Internal loading on roof trusses

[Why Not Indeed]

Fink/"W" trusses on ~500mm centres. Old ply flat roof about 350mm below. The bottom chords are 34x95mm section and about 8m long:

How much load can the bottom chords take as-is? The intention is to board it out and use it for storage.
If the answer is "they won't even support the weight of some boarding" or similar, I assume I'll be able to put some props between the bottom chords and the flat roof to increase the maximum loading. I am only really interested in loading the central "A" as the outer sides will be awkward to get to.

 

[Leofric]

Assuming this is all existing the trusses will not have been designed to take additional loading Any additional loading on the flat roof will depend on the size , spacing and span of the joists . As usual , building regulations approval with structural engineer's calculations are required for structural alterations to a house.
ps what is the building?

 

[Why Not Indeed]

The sketch is how it stands now. To be clear, this is just a view of the top 1/3rd of the house; ground floor and 1st floor aren't shown.
The building is a house.
Ceiling joists are 220x50mm on roughly 350mm centres. There is a single-skin internal wall running E-W [omitted for clarity]. To the north the joist span is a clear 3600mm, to the south 5600mm with one stud wall perpendicular to the joists.
Same pattern of joists on the first floor.

 

[tony1851]

Trusses are generally designed to take a live load for storage of 0.25kN/m².
Propping a truss at the 1/3 span node points is not a good idea because it will induce stresses in the joints and members which they are not designed to take.

 

[Leofric]

Trusses are generally designed to take a live load for storage of 0.25kN/m².
Propping a truss at the 1/3 span node points is not a good idea because it will induce stresses in the joints and members which they are not designed to take.

Truss members are connected at 1/3 span node points by gang nailed plates , if support is added at these points won't it reduce the stress in the plate connections created by the new floor loading and , if support is added at mid span of the horizontal ties wouldn't this reduce the bending stress in these members created by the loft floor loading ?
regards

 

[tony1851]

@Leofric - I fully understand where you're coming from, and intuitively, adding supports at the node points on the bottom chord would suggest it improves matters.
But structurally, doing this will induce different forces in the members and joints, depending on how much load is added. For example, some members which are designed to be in tension (eg the bottom chord) will be subject to compression, for which the member may not necessarily be designed, bearing in mind that they are only 34mm thick (though if the boarding is firmly nailed down, that will help to keep the bottom chord stable and prevent buckling).

Trusses are designed on computer programs and adding supports will invalidate the calculations provided, which will invalidate the warranty. However, I would have to hold my hands up and say that, as long as the OP doesn't use the new floor to support his lathe, milling machine and anvil for his metalwork hobby, the floor will almost certainly be fine with the extra supports.

 

[Why Not Indeed]

How about, as well as props, a timber below the bottom chord to reduce the rotation on it:

The base of the "W" is a hair over 2400mm and as that's a common timber length then this might work in my favour. I don't want to go too nuts with the propping as I intend to lay fibreglass insulation on the "flat roof". I am assuming the rubber layer on top of the flat roof will provide a vapour barrier.

 

[tony1851]

I think a beam would be superfluous.

Here's the reasoning:

1. With the truss as it is, the bottom chord will be in tension. You know this by intuition because if you could pry off the gang nail plates in the middle, the two halves of the truss would tend to move outwards away from each other. As it is, the bottom 34 x 95 chord will be fine to take pure tension.

2. Putting a prop under each node will tend to cause a reversal of stress in the middle third of the bottom chord once it's loaded and - like a beam - the upper fibres will be under compression rather than tension. As the span is 2400 and the width of the member only 34mm, there will be a tendency for it to buckle or twist sideways. The best way to prevent this is to firmly hold the top of the chord in position so that it can't try to move out-of-plane, which is most simply achieved by fixing boards across the tops.

3. So if you prop the trusses directly over the flat-roof joists which are nearest the nodes, and fix plywood or OSB boards along the tops of the chords (suggest screwing into pilot holes rather than risking splitting by nailing as they are very thin) then that will hold the chords firmly.
(And be sensible with the loading!)

 

[Why Not Indeed]

Won't be nailing anything, screwing all the way.
I suspect getting sheet material up there is going to be a bit of a trial so I was consider T&G floorboards or the smaller "loft panels". Does it need to be as "decent" as OSB or ply? I suspect the answer is going to be Yes...

 

[Why Not Indeed]

I've [finally] finished one end of the loft and am pleased with the result. No squeaks, creaks or obvious flexing when walking on it [feels more solid than, say, my son's bedroom floor, but that'll be another thread some day]. Couldn't have hoped for better. Thanks to all who offered advice.