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Maintaining 1/8” gap on exterior t&g siding

4/17/19       
John L

What type of tongue and groove mil specification and installation method would you suggest for exterior vertical tongue and groove edge matched genuine mahogany siding?

Customer is looking to maintain a clean minimum 1/8” gap without it closing up due to seasonal expansion. The mahogany is 3/4” flat sawn 4” wide which will be prefinished all sides & edges. I determined the tangential expansion worst case scenario to be .06” from its kiln dried state of 6% MC to a worst case MC of 15% during peak humidity, 10% being a more realistic number.

Would you mill the tongues 1/8” deeper than the grooves to provide an expansion stopping point and space the boards an additional 1/16 or .0625” to compensate for seasonal expansion? Or would you rebate the face above the tongues and space accordingly? Or is there another way?

How would you fasten them to allow the boards to expand and contract to maintain a consistent gap and what type of fasteners would you recommend? I’m concerned with panelization and an uneven gap when contracting during the dry season. Any ideas? Buckling is also a concern, but if my math serves me correctly, the 1/16” spacing “should” be fine.

Thank you in advance for any responses.

4/18/19       #2: Maintaining 1/8” gap on exterior t& ...
Haroldd Pomeroy

In doors, I pin the tongue and groove panels in the middles to average out the wood movement. Having narrow wood will help.

Some education on wood movement ahead of time will calm the customer's anxiety when the wood moves, because it will.

Finally, a set of feeler gauges measuring 1/32" to 1/4", all labeled as 1/8", would help.

4/18/19       #3: Maintaining 1/8” gap on exterior t& ...
John L

“Finally, a set of feeler gauges measuring 1/32" to 1/4", all labeled as 1/8", would help”...haha...good one...forgot the dark pair of sunglasses..lol.

Yes, averaging out the wood movement is what I’m looking to achieve and wondering if face fastening vs blind fastening is the better approach.

4/19/19       #4: Maintaining 1/8” gap on exterior t& ...
Chris H.

Website: https://www.csaw.com/lamello/

Love Haroldd's idea of the 1/8" spacers!

Agreed, laws of Nature apply to all customers, so there WILL be seasonal expansion and contraction. However, you mentioned flat-sawn material. Is quarter-sawn available? That will shrink less across the grain than flat-sawn. And as Haroldd mentions, fastening in the middle of each board is a good best practice too, like fastening a breadboard end on a farmers table. Acclimatization of the material is also a good idea. I found a pretty detailed link below about seasonal movement for you. Good luck on this one! When I had my shop and would field requests like this (which fly in the face of the laws of wood science) I would exempt that part of the project from my warranty. Just saying.

Wood Movement

4/19/19       #5: Maintaining 1/8” gap on exterior t& ...
John L

Thanks Chris. I did consider quartersawn but the difference in expansion across the width for flat vs quartesawn was only .0163” based on the expected MC change. The shrinkage calculator resource on this site was very helpful.

It would probably be a good idea to sticker the material in a garage or covered location to allow it to acclimate as you mentioned so any dimensional changes will be less drastic, although time is of the essence...you know how that goes.

I did decide to have the material sent back to the mill and have them rebate the board faces above the tongues 1/16” in the event the boards do compress, so at least the gap won’t totally disappear.

The existing siding being replaced was supposed to have a nickel gap and was spaced 3/32”, yet compressed, not looking too attractive. Subsequent refinishing resulted in bridging of finish at the non-existent nickel gap, water getting trapped/not shedding, resulting in finish failure at edges. I’m hoping to avoid a repeat scenario.

I’m not the contractor doing the installations, the client and architect deferring to me for sound advice. I’m guessing the installers will be utilizing wire nails, toe nailing above the tongues. I’m wondering what options there might be for fastening. The installers were concerned with buckling. Although science dictates that 1/16” spacing would work, I just bumped up the spacing specification to 3/32” to appease their concerns.

Thanks for the link!

4/20/19       #6: Maintaining 1/8” gap on exterior t& ...
Adam

Think of it as flooring. It will not buckle.

My mind of expansion was completely changed by a massive water leak in my old carriage house workshop. The floor is 32’ x 32’. 6” wide x 16’ southern yellow pine was installed with typical Bostich flooring staples. No face nailing.

There was a massive water leak on the 2nd floor. It had 1” of standing water on the floor for 1 week. The pine floor only buckled at 2 location(4 boards) across 32’. The rest looked perfect.

1st thing regardless of how you mill it would be to leave it outside for 2 weeks to acclimate to the outdoor moisture content.

I would assume the wood is going to expand rather than shrink. Depending on your location the gaps may change during the seasons.

Think of this project as vertical flooring. Making the groove any deeper(1/16” typical) achieves nothing. The shoulder matches the tongue.

I would gap it with 1/8” spacers and be done with it. The gaps may close to a 1/6” and expand to 3/16” throughout the year.

It’s wood it moves end of story.

4/20/19       #7: Maintaining 1/8” gap on exterior t& ...
Russ

Website: http://www.mr-moulding-knives.com

Is this blind nailing joint an option? We offer many other siding designs you may want to review and consider. Happy Easter Russ @ MR

tongue-and-groove

4/20/19       #8: Maintaining 1/8” gap on exterior t& ...
Pat Gilbert

Adam

Floors buckle all the time?

4/21/19       #9: Maintaining 1/8” gap on exterior t& ...
Adam

Pat ,

What?

4/21/19       #10: Maintaining 1/8” gap on exterior t& ...
Pat Gilbert

You state "It will not buckle."

I'm saying that floors do buckle

I'm trying to understand your take on this

4/22/19       #11: Maintaining 1/8” gap on exterior t& ...
Walter Member

All six sides of the siding should be finished to reduce the movement of moisture in and out of the wood.

Knowledge Base Article

4/22/19       #12: Maintaining 1/8” gap on exterior t& ...
Gerry

This floor never got wet but the space under it flooded. The floor gained a lot of moisture. I calculated in the 40' of width it moved between 3 and 4 feet.


View higher quality, full size image (1280 X 960)

4/22/19       #13: Maintaining 1/8” gap on exterior t& ...
LJ

I'd tell the customer 1/8" is too tight a reveal to maintain consistently throughout the year. Even an extra sixteenth would make all the difference visually. I would mill it with an 1/8" groove when pushed together hard and have the installer use a 3/16" spacer. Room to grow and room to shrink.

4/22/19       #14: Maintaining 1/8” gap on exterior t& ...
John L

Thanks LJ. We finally settled on milling it 3/16” and spacing it an additional 3/16”....very close to your suggestion.

Will settle for a 3/16” vs” 1/8” minimum gap. Should be good.

4/22/19       #15: Maintaining 1/8” gap on exterior t& ...
BH Davis  Member

How much time to you have to let the wood acclimate to the outdoor air. In most situations the wood will begin to take on moisture as soon as it comes out of the kiln.

If you don't already have one pick up a moisture meter and find out the MC of the wood at the time of the install. The 3/16" is overall a good idea but if the wood has acclimated to 9% to 10% when installed you could likely have good results with the 1/8" gap. Depending of course upon your location and real summer max. moisture levels.

Most mahoganies (real or substitute) will be fairly stable. Quarter sawn would be better but as you implied might not be worth the extra cost. I'd think the numbers you got for expansion would more likely relate to each 12" of width vs. 4" of width. I could be wrong on that though but thought I'd bring up the point.

BH Davis

4/22/19       #16: Maintaining 1/8” gap on exterior t& ...
John L

Correction, the final spec was 3/32” milled gap and 3/32” additional spacing...


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