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I need to build up my attic framing for three reasons, but first, I can think of two ways in which to do this.

The Methods

Method 1

Put new framing on top of the original framing along the existing joists. Parallel Joists

I would be stacking the new joists directly on top of the old joists. In theory, this should make them similar to 2x8s (I know this isn't quite true, and I have the beam theory to prove it).

Method 2

Put new framing on top of the original framing against the existing joists. Perpendicular Joists

I would be stacking the new joists across the tops of the old joists. In theory, I've made a lattice structure of sorts.

The Uses

Insulation

I am going to lay 2x4 fiberglass insulation in the original joist bays, but that only gets me ~R-13, and the guideline for my region is >R-30. Blown-in is not an option I am considering, by the way. I've heard that laying rolls/batts perpendicular to each other when stacking them is the appropriate method, so method 2 seems to make sense for this application.

Flooring

Right now, when I need to do work in my attic I need to tiptoe along the tops of the joists. There is no flooring of any kind. I want to fix that situation by laying down some OSB (in some places, at least). I don't think the method I use will affect this, but there is one double-wide joist bay (there's a wall under it but no joist) so I need to add some perpendicular reinforcement there anyway, so method 2 seems to make sense again.

Reinforcement

I am adding some cabinets that need to hang from the ceiling as sit quite far from any walls. I expect the cabinets and contents to weigh ~400lbs, and I don't doubt the existing joists could take the load (they have a 160" span), but I'd rather reinforce the framing. I had considered sistering joists where the cabinets would be mounted (method 0, not shown), but I think methods 1 and 2 effectively supersede sistering. Method 1 will increase the effective joist stiffness and strength while method 2 will distribute the load across multiple joists. I'm not sure which is better in this case.

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  • It doesn't sound like you're using the space for storage. I would be adding some cross-members for your cabinets and whatever the wide joist bay needs and stopping there. Adding an entire floor system seems like vast overkill, and it makes insulating tougher. – isherwood Jan 22 '18 at 21:39
  • @isherwood I am using it for storage, but it is going to be quite minimal. My guess is that 20% of the attic floor area will have a storage load of 1-2psf. Why would framing the entire area make insulating more difficult? It seems like everything should just roll into the bays... – Hari Ganti Jan 22 '18 at 21:46
  • I assume that you have truss webs or W bracing. Adding lumber makes things more complicated, and your R-value is reduced as well. – isherwood Jan 22 '18 at 21:52
  • Assume I have what now? The roof trusses are literally triangles. There are no other truss elements. I do understand that the R-value of wood is pretty crappy, but I'm ok with a slight reduction in insulation for better structure, at least where needed. I do see your point that it isn't needed everywhere. – Hari Ganti Jan 23 '18 at 19:46
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    Fair enough, but the fact that there are no truss webs raises concern about the extra weight you're adding. I can't offer specific advice on that from here. Use good judgement. – isherwood Jan 23 '18 at 19:48
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Method 1 and Method 2 do NOT strengthen the existing joist. They merely add “dead weight”.

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    Method 1, assuming the new joist can't slip off sideways but can slip longitudinally relative to the old joist, doubles both the bending and shear capacity; it doesn't improve the end bearing capacity. If Method 1 is carried out in such a way that longitudinal slip is prevented it quadruples the bending capacity. Agreed that Method 2 doesn't strengthen, and merely redistributes local loads. Increased dead weight is a good point though. – AndyT Jan 24 '18 at 15:14
  • @AndyT In Method 1, how does sitting one joist on top of another joist “doubles both the bending and shear”? (I assume you mean vertical shear, not horizontal shear.) you’re not increasing the “working section modulus”, unless you connect the two joist together, like a glu-lam. – Lee Sam Jan 25 '18 at 8:43
  • Take a simply supported beam (say a 30cm ruler spanning between two books). Put a weight in the middle (say your mug of coffee). Measure the deflection. Take the mug off and add another ruler on the top of the first one. Put your mug back on. Measure the deflection - its half. This is because when you put the mug on the top ruler, it causes the top ruler to bend. But there is something stopping the top ruler from bending: the bottom ruler. So the top ruler pushes the bottom ruler to bend too. – AndyT Jan 25 '18 at 9:21
  • Given that the two rulers have equal vertical deflection, (and assuming they were identical rulers) we can see that they have the same stress and are carrying the same load. The load carried by each ruler is therefore half the weight of the mug. – AndyT Jan 25 '18 at 9:23
  • PS Yes, I meant vertical shear. I don't know of many people who put horizontal loading on their attic floors. ;) – AndyT Jan 25 '18 at 9:24

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