Calculating weight of soil for rooftop planters
I'm building a rendered planter (using light concrete blocks) on what will effectively be a roof terrace and have been asked by the roof supplier (beam and block roof) to confirm the imposed load of these planters. For initial quote they have assumed 7.5KN/m2, so I guess they are assuming a dead weight of 750kg/m2.
I had thought they'd have their own tables to calculate the load, but I guess their engineers are playing it safe by asking me to confirm the weight.
I can calculate the weight of the blocks from which the planter will be constructed. But I'm unsure about the weight of the soil. Do people generally use a specialised soil for such applications, i.e:
Also I'd need to take account of retained water as they'll be exposed to the elements. I have planned to build in drainage. I will do this by lining them with DPM (which will also protect the exterior render), lay some perforated drainage pipes to a fall, fill with drainage pea gravel to above pipes, cover with geotex then fill the rest with soil. Does this seem like a reasonable approach to prevent them becoming saturated in heavy rain (I'm in the UK).
The exterior dimensions of the planter are 7200mm (l) x 800mm (w) x 645mm (h), interior dimensions 700mm (l) x 600mm (w) x 645mm (h) (filled to approx 600mm).
My calculations suggest (ignoring pea gravel drainage layer for now):
Volume of Soil: 7m * 0.6m * 0.6m = 2.52 m3
Volume of Soil / m2: 0.6 m3 of Soil per m2
Some "googling" suggests a m3 of soil weighs upto 1700kg (not sure if this accounts for any retained water). Based on this:
Imposed load of planter (based on Soil alone): 1700kg * 0.6 = 1020 kg/m2 = ~ 10 KN/m2
If I were to use the "roof soil" I linked to previously it appears I could significantly reduce this. I've used their online calculator as they suggest it takes account of compaction. And it estimates I need 101 25L/25KG bags (of course I'd buy the bulk bags, but they don't state the weight of these).
Total Weight of Soil: 101 * 25 kg = 2525 kg
Surface area of Soil in planter: 7 * 0.6 = 4.2 m2
Imposed load of planter: 2525kg / 4.2 m2 = 601 kg/m2 = ~6 KN/m2
To give some background/context:
I'm in the process of renovating my smallish town garden which is significantly below the ground floor of my house. I've gained planning permission to raise the entire garden to the houses ground floor level with a combination of decking and artificial grass.
As part of this I'm also building a storage shed (sub structure, that's how much height I have to play with) on the rear boundary, upon which the artificial grass will be laid. The sheds dimensions are 7200mm x 3555mm, constructed from reinforced concrete base, 1.2m reinforced concrete walls, then dense concrete block walls to bring it to 2m high. The roof will be beam and block, with 100mm screed.
So far everything has been straight forward, but the planter is also to be built on the rear portion of this roof.
I've attached a basic Garden design drawing which may help you better visualise the layout.
Thanks in advance for any help :)
Comments (9)
Related Discussions
Question for Al re: soil
Comments (6)I'm going to stick with my calculations (besides, I already have the advantage of knowing what it weighs). Schmily was asking about a well aerated mix. Dry sand weighs from 90-110 lbs/cu ft. If we say it weighs 100 to make it easy and sand is about 15% pourous, even if all the pores were to retain water, it would only gain 15% of 1 cu ft, which equals a shade over 9 lbs. Loam and clay are lighter than sand at about 90 and 75 lbs/cu ft dry, and are less porus than sand, so will gain less weight at field (or container) capacity. All the above for mineral soils. For the open soils I use, the calculation for weight, based on the heaviest ingredient, pine bark which is already 50% saturated and that weighs in at 40 lbs/ 2 cu ft bag, would look like this: 20 lbs (weight of 1 cu ft damp bark). If the soil is at 65% total porosity and 30% is air porosity (macro-pores which will drain) it means the soil will gain .35 x .5 cu ft H2O @ 62.5 lbs/cu ft or another 11 lbs of water. Knock off about 5 lbs because the actual soil will have perlite & peat, both lighter than bark, and multiply by 1.25 cu ft & you get about 32.5 lbs/1.25 cu ft. If you are using sand or garden soil in your soils, I can easily see where they might weigh the 50lbs/ 1.25 cu ft you suggest. I have weighed a 5 gallon bucket of my bark-based soil after it's mixed & moistened & it weighed about 23.5 lbs, including the bucket. Al...See MoreRooftop and Self-Watering Garden Projects
Comments (11)Nice pictures, very extensive setup you have there. I guess I'm going more for a raised-bed scale type of setup, only on a slanted roof, along with being self-collecting and self-watering. Load - I've played around with it as I've proceeded. I have 3 water berms, which hold at least 5 gallons each. The mix is only 4-6 inches deep, at most 2-3 lbs per 12 cu " . If you think of it as a cubic foot being a 5 gallon pail there is probably a gallon of water and 1.5 gallons of mix at most per cubic foot. So I don't think I'm at even 20 lbs per sq. ft of roof in use, and I'm using about 2/3 (25 sq ft)of each side of the roof. I don't really know what this roof is rated for, but it has 2x4' trusses every 2 feet under 3/8" OSB ... seems pretty sturdy ... from what I read, most garages are rated for 25-30+ lbs, but I'm trying to get up to speed on how to caluclate loads more accurately. Squash Growth - Yeah, I got a little wild, but wanted to make sure I got my money's worth. I'd estimate each pair of plants has about a gallon of space. They're doing fine so far. Got a good rain a few days