.

Ceilings would be moving up and down like crazy from winter to summer as the RH in the house varied.
The result would be that the fasteners holding the drywall would be torn out of the drywall.

Joist would be getting longer an shorter, siding would pop off the house.

It is an idea so dumb it is embarrassing to have someone with PE after their name make the claim.

brickeyee

Your point about RH of winter air seems well taken. What was missing from the Carson Dunlop explanation, I think, is that attic air in a home is often more humid than air outside the attic due to leakage from the conditioned to unconditioned space.

Here's how another P.E. describes truss uplift:

"A problem that can occur with this system causes cracks in wallboard over time. In recent years, attic insulation usage has increased, which ends up covering the bottom chord of the truss. This tends to keep the bottom chords warm and dry while the top chords absorb moisture and expand. The top cord expansion causes the top chords to lift, pulling up the bottom chord as shown in Figure 1, hence the term truss uplift. " Charles C. Roberts, Jr., Ph. D., P.E.

My untrained guess would be that the lengthwise shrinkage of grain does not relate to the length of the wood but to which portion of the tree the wood was cut from.

Here's a longer explanation:

"Truss uplift is caused by wood's natural response to moisture exposure. This response occurs when humidity changes differentially in two areas (an attic floor and attic ceiling, for example). A truss's top chords expand as their moisture content increases.

Problems resulting from truss uplift are most common with flat-bottomed trusses in homes during winter when a truss's bottom chords, often covered by insulation, are kept warm and dry while its top chords are exposed to moisture from condensation. During winter months, warm, humid air emanates from buildings' interior use and occupants. The uplift movement occurs because, as the truss's upper components resize themselves under the differential moisture conditions, internal tensile stress occurs in the truss's web members, which then pulls up the bottom chords (and the attached ceiling drywall), resulting in a separation along the wall-to-ceiling juncture of the drywall.

To put this into perspective, consider a truss with top chords made of two members, each measuring 30 feet in length. The total movement anticipated from winter swelling caused by the truss's moisture content increasing is a 0.72-inch expansion per side, or a 1-inch total rise of ridge above the bottom chords. And once a crack forms inside a home, additional moisture generated by occupants and their activities can escape into the attic, travel to the cold attic ceiling surface, condense on the truss's top chords and cause further truss uplift. " Lucas J. Hamilton manager of building science applications, CertainTeed Corp.'s Insulation Group

For interest, I will forward this thread with a request for comments to Carson Dunlop.

lzerarc
As I said, different places, different practices. I've changed only reluctantly. Seeing those spindly stuck-together 2x2s gives me a visceral reaction and I have to rationalize using roof trusses. However, my engineer says they're much better done now than ever and with wall lengths less than 50 feet uplift is rare. None of this applies to warm climate homes.

brickeyee
Uplift is caused by the lower cord not bending stiff enough to resist upwards bending from the loads placed on each end.

It is a really a design issue since uneven roof loading was not accounted for correctly.

So all the engineers are wrong about moisture? They just don't know how to design correctly for roof loads?

The commonly accepted explanation for why a truss bottom chord rises in the winter has always seemed contradictory to me and I doubt the truth of it has ever actually been tested.

But until someone solves the problem it will affect a small percentage of trussed roofs so it must be addressed in the construction detailing. That's all the OP needs to know.

Maybe someone should start a thread that addresses the mysterious truss issue.

For me, the savings of trusses is more than offset by the inflexibility for later renovation. When I first discuss a house renovation I ask if the house has brick cladding or roof trusses. If it does I warn the owners that their options will be limited and the cost will be great. There was a time when people moved from one house to another when their needs changed but the recession and tax changes have made that less attractive. There was a time when I would not take on a project with brick or trusses but that has changed too.

"So all the engineers are wrong about moisture?"

If they are ascribing change in length of wood with moisture content they are indeed wrong.

Wood does NOT change in length with moisture content to ANY significant degree.

This is form the Wood Enginering Handbook, USDA, p. 3-8

"Longitudinal shrinkage of wood (shrinkage parallel to the grain) is generally quite small. Average values for shrinkage from green to ovendry are between 0.1% and 0.2% for most species of wood."

That is simply not enough movement to matter.

A 10 foot board will change length by 0.012 inches to 0.024 inches from wet in the tree as cut to oven dry.
Movement in use is way smaller since the wood never gets to fiber saturation (around 30%).

Whomever is claiming boards change in length need to actually read the Wood Engineering Handbook (especially chapter 3) and understand HOW wood changes shape and size with moisture content. It is not anything new.

That is simply not enough movement to cause anything like truss lift.
The OTHER dimension of boards vary (sometimes by up to 10-15%) but the members are not wide enough to accumulate anything approaching an inch of movement.

There must be another reason.

"They just don't know how to design correctly for roof loads?"

The ones I saw failed to account for uneven roof loading.
They designed for a uniform load over the entire upper cord of the truss.
While that may be one example of a 'worst case,' a snow load on only half the roof is going to produce some interesting movement not accounted for by a uniform load.

Since we know the middle of the lower cord is rising, downward loads at each of its ends (especially past the bearing point of the wall) would be a likely place to start looking.

It would not be the first time 'standard methods' have been incorrectly applied.

And by the way, I am a PE.

Here is a link that might be useful: Wood Handbook, Chapter 3

The only trusses I ever installed that came with a note explaining the dire consequences of "uplift" were studio (so-called) trusses. They were flat-ceiling on one half of the span, and scissor trusses on the other half. Perhaps this style of truss had an extreme case of seasonal change, because of non-symmetry, and no continuous bottom chord. The engineering note called upon us to use a style of hurricane tie that had elongated holes to account for movement, to which we responded "h3II no" and nailed them securely. I visited that house for about three years afterward, and there were no issues with those ceilings other than some nail pops, though not in excess.
Casey

"Perhaps this style of truss had an extreme case of seasonal change, because of non-symmetry, and no continuous bottom chord."

The lack of a continuous bottom chord it a lot more likely to cause a problem than anything else.

the idea that wood changes length with water content and create "seasonal change" is just wrong.

0.01% to 0.02% dimension changes are just NOT significant.

It is far more likely that the wood is simply lower grade and warping like crazy.
The thin sections used in truss design while saving material are more prone to warping.
The wood grading criteria do not take warping into account, they just look for knot defects and other defects in the wood itself.

the fast growth timber available now does not help.

how many times have you seen a stud not restrained by surrounding framing turn into a hockey stick as its moisture content changed?

Even in conditioned space the wood can continue drying and warp badly. The normal drying levels are NOT the final levels the wood will be at in conditioned space (like in walls).

Trying to get a true level 5 finish on studs that have moved is a real PITA.
I have had to send crews with lasers and hand power planers in to make studs co-planer and flat before applying drywall.

Nothing is worse than getting to almost the end of the job and having a huge bulge or hollow show in what is supposed to be a level 5 wall.

The next time someone demands this type of finish I am going to tell them that steel studs MUST be used.