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Old 11-28-2011, 08:10 PM   #1
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Attic catwalk - step over HVAC duct


I am building an attic catwalk, which is NOT the problem. I'm mounting a 16" 2x4 on top of the existing 2x4 truss joists, using a 5/8 plywood gusset on one side, glued and screwed all together. Then laying 3/4 ply atop the 2x4, screwing it in with 2" deck screws. No problem.

What I'm struggling with is building a step over for the HVAC ducting. I have two parallel ducts to get over, so I have to span a full 24 inches, which is my joist spacing.

I plan to build a 2x4 frame for the step over, then put the 3/4 plywood on top of it. The question is: how do I construct the top corners of the frame? Do I over place the uprights on either side, with the piece going across (perpendicular to the joists) between them? (see attached Figure A).

Or do I place the uprights on either side and lay the cross piece alongside the upright? (attached Figure B).

(the figure omits the plywood and the 2x4 atop the joist, I just put the joist in there to show the angles)

And how should I attach them? For Figure A I was going to use Strong Tie 66L (an "L" connector), but not sure that's enough.

For Figure B I could use a number of structural screws (GRK R4: 10 x 2.5").

For the bottom of the step over frame, I'll just toenail (or screw) that into the mounted 2x4 atop the joist. I might even put a cleat there to hold it on. I'm not too worried about that part.

I'm probably stressing too much about such a minor project, but just want to make sure it's never going to fall down and crush my ducting, as well as causing someone to fall off the walkway and through the ceiling.
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File Type: pdf 2011_11_28_18_05_13.pdf (16.3 KB, 125 views)


Last edited by M3 Pete; 11-28-2011 at 08:14 PM.
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Old 11-28-2011, 08:44 PM   #2
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Attic catwalk - step over HVAC duct


The cross pieces should lay on top of the legs, rather than hang on the sides or edges.

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Old 11-29-2011, 12:06 PM   #3
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Attic catwalk - step over HVAC duct


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Originally Posted by Wildie
The cross pieces should lay on top of the legs, rather than hang on the sides or edges.
That was my initial thought since I remember an old framer telling me to always think through gravity and support everything from the bottom when possible. But I thought it might not be as stable since it is unbraced on the sides. I can put a 2x4 or even a 2x6 between the two sides to stabilize them.

But my remaining question is how to fasten the legs and cross pieces. Toe nail? Long screws like 6" TimberLok through the cross piece and into leg? Strong tie L bracket? Something else?

Since it is only 24" across, I could lay the cross pieces flat (yes I know that is not the strong way to do it, but the 3/4 plywood will pretty much hold the weight on its own over a 24" span). Then I could turn the legs 90 degrees and nail through the cross pieces into the legs. But that seems like a less stable corner Nd might push over without additional bracing. Thoughts?
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Old 11-29-2011, 12:48 PM   #4
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Attic catwalk - step over HVAC duct


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That was my initial thought since I remember an old framer telling me to always think through gravity and support everything from the bottom when possible. But I thought it might not be as stable since it is unbraced on the sides. I can put a 2x4 or even a 2x6 between the two sides to stabilize them.

But my remaining question is how to fasten the legs and cross pieces. Toe nail? Long screws like 6" TimberLok through the cross piece and into leg? Strong tie L bracket? Something else?

Since it is only 24" across, I could lay the cross pieces flat (yes I know that is not the strong way to do it, but the 3/4 plywood will pretty much hold the weight on its own over a 24" span). Then I could turn the legs 90 degrees and nail through the cross pieces into the legs. But that seems like a less stable corner Nd might push over without additional bracing. Thoughts?
If I were to do this, I would consider making a square frame from 2X4s with another 2X4 running diagonally from one corner to the other.
I would use 3" deck screws to fasten them together.
Although framing plates would work also!
The box frame would be screwed to the ceiling joists and the plywood deck screwed to the top of the frame.
Gussets could be used between the plywood deck and the box frame to prevent 'yaw' movement.
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Old 11-29-2011, 12:51 PM   #5
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Attic catwalk - step over HVAC duct


Quote:
Originally Posted by M3 Pete View Post
That was my initial thought since I remember an old framer telling me to always think through gravity and support everything from the bottom when possible. But I thought it might not be as stable since it is unbraced on the sides. I can put a 2x4 or even a 2x6 between the two sides to stabilize them.

But my remaining question is how to fasten the legs and cross pieces. Toe nail? Long screws like 6" TimberLok through the cross piece and into leg? Strong tie L bracket? Something else?

Since it is only 24" across, I could lay the cross pieces flat (yes I know that is not the strong way to do it, but the 3/4 plywood will pretty much hold the weight on its own over a 24" span). Then I could turn the legs 90 degrees and nail through the cross pieces into the legs. But that seems like a less stable corner Nd might push over without additional bracing. Thoughts?
If I were to do this, I would consider making a square frame from 2X4s with another 2X4 running diagonally from one corner to the other.
I would use 3" deck screws to fasten them together.
Although framing plates would work also!
The box frame would be screwed to the ceiling joists and the plywood deck screwed to the top of the frame.
Gussets could be used between the plywood deck and the box frame to prevent back and forward movement.
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Old 11-29-2011, 04:36 PM   #6
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Attic catwalk - step over HVAC duct


Quote:
Originally Posted by Wildie View Post
If I were to do this, I would consider making a square frame from 2X4s with another 2X4 running diagonally from one corner to the other.
I would use 3" deck screws to fasten them together.
Although framing plates would work also!
The box frame would be screwed to the ceiling joists and the plywood deck screwed to the top of the frame.
Gussets could be used between the plywood deck and the box frame to prevent back and forward movement.
Wildie, I appreciate the help.

