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Old 10-18-2010, 09:42 PM   #1
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Sistering Floor joists


I have a 20' x 32' one story hunting cabin on piers. Floor and sill framed out with 2x6s. Double 2x6 sill and center beam. 10' 2x6 joists 16" on center between. Floor has been bouncing for about 20 years and I want to stiffen it up. All wood is good , no rot. I have about 2 ' at low end and 3'-3 1/2' at high end under camp to work. Have a little sag under wood stove area from weight but nothing I can't live with. Don't realy want to jack. Can I sister 10' joists to stiffen up floor? What size: 2x6, 2x8, or 2x10. Every one or every other one? How would they be fastened? I've heard PL between then 3 3" deck screws about a foot or two apart for the 10' length. Would running a couple of 2x4s under each 10' section perpendicular to the joists the lenght of the camp and nailing each on to the bottom of each joist do anything?? Should have spent the extra money to get 2x8s or 10s for the deck. Its been ok for 20 years just sick of the bounce.

Any advice would be appreciated.

Thanks
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Old 10-19-2010, 01:20 AM   #2
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It is a bit puzzling that a floor 2x6, 16" o.c., 10' span bounces. It shouldn't if the wood is in good condition, therefore the culprit has to be somewhere else. Either the deck on top is not nailed properly or the connection of the joists at supports is flimsy or the supports themselves are bouncing.

First I would check and strengthen the center beam and the sills supporting the joists at the perimeter.

Regardless of what else you will do there attaching the perpendicular 2x4's at the bottom to the joists, as you mention, is always a good idea.
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Old 10-19-2010, 07:05 AM   #3
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Using this calculator: http://www.awc.org/calculators/span/...al+Span#answer
for #2 SPF, 2 x 6 @ 16" , 50 lb live and 10 lb dead loads, the Maximum Span lists at 8 ft 8 inch only.

The bouncy floors don't surprise me at all.
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Old 10-19-2010, 07:24 AM   #4
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Thanks Engineer10,

So you know the sill I'm talking about are not sills laid flat and the joists on top of the sill as in house construction. We made 2 10' x 32' boxes, nailed the joists into the vertical 2x6 "sill", joined the boxes in the middle (double 2x6 center beam), then added another 2x6 to the outside of the box to double the "sill" Mabe it should be described more as a headder not a sill. No joist hangers either. Everything is still tight as far as I can tell, plywood floor 3/4" (tung and groove) etc. There is no bounce and minimal vibration if you stamp on the floor over the center beam or around the edges. Only in the middle. Where it is supported it seems to be ok. I really think the problem is the fact there are 2-10' x 32' spans with only 2x6s supporting. I only have three rows of 5 piers under. One row on each side and one row under the center beam. Should have put two more rows of piers in. Shoulda woulda coulda. Does this give you any more to go on??

Thanks again
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Old 10-19-2010, 12:42 PM   #5
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From your description the center beam and sills/outside beams are made out of 2x6's nailed together and the span between piers supporting the beams is 8' center to center.

These are definitely weak undersized beams to support such a floor and this creates excessive deflection and bouncing. When standing right on top of the beam it is fine but when you are on joist then the interaction of joist/beam is what comes into play and makes the bouncing annoying.

A joist with 40 psf residential live load and no. 2 wood should span 9'-9" in dry conditions and under wet conditions it is 9'-5". Considering that the actual measured length of the joist is probably around 9'-6" between beams then this is OK. If the joist wood grade is no. 3 (it shouldn't) then the joists are a problem since the maximum allowable span to avoid bouncing is around 8'.

The first step I would take is to put nailed wood blocking between joists in 4 or 5 places at the joist's center and then add the 2x4 continuous at the bottom nailed to the joists with a couple of nails and to the blocking with 4 nails. This should spread the weight of a person among several joists and it also dampens the interaction with the beam. There will be a reduction in bouncing but it will not be completely eliminated because the beams are too small. Next would be to strengthen the beams by adding extra wood on the sides or underneath.

Pictures underneath the floor to show how the joists frame into the beam and sills and the bearing of the beam on the piers will help to make additional suggestions how to strengthen the beams.
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Old 10-19-2010, 06:52 PM   #6
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whats the flooring, sub flooring?


And I would double and block. If theyre exposed at all to the elements, might even consider PT lumber
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Old 10-19-2010, 08:08 PM   #7
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Thanks II Weeks,

Nothing exposed, just in open air under camp. All wood is solid and good. Flooring is 3/4" tung and groove plywood. I don't know if it's a certian type of "sub flooring" or not. Everythings been there for 20 years and looks like the day it went in. So your saying to sister and block. Again, what is the best size to sister with and how to fasten. I'll try and get pictures for next week.

Thanks again
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