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Solar Dave 09-28-2008 01:42 PM

Best caulk for Hardie Plank siding?
I am planning to install horizontal Hardie Plank lap siding and the Hardie info says use a latex sealant per ASTM C834 or C920 (Grade NS, Class 25).

Can you tell me what gap you use between butt joints on horizontal lap siding and the best caulk for the joint?

How much expansion / contraction is going to take place with this siding?

Thanks much.

Solar Dave

Termite 09-28-2008 02:01 PM

I believe they make a special caulk just for fiber cement now.

1/8" gap at butt and trim joints.

buletbob 09-28-2008 02:54 PM

Yes they do! its made by Tight-Bond Its called tight bond weathermaster caulk, it comes in 125 variety of matched colors. Excelant gap filling and will not slump into the gap. BOB

DangerMouse 09-28-2008 03:45 PM

i'm curious too,---> How much expansion / contraction is going to take place with this siding? <--- and shouldn't there be flashing in there somewheres?......but what do i know? i'm just a cartoon mouse......


Termite 09-28-2008 03:57 PM

When installing any lap siding, it is always a good practice to "flash" behind each butt joint with a small piece of 15# felt. Same thing at the trim and the ends' joint.

Clutchcargo 09-28-2008 05:53 PM

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From my experience, there's minimal expansion/contraction with this product. I believe the specified gap is mainly so that you can get caulk between the planks.

mikey48 09-28-2008 06:13 PM

Per Hardi's instructions if you flash behind the joint you can butt them up. I still caulk them

Termite 09-28-2008 08:02 PM

Yeah, I would never butt them tight. The flashing is the guarantee that if water gets in, it will be shed down. It would be insane to install any kind of lap siding without a tin shingle or building felt behind the joint. I look at the caulking as an added means of protection.

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