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Hardwood floor thickness for loft...

Derek Krzanowski Member

I've been asked to make a custom hardwood floor out of white ash and as close to 6/4 finished thickness as possible as it is for a loft area where they want the underside to look like traditional tonque and groove as it will be visible from below. The reason for the thickness is that the beams are 4' on center and the unsupported distance between beams is roughly 3' 6". I'd love to hear opinions or even better would be an engineers opinion on if that is realistic or not. I've found a ton of formulas for calculating the maximum load however I'd have to go back to school to figure them out.. Lol. My basic test of taking an 18" wide by 4' long by 1-3/8" thick slab and sticking it on 4x4s and jumping on it showed almost no deflection but that is far from a definitive answer... Does anyone have experience or knowledge on this that they'd be willing to share?

3/10/20       #2: Hardwood floor thickness for loft.. ...
james e mcgrew  Member


I am not an Engineer and never profess to be one, That said get an email from the drawings Engineer to sign off on it directing you to Install as drawn, if something were to go wrong the liability is gonna fall on the responsible party

3/10/20       #3: Hardwood floor thickness for loft.. ...
David R Sochar Member

Use the Sagulator to help predict what will work.

6/4 Ash should dress to 1-1/4", maybe a little thicker. According to my run thru the Sagulator, you will be fine. Assuming a live load of 450 lbs on a 42" x 42" floor....

The Sagulator

3/10/20       #4: Hardwood floor thickness for loft.. ...

I've built numerous post and beam houses
where we used 1-1/2 native pine T&G for
loft flooring on 4X10 joists 4 foot OC and
never had any owner complain about
springy behavior. 8/4 ash would be way
way stronger - I wouldn't hesitate at all
to use it as you describe.

The complaints I did get had to do with sound and dirt -
every step is heard perfectly
from the floor below, and eventually, dust/dirt
works its way into the T&G joints and when
the low humidity in winter has the flooring
shrunk to its narrowest width, the dirt finds
its way through and onto whatever's below

hope this helps

3/10/20       #5: Hardwood floor thickness for loft.. ...
Rich  Member


Are the Beams rated to support living space?

3/10/20       #6: Hardwood floor thickness for loft.. ...
Peter Gagliardi

1 1/4"-1 1/2" is well more than enough. Don't overthink it. 2x6" t&g #2 spruce is used for this all the time at 1 1/2" finish.The Ash is considerably stiffer.
The dirt working thru is a real problem, though the Ash being kiln dried should stay much tighter.

3/11/20       #7: Hardwood floor thickness for loft.. ...
BH Davis  Member


How about a sub-floor first? That could be 4/4 or 5/4 T&G material. Then a layer of the typical laminate flooring 1/16" thick sheet cushioning material and finally a 5/4 or 6/4 top layer of ash flooring. If you ran the ash at 45 degrees to the sub-floor it would be even stronger.

The whole system would look good from below, have plenty of strength, solve the dirt drop issue and be quieter due to the sheet of cushioning material.

BH Davis

3/12/20       #8: Hardwood floor thickness for loft.. ...
Derek Krzanowski Member

Wow, thanks to everyone for all the awesome information! The sagulator is exactly what I needed! I also like the idea of making a subfloor from the same material and running it at a 45 degree angle, I'll mention that to the homeowners and see what they think. Thanks again, much appreciated!

3/12/20       #9: Hardwood floor thickness for loft.. ...
BH Davis  Member


If you are going to run the sub-floor at 45 then take another look at the Sagulator. Your spans are going to be greater so thicker material will likely be in order for the sub-floor.

BH Davis

3/13/20       #10: Hardwood floor thickness for loft.. ...
Derek Krzanowski Member

BH Davis, good point, thanks!

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