ago, and they are showing good growth. The top berm dries right up, the bottom one doesn't seem to, at least not yet. Too early to tell if the system is wicking. As wicks, I have halves of gallon jugs cutting up through an aeration bench of flexible plastic on one side, 1/2 inch wire on the other. Mix - there's nothing terribly exact. I'm trying to do it on the cheap. One side is mostly leaf compost and a horse bedding / manure mix. The other side is both of those and a lot of double shredded pine in there, too. I threw a few pounds of Milorganite (CRF) around. I can see with all your smaller containers that change out is an issue. This year, I guess I'd be happy to see everything hang together and produce a decent crop. I can't see going with perlite, peat moss and some of the fancier things on something of this scale right off until I have a better understanding of the "gritty mix" or such sytems and their applicability. In just toying with leaf compost only last year, everything (some tomatoes and beets) was ok for a few weeks, but then went yellow, I assume for lack of N, if not a lot of other things. I have some tomatoes, canteloupe, sweet corn, and pole beans on the east side. I'm trying this out for a few reasons. One, there are some real practical advantages to getting things up away from pests. Also, I think 20 foot squash vines might be more manageable up there than on the ground. We'll see in they need extensions or trellising. In general I think we may need to do things like this to remain a free society, frankly. I'm thoroughly perturbed by the Malthusians , and this is my statement against such ideas. I also looking into integrating a fish pond into this sort of thing as an aquaponics setup. Looking for better, lighter, sturidier materials. Any suggestions, especially with fertilizer regimen, and possibly wind and sun screening are welcome....See MoreHelp needed to calculate soil weight in boxes
Comments (36)Let's assume you fill the boxes with sand. Keep in mind that sandy soils have a higher bulk density than nearly anything else you would fill these boxes with, including clay. Clay IS NOT HEAVY; clay is cohesive. It sticks to itself so that it only feels "heavy" when plowing or otherwise working the soil. But I digress, build the structure strong enough to fill with sand, and you should be ok filling with anything else. Let's first make some assumptions. We'll say the bulk density (dry mass of soil/total soil volume) is 1.7 g/cm3. This is typical for single-grained structureless sandy soils. Second assumption: particle density (dry soil mass/volume of the solids) is 2.65 g/cm3. This is a typical estimate for the particle density of sands, which is essentially quartz. Knowing these assumptions and the total volume weÂre dealing with, we can calculate the total mass of the solids, total pore space, and the mass of water that will fill that pore space when the soil is saturated. OK, calculate the volume of the boxes. 8 ft x 2 ft x 2.5 ft = 40 ft3. If you have four of these boxes then you have 4 x 40 ft3 = 160 ft3. If you fill these boxes to 1.7 g/cm3 with sand, the total mass will equal the bulk density times the total volume. First, convert your units to metric. 160ft3 x (1m/35.31 ft3) = 4.53 m3. Now you can calculate total mass of the solids. 4.53m3 x 1.7g/cm3 x (1kg/1000g) x (1,000,000 cm3/m3) = 7701 kg. So, all that sand in 160ft3 at a bulk density of 1.7 g/cm3 will weigh 7701kg when completely dry. That will never be the case though. If the soil becomes saturated, then all the pore space will be filled with water. We can calculate the total pore space and, assuming the density of water is 1 g/cm3, calculate the mass of water filling those pores. Total pore space is equal to one minus the quotient of bulk density over particle density. Porosity = 1  (1.7 g/cm3 ÷ 2.65 g/cm3) = 0.358. This number means that for every 1 m3 of total soil volume, there is 0.358 m3 pore space. Just multiply the porosity and the total volume and you get the total pore space. 0.358 x 4.53m3 = 1.62 m3. The density of water is 1000 kg/m3. 1.62 m3 of water would then weigh 1620 kg. Add the mass of the solids and water, 7701 kg + 1620 kg = 9321 kg. There are 2.20 lbs per kg. 9321 kg x (2.20 lb/kg) = 20549 lbs. I would say thatÂs the maximum mass you should expect from your solids at saturation. Keep in mind that I study soils, not engineering. Speak to a license engineer about how to best support your roof safely....See MoreRecommend decking for Chicago rooftop deck
Comments (20)Perfect, makes sense now. Looking around at my neighbors to see what they are doing since 2 are building decks right now. One is running 2x4 stringers flat sitting on little pink foam pads and the other is setting crazy thick stone tiles on bison deck supports. Your way sounds the most solid, though. For screws, do I need to go with 1 5/8" SS screws? I'd like to use the brown headcotes because the smartbit looks pretty idiotproof for a guy like me, but I understand that these are only in 2 1/2" lengths? I'm worried about screwing into the roof with the 2 1/2" screws at the lower spots. Then again I'm worried that the 1 5/8" might not be long enough if I end up going with 5/4 ipe (plus I'd have to use 2 bits for the drill/countersink unless theres some other tool out there). 2" seems like it would be perfect but I've never seen the pros on here discussing any 2" SS screws. Any recommendations? I don't want to plug, just facescrew flush with colored screw heads to blend in....See More
gb222222
7 years agogb222222
7 years ago
theparsley
7 years agolast modified: 7 years agoTim Walker
7 years agogb222222
7 years agotoxcrusadr
7 years ago
mad_gallica (z5 Eastern NY)