# Analysis of the Weight of the Smart Roof Deck Garden

A garden can weigh more than you might think due primarily to the amount of soil being used and the soil’s ability to retain water.  Most roof structures are only designed for snow and rain loads, not to support the additional loads from planters, wet soil, and people using the garden. Because the roof we worked on is relatively old it can probably only hold 30 lb/ ft², which is why the deck sits on the masonry bearing walls.  In this case, the capacity of the roof itself is irrelevant, and we need to focus on the capacity of the deck.  With help from our civil engineering friend Kartik Patel and our own online research we found that a standard deck built with 2 x 8 ft joists spaced at 16 inches on center should be able to comfortably hold 50-60 lb/ ft² of live load in addition to its own weight (60 lb/ ft² is the minimum live load required for a deck as defined by the American Society of Civil Engineers (ASCE)). Below we summarize how we calculated the weight of the garden and how we checked that the deck would be able to support the weight.

The basic formula for the weight of the garden is  soil mixture + lumber = total weight

Note that we neglect the weight of the plants and the irrigation system since they are relatively small.

Soil Mixture

Our soil mixture of 1/3 vermiculite, 1/3 peat moss, and 1/3 compost proved to be relatively light compared to other soil mixtures we experimented with that included top soil.  This is the soil combination called “Mel’s Mix” in his book Square Foot Gardening.  However, ready-made Mel’s Mix was not available at our local Home Depot so we bought bags of vermiculite, peat moss, and compost separately for the bottom half of our garden.

Because our garden consists of nine 3 x 3 ft planters, it will cover 81 ft²:

We know how many cubic feet of soil we need using a soil depth of 6 inches:

Now combining the weights of each soil type based on the proportion they were used in and the weight of the lumber, we can approximate the weight of the garden.

 Soils Dry Density (lb/ft³) In Bag (ft³) Weight in Bag (lb) Coverage Depth (ft) Coverage Area (ft²) Out of Bag (ft³) Peat Moss 6.7 3 40 0.17 36 6 Vermiculite 7 NA NA NA NA NA Compost 51.9 NA NA NA NA NA

Table 1.  The above table lists the weight per ft³ for the three types of soil we used.  We had a bag of compressed peat moss (meaning when it is taken out of the bag it expands).  On the bag it told us how many ft³ it covers in and out of the package, how much area it takes up, and that it expands to double the size. If you multiply peat moss’ coverage depth and coverage area you get the expanded, out of bag volume of soil which is double the volume in the bag.  Dividing the weight by the out of bag volume then gives the dry weight density of peat moss.  We estimated the density of vermiculite off of a website.  Compost varies in density because its components can vary depending on where you buy it from, so we estimated its dry density based on information we found online.

Combining the densities for each soil and multiplying against the volume of the soil we get

for the weight of the bottom half of the soil mixture in the garden.

For the rest of our soil we ended up finding a local Philadelphia seller that had a similar soil mix to Mel’s that replaced vermiculite with rice hulls and added worm castings. We weighed several cups of the new mixture and came up with an average density of 3 oz/cup. This translates into a weight of 908.88 lb if we were to use this soil mixture for the entire garden.

The total soil weight between both mixtures used is therefore

Lumber

We created a prototype planter (see this link for build instructions and details) and weighed it at 35 lb. With 9 planters, this is a total lumber weight of 315 lb.

Moisture

Moisture and water weight is considered a live load meaning it is not an intrinsic weight of a structure.  When the soil is saturated the garden will weigh more.   We found that standard wet soil is 100-120 lb/ft³.  For a 6 inch coverage we should use 50-60 lb/ft².  With the area of our soil coverage in the garden calculated above, this brings our wet weight of the soil mixture to 81 ft² * 60 lb/ft² = 4860 lb, which is significantly higher than the dry weight.  This is a very conservative estimate because our soil mixture is quite a bit less dense than standard soil, but it gives an upper limit to the saturated soil mixture weight.

The pine used to create the planters retains water as well, so we let a prototype planter sit outside for a month with soil in it, then emptied it and weighed it at 43 lb – about 8 lb more than its dry weight. This changes the lumber weight from 315 lb to 387 lb total.

Weight Analysis

The dry weight of the soil mixture and lumber is

896.7 lb + 315 lb = 1211.7 lb

which is equivalent to 15 lb/ft².

After moisture and water weight are taken into account, the total weight of the garden is approximately

4860 lb + 387 lb = 5247 lb

which is equivalent to 65 lb/ft².  Although this exceeds the 60 lb/ft² live load the deck is likely designed to handle, this also uses a very conservative estimate for the weight of the wet soil mixture.  Following this analysis, we again consulted Jennifer Pazdon, a licensed structural engineer, if she thought the deck could hold the additional weight from the garden based on our analysis and she said it would be okay.  Based on these calculations and her feedback we moved forward with the installing the garden.