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I have a (I believe it's called) "W" frame loft:

W-Frame Loft W-Frame Loft

(sorry about the quality of the pictures - maybe I should ask in the photography Stack Exchange how to take such pictures)

There are no floorboards, just ceiling joists, but I had an idea that I could create some additional storage by lashing some lengths of chipboard between the frames (making an upside-down A on the two arms of the W).

My question is: could this affect either the structural integrity or the airflow in the loft; or, generally, are there any reasons this should not be done? Further, if there are no reasons to not do it, then are there any considerations to bear in mind (for example, it occurred to me that I shouldn't screw anything in place to allow the beams to expand and contract).

EDIT:

Following a request in the comments for more detail, I have (badly) sketched what I'm trying to achieve. This illustrates the loft / attic space currently:

Loft1

And this illustrates what I'm contemplating doing:

Loft2

  • With regards to the pictures : Your white balance is off , contrast is really wrong on the left. Given the quality of the pictures and not sure if I am looking sideways or [ down the middle - assumed] you will be changing the airflow dynamic - structurally there would be no harm that I am aware of. – Ken Dec 13 '17 at 15:36
  • Is that a loft (a space intended for use by humans) or is that your attic? – isherwood Dec 13 '17 at 17:29
  • Just for storage - no humans will be living there :-) – Paul Michaels Dec 13 '17 at 17:47
  • That didn't really answer my question. Calling it a loft implies that it was designed for foot traffic, furniture, etc. Is this simply your roof attic? – isherwood Dec 13 '17 at 21:07
  • Yes. It doesn’t have floorboards, and isn’t really tall enough for anyone to even stand in. It was not designed for foot traffic. – Paul Michaels Dec 13 '17 at 23:24
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+50

Trusses are engineered to support loads that are applied in certain ways. It would often not be practical to make a truss support all of its required loads without it also being able to support some additional load applied in other places, but it's generally unwise to expect trusses to support much additional loading beyond their own weight. For example, a truss which intended only to hold up a roof above it (and not a ceiling below) might well use a thinner bottom board than would be required to support a ceiling, and one which is intended only to support a ceiling might use a thinner board than one which is designed to support both a ceiling and a storage area.

Adding loads to a truss which are small relative to the weight of the truss itself (e.g. an attic light fixture) should be safe, but adding loads beyond that could be dangerous. Note that applying a load in the middle of a truss member will create stress on that member proportional to the ratio of its length to its thickness. If a truss has an 8' long 2x4 board with nothing attached to the middle of it, each pound of loading to the middle may increase the stress on some parts of the board by more than twenty pounds. It's not hard to design trusses to withstand interior loading, but such trusses would cost more than those that aren't designed for such use.

  • Based on this, am I right in thinking that the stress you mention would be quartered if the shelve was spread across 4 trusses (Therefore 1 lb of weight would add 4 - 6 lb of stress)? When you say stress, what does that actually mean - are you talking about weight which would cause the wood to buckle over time? Also, are there any methods of shoring up the trusses - for example, some kind of metal support ? – Paul Michaels Dec 21 '17 at 17:53
  • @pm_2: My description is somewhat over-simplified, but the primary stresses would be longitudinal tension and compression. Wood is strong in both those modes, but trusses use pieces of wood which are relatively thin for the amount of stress they have to handle. If you put a shelf across four trusses and someone places a heavy object near one end of it, most of the weight will be concentrated on one truss. You might know that you're intending the shelf to store only inflated party balloons (which would be unlikely to overload the trusses) but that doesn't mean the next homeowner won't... – supercat Dec 21 '17 at 18:40
  • ...think that one end of the shelf wouldn't be a great place to store his large collection of lead bricks. – supercat Dec 21 '17 at 18:41
  • In that case, would screwing some form of metal support to the wood help to strengthen it? – Paul Michaels Dec 21 '17 at 19:24
  • @pm_2: It probably would, but it's difficult to quantify by how much. I'm not sure how one could add such supports in a way that would not leave an inspector wondering if it was adequate. – supercat Dec 21 '17 at 23:27
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Fastening flooring to the ceiling joists is okay. (This is what would be done if you were going to add a second floor anyway.) The roof over the loft is built with trusses, which you show in your pictures. DO NOT CUT THE TRUSSES. I cannot stress that enough. Cutting the trusses will definitely effect the structural integrity of your home/roof.

Adding items for storage in the loft space should be okay as well, as long as it's not a bunch of extremely heavy objects all resting on one or two of the trusses. But for some storage boxes or whatever you need, it should be fine. (To put it in perspective, if you wanted to install a furnace in that attic space, you would be able to rest the weight of the furnace on a couple of those trusses.)

Hope that helps!

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    If a person was adding a second floor these roof trusses would be removed and replaced with joists designed to carry floor loads. I have concerns about using attic space for storage. Drywall damage and eventual ceiling joist warpage come to mind. – isherwood Dec 13 '17 at 17:35
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Very dangerous...I wouldn’t do it. You’re putting “unrecognized loads” on the chords (and connections) that were not designed into the design of the truss.

Those chords and connectors are designed to carry specific loads. If one chord fails, the entire truss fails.

By the way, if you’re putting books up there, the Code lists books as one of the heaviest loads to design for on a per square foot basis.

Also, don’t attach anything to the side of a truss. That will cause it to buckle.

  • Not "very dangerous": millions of garages have random objects stashed in the roof structure. The key is moderation. – Spencer Joplin Dec 17 '17 at 23:18
  • @SpencerJoplin You’re putting the web chords in “double bending”. If you know how much stress those web chords can take, then, yes, add that additional stress. – Lee Sam Dec 18 '17 at 0:36
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It should not be a problem to add some floor board. It can be secured if you like. Just don't cut any of the existing support members.

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    I disagree. Roof trusses not engineered for floor loads don't have the bottom chords or gusset size necessary to prevent damage. – isherwood Dec 13 '17 at 17:36
  • I think there is a misunderstand of the intent of the OP. I do not think they want to add floor boards.. I believe they want to add some boards that will be cradled in the V of the truss's thereby making a shelf. This would only add downward force of gravity, (if done properly) which is what the truss is designed to do anyway, ( not that i am saying it is acceptable ) Would not a v gusset on each truss ( or a 2x4 attached horizontal between the two downward legs of each truss ) add strength, prevent lateral force and provide a surface for a shelf to sit on? – Alaska Man Dec 13 '17 at 19:13
  • In my scenario the weight on the shelf would be transferred to the ceiling joists and they may not be rated for the extra weight of the shelves and its contents. I was only trying to emphasize what i interpreted the OP's question to be. – Alaska Man Dec 13 '17 at 19:23
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    There's much more happening in that scenario than a simple downward load transfer. – isherwood Dec 13 '17 at 19:23
  • @Alaskaman: Diagonal boards within a truss are designed to withstand forces which are parallel to their length. Unless the house is at a really weird angle, the force of gravity isn't going to be parallel to the length of a truss' diagonal supports. – supercat Dec 17 '17 at 3:57

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