I can't do a full box across the span because I have the two ducts running underneath, parallel to the joists. So the bottom chord of the box, as well as the diagonal, are not possible.

What about a three-sided box (two legs and a top), with triangular plywood gussets at the corners? They might even be stronger than the Strong-Tie L brackets.

Then take plywood or a 2x6 or even a 2x8 "blocking" for each end of the two parallel boxes to hold the assembly square and prevent the yaw movement.

I attached a very crude sketch of what I mean. The plywood decking and joists are omitted.
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File Type: pdf 2011_11_29_14_29_31.pdf (13.6 KB, 74 views)
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Old 11-29-2011, 06:44 PM   #7
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Attic catwalk - step over HVAC duct


Quote:
Originally Posted by M3 Pete View Post
Wildie, I appreciate the help.

I can't do a full box across the span because I have the two ducts running underneath, parallel to the joists. So the bottom chord of the box, as well as the diagonal, are not possible.

What about a three-sided box (two legs and a top), with triangular plywood gussets at the corners? They might even be stronger than the Strong-Tie L brackets.

Then take plywood or a 2x6 or even a 2x8 "blocking" for each end of the two parallel boxes to hold the assembly square and prevent the yaw movement.

I attached a very crude sketch of what I mean. The plywood decking and joists are omitted.
I assume that the catwalk is to perpendicular to the ductwork and the ceiling joists.
How high would you like the c/w to be? And how far above the ceiling joists is it to the top of the ductwork? And how much headroom do you have overhead.
If you could post a photo or at least a sketch showing all the details, it would easier to envision.
Gussets as per your sketch would work I'm sure. Gussets on the sides, between the legs and the deck would be a good idea, also!
How would you plan to fasten the legs to the ceiling joists?
By the way, don't nail into the joists, because the pounding may pop the drywall nails.
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Old 11-29-2011, 07:29 PM   #8
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Attic catwalk - step over HVAC duct


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Originally Posted by Wildie View Post
I assume that the catwalk is to perpendicular to the ductwork and the ceiling joists..
You assume correctly.

Quote:
Originally Posted by Wildie View Post
How high would you like the c/w to be? And how far above the ceiling joists is it to the top of the ductwork? And how much headroom do you have overhead..
I'd like it to be low as possible, just for weight, stability, etc., but I do have a fair amount of headroom. I think the ductwork is about 10-12 inches over the joists, since it is 8 to 9 inch duct with insulation, and sitting atop the R-19 batt ceiling insulation.


Quote:
Originally Posted by Wildie View Post
If you could post a photo or at least a sketch showing all the details, it would easier to envision..
I'll try to do that tomorrow, but I'm pushing my drafting skills as it is.

Quote:
Originally Posted by Wildie View Post
Gussets as per your sketch would work I'm sure. Gussets on the sides, between the legs and the deck would be a good idea, also!
How would you plan to fasten the legs to the ceiling joists? .
I was going put the legs atop the plywood catwalk, then toe nail them through the plywood into the joists below, and maybe use cleats on the plywood just to hold everything.

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By the way, don't nail into the joists, because the pounding may pop the drywall nails.
When I say "nail" I mean screw, for that exact reason.
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Old 11-29-2011, 08:05 PM   #9
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Attic catwalk - step over HVAC duct


I was going put the legs atop the plywood catwalk, then toe nail them through the plywood into the joists below, and maybe use cleats on the plywood just to hold everything.

I'm not following you here! I'm thinking that the legs would be underneath the plywood.
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Old 12-01-2011, 04:57 PM   #10
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Attic catwalk - step over HVAC duct


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Originally Posted by Wildie View Post
I was going put the legs atop the plywood catwalk, then toe nail them through the plywood into the joists below, and maybe use cleats on the plywood just to hold everything.

I'm not following you here! I'm thinking that the legs would be underneath the plywood.
OK, I attached a sketch, which is not even close to scale or proportion. The catwalk is 16" wide.

The 2x4 atop the joist is a joist extension to allow room for R19 insulation below the catwalk. It is attached with a single 5/8 plywood gusset along the side, glued and screwed to both the joist and extension.

The legs for the step-over have to be atop the plywood of the main catwalk so I can fasten the plywood to the joist extensions. Otherwise I'd have to hang a 2x4 or some other support off of the joist extension to fasten the main catwalk. I figure I can toe nail (screw) the legs through the plywood and into the joist extension.

I'm counting on the triangular plywood gusset at the top corner to provide the end-to-end stability. But I will probably use a 2x6 as shown on the "end view," which will provide additional support for that corner by fastening the cross piece to the leg. In fact, that's the only fastener other than the triangle gusset. 3" deck screws would not fasten the leg and cross piece alone, unless you angled them in from the side or end.

I will probably use 3 1/2" deck screws through the triangle gusset and into the 2x6 where possible. And 2" screws for the rest of the triangle.

With the plywood "blocking" shown on the "end view," the assembly will be solid from side-to-side.
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File Type: pdf 2011_12_01_14_30_02.pdf (31.3 KB, 84 views)
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Old 12-01-2011, 11:01 PM   #11
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Attic catwalk - step over HVAC duct


That sketch explains it all. One picture is worth a thousand words, its said!

So at the end of the day you will be laying the plywood sheeting onto the extended ceiling joists and constructing a bridge over the duct work.
What you have shown should work OK!
Another option for the bridge would be make a plywood box to form the bridge. U shaped sections, cut to accommodate the ducts would be cut in the two sides.

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