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Old 08-07-2013, 01:12 PM   #1
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1x4 joists in my greenhouse??


I'm new to this forum, and this is my first post. I'd be very appreciative of any experienced members who could answer my question.

I recently bought a house which has an 8'x12' greenhouse already built in the yard. I have attached a couple of pics, which I hope you will be able to see.

I plan to place 9 27-gallon planter boxes and 2 50-gallon water tanks inside the greenhouse, and set it up as a self-watering wick system. The large water tanks will also act as thermal mass to help regulate temperatures, especially in winter.

What I don't know is whether the the floor of this structure is strong enough to hold the weight of the water tanks and the soil.

The floor is made of 3 beams that are 4x6 posts, twelve feet long, and spaced 4 feet apart. The joists are only 1x4's, spaced 16 inches, running perpendicular to the beams. On top of the joists is a subfloor OSB. The lumber is all incised and treated. I can't tell for sure, but it appears to be cedar.

I've found calculators that will tell you the minimum span length based on the joist material, and assuming that you're building a house. But, I can;t find anything that will tell me the load carrying capacity of a floor that is already built like this one. Does anyone know how to calculate the maximum weight that I can place inside without destroying the floor?
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Old 08-07-2013, 01:25 PM   #2
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1x4 joists in my greenhouse??


Sure hope that's a typo, 1 X 4's?
Even 2 X 4's would be way undersized for floor joist.
Just the water would be 8.3 X 100 + 830 lb.

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Old 08-07-2013, 01:55 PM   #3
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1x4 joists in my greenhouse??


I just went back out and examined and I found a spot where I can reach under and actually measure the thickness of the joists. They are 1 3/8" thick....so 1 3/8 x 4 if such a thing exists....apparently it does. It's not the first time I have found something structurally weird about this property.

Anyway, from what I can gather, if the joists were 2x4, then it would be built appropriately and be able to support at least 40 psf. It's 96 sq ft on the floor, so that would be more than sufficient to hold my water and planters. I recken the water and planters and accompanying materials will weigh approximately 1 ton, and a floor that size built to code should support at least 2 tons, if I am not mistaken.

But, the joists are only 1 3/8" thick. So, how much less weight can it hold?
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Old 08-07-2013, 01:58 PM   #4
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1x4 joists in my greenhouse??


Here is the calculator I have been using. : http://www.awc.org/calculators/span/...rcalcstyle.asp

According to this, if I have 2x4 joists, I'll be ok as long as the span is not more than 4 feet. The span between beams in this floor is exactly 4 ft. But there is no way to calculate for thinner joists.
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Old 08-07-2013, 02:04 PM   #5
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1x4 joists in my greenhouse??


IF it were ours, we'd place the holding tank outside & not worry about weight,,, faster, just as efficient, & LOTS less $$,,, i wouldn't trust existing load capacity
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Old 08-07-2013, 02:15 PM   #6
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1x4 joists in my greenhouse??


That is considered a 2 x 4. actual measurement, (when new), was 1-1/2" x 3-1/2 " so they would be a little smaller when dried. load supporting ability should have this taken into consideration. More important is how well the 4 x 4s are supported, and how well the boxes and tanks can distribute the weight. You could place larger timbers on the floor over the 4 x 4s if they were well supported, and take the load off the joists.
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Old 08-07-2013, 02:28 PM   #7
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1x4 joists in my greenhouse??


water weights 8.33 lbs per gallon and soil/dirt is typically 100 lbs per cubic foot if memory serves me correctly
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Old 08-07-2013, 03:07 PM   #8
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That is considered a 2 x 4. actual measurement, (when new), was 1-1/2" x 3-1/2 " so they would be a little smaller when dried. load supporting ability should have this taken into consideration. More important is how well the 4 x 4s are supported, and how well the boxes and tanks can distribute the weight. You could place larger timbers on the floor over the 4 x 4s if they were well supported, and take the load off the joists.
In order to do this I would need to pull up the OSB subfloor first, correct? I was hoping to not have to deal with that.
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Old 08-07-2013, 03:10 PM   #9
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IF it were ours, we'd place the holding tank outside & not worry about weight,,, faster, just as efficient, & LOTS less $$,,, i wouldn't trust existing load capacity
I don't think that will work. We have very long winters here in Bend. At least some of the water in the tanks would freeze, if not all of it. Keeping the water inside the greenhouse is necessary I think, if I am going to use a wick system. If the floor won't hold it, I'll probably just have to water the plants manually.
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Old 08-07-2013, 04:27 PM   #10
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1x4 joists in my greenhouse??


