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Hardie and rain screen details

canada_deck | Posted in General Questions on
I’m working out the trim and siding details on my project and would appreciate any thoughts on these details. It’s an insulated out-building.  Tyvek as the WRB. 3/4″ PT strips as the rain screen. Perforated aluminum J channel as the bug screen (top and bottom – ventilated rain screen.) Hardie plank siding and hardie trim.

Trim details:

1) Waterboard and rain screen bottom location.
The shed is on concrete footings over top of a concrete slab.  The shortest footing is about 3″ off the ground.  The rim joist is pressure treated.  
Current plan:
I will extend the furring strips all the way to the bottom of the rim joist (there will be a rain screen behind the water board.)  The bug screen will be at the very bottom of the assembly.  Flashing over the waterboard will only go back to the outside surface of the furring strips.
Other option:
I could attach the waterboard (5/4 hardie trim) directly to the rim joist (no rain screen behind rim board.)  Above the waterboard, there would be flashing that goes all the way back to the wall. Above that, there will be a 1/4″ gap and then the bug screen and bottoms of the furring strips.
The thing that bothers me about this alternate plan is that it means that the water board will be inset vs the corner trim and siding. On the other hand, I do like how the bug screen would be up higher and behind a 1/4″ gap instead of being 3/4″ wde and just inches off the ground.
2) Electric light mounting blocks
I’ll need two mounting blocks that are each about 7″*10″
Current plan: I have a strong preference for a large sturdy octagon box (1.5″ deep.)  So I was going to build my trim block as follows:
Layer 1: 3/4″ plank that is 7″ wide.  I will drill a ~4″ hole in the middle to hold the octagon box.  The WRB will be cut around all sides of the box.  On the sides, I will use flashing tape that will run up the 3/4″ edge of the plank.  There will be metal flashing on the top that goes all the way back to the sheathing.
Layer 2: 5/4 (1″) thick hardie will then be screwed on top.  It will also have a ~4″ hole through the middle.
Other option:
The other option is to use a small pancake box in the 5/4 hardie and mount it on furring strips with only a small hole in the WRB for the electrical wire.  
3) Door
Due to some poor planning, the door is inset about 1/2″ into the wall (it is not flush with the outside sheathing.  It’s an awkward distance. Anything I try to do with wood or hardie will be frail because it will be a piece (or lip that is only about 1/2″ * 1/2″.)  I was thinking I might just fill the gap with caulk but it seems a bit too large for that.  My latest plan is to use some tile trim between the sheathing and the door jamb to cover that 1/2″ gap.  I will then trim out the door using a sandwich of 4/4 and 5/4 hardie trim boards to get the 1 3/4″ depth.
4) Screws vs nails
I will use siding nails for all the siding but I don’t have a 16 guage nailer.  I am planning on using trim screws instead.

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  1. Expert Member


    I use option #1 for water tables. If you go with #2 I would increase the gap from 1/4" to at least a half. I don't think there is much point building a fully ventilated rain-screen and then restricting the intake.

    Your electrical block will work. Did you consider buying a pre-made one from the big box store?

    Why not glue a piece of matching stock to the door to make up the width? Hold it in place with brads or pin-nails until it sets up. After sanding and priming you shouldn't be able to tell it was added.

    Trim screws are fine, just a lot slower.

    1. canada_deck | | #2

      Thanks. Re the water table - that sounds good. For reference, this is where it will go. I also like your idea re the door.

      Re the electrical block - I looked at the sturdimount but I don't think it is deep enough and I don't think I can quickly source an Arlington box. I will have a bunch of extra 5/4 Hardie so I figured I'd build it from scratch.

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