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Here is the calculator I have been using. : http://www.awc.org/calculators/span/...rcalcstyle.asp

According to this, if I have 2x4 joists, I'll be ok as long as the span is not more than 4 feet. The span between beams in this floor is exactly 4 ft. But there is no way to calculate for thinner joists.
Those on-line things have limited utility. Usually they are programmed for uniformly-distributed loads; yours seems to be a fairly high localized load, so the result may not work.
Short of re-building the floor, you need to consider spreading the load out as much as possible. Can you glue and screw some thick (1") plywood over the OSB? This would stiffen the boarding considerably and help to spread the load a little more evenly over your joists.
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Old 08-07-2013, 07:36 PM   #11
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Those on-line things have limited utility. Usually they are programmed for uniformly-distributed loads; yours seems to be a fairly high localized load, so the result may not work.
Short of re-building the floor, you need to consider spreading the load out as much as possible. Can you glue and screw some thick (1") plywood over the OSB? This would stiffen the boarding considerably and help to spread the load a little more evenly over your joists.
Thanks. That's what my Dad suggested, but I told him it wouldn't help...I guess I need to listen to Dad sometimes.
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Old 08-07-2013, 08:44 PM   #12
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1x4 joists in my greenhouse??


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In order to do this I would need to pull up the OSB subfloor first, correct? I was hoping to not have to deal with that.
If the 4 x 4s the building is made of are well supported by blocks, Just put maybe 4 x 6s on edge, on the existing floor over, or crossing, (over the joists), the 4 x 4s The weight is transferred to the blocks under the 4 x 4s taking some weight off the joist span. It would be something like adding 1" of plywood. If it isn't well blocked, short of jacking it up a little and adding blocks, probably not much will help enough.
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Old 08-07-2013, 09:25 PM   #13
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1x4 joists in my greenhouse??


Also covering that floor with some type of flooring like sheet linoleum would be a good idea.
OSB that close to the ground is already soaking up moisture from below, add water on top and it's a mess.
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Old 08-07-2013, 10:14 PM   #14
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1x4 joists in my greenhouse??


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Also covering that floor with some type of flooring like sheet linoleum would be a good idea.
OSB that close to the ground is already soaking up moisture from below, add water on top and it's a mess.
Thanks. I'm planning to lay down a 4 mm vapor barrier and staple it down, then lay down that green atro-turf looking stuff they sell at lowes on top of that.
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Old 08-07-2013, 10:21 PM   #15
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1x4 joists in my greenhouse??


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If the 4 x 4s the building is made of are well supported by blocks, Just put maybe 4 x 6s on edge, on the existing floor over, or crossing, (over the joists), the 4 x 4s The weight is transferred to the blocks under the 4 x 4s taking some weight off the joist span. It would be something like adding 1" of plywood. If it isn't well blocked, short of jacking it up a little and adding blocks, probably not much will help enough.
Actually, the beams that support the floor joists are 4x6's, not 4x4's.

How do I know how many blocks are enough? How far apart can the blocks be?

Also, I'm not sure I'm following you. You're saying to lay 4x6s on top of the OSB floor? Should they be parallel or perpendicular to the joists? I don't understand how that would help because would any weight from the top still be supported by the joists that are between the OSB and the beams? It's the joists that I am worried will fail, more than the beams....I think.

Also, how would adding 4x6s be similar to adding 1" of plywood?

(sorry much of this is new to me and so I'm not following well)

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