119) EngineeringDesignConstruction of a Rooftop Garden

119) EngineeringDesignConstruction of a Rooftop Garden

by Gail on April 1, 2011

(Warning. This is a long, detailed report of the requirements for a rooftop garden and the report of our process. I am well aware that blog posts should be short/concise, but this is not an E-How version. I am posting this for people who may wish to design such a garden themselves and need all the detail I wish we had had when we began.)

When our engineer and I were doing research about building a rooftop vegetable/flower garden on the studio building, we found little information. Wikipedia makes an applicable  distinction between a green roof and a rooftop garden:

Roof garden vs. green roof

A roof garden is actually very different from a green roof, although the two terms are often and incorrectly used interchangeably. A roof garden is an area that is generally used for recreation, entertaining, and as an additional outdoor living space for the buildings residents. It may include planters, plants, dining and lounging furniture, outdoor structures such as pergolas and sheds, and automated irrigation and lighting systems. A roof garden reestablishes the relationship between humans and nature that can be lost in urban environments.

It is different from a green roof in that the considerations are primarily of an aesthetic or recreational nature, whereas a green roof is usually constructed to cover a large area in the most economical and efficient means possible with an emphasis towards improving the insulation or improving the overall energy efficiency of cooling and heating costs within a building.

The panels that comprise a green roof are generally no more than a few inches up to a foot in depth, since weight is an important factor when covering an entire roof surface. The plants that go into a green roof are usually sedum or other shallow-rooted plants that will tolerate the hot, dry, windy conditions that prevail on most rooftop gardens. With a green roof, the plants layer can shield off as much as 87% of solar radiation while a bare roof receives 100% direct exposure.

The planters on a roof garden, on the other hand, can generally range anywhere from 6 in up to 3 ft (0.15 to 0.9 m) in depth, depending on the weight-bearing capacity of the roof, and would be placed more for aesthetic purposes. These planters can hold a range of ornamental plants, anything from trees, shrubs, vines, or an assortment of flowers. Since the planters on a roof garden are placed in random fashion, it would be much less likely to provide the environmental and energy benefits of a green roof.

I went to the Light House Sustainable Building Centre, but there was little help there. They did have some models of green roofs from BCITs Centre for Architectural Ecology. but the models didnt include a full garden on top of a building nor was there an indication of supporting substructure. Communication with the BC Landscape and Nursery Association (BCLNA) revealed a list of landscapers who have qualified to design green roofs. I ordered their publication, BC Standards for Green Roofs. The information therein did not address the design of the building to support a rooftop garden. From a qualified BCLNA rooftop landscaper, I learned:

The best way to proceed with designing your studio is to first determine the live weight of your roof. This is determined by your requirements for soil or growing medium on the roof. If you plan on growing vegetables and shrub then you will need at least 12 inches of soil but 18 would be better. The weight depends on the type of soil used, the rule of thumb is for a light weight growing medium (not soil) is 60 pounds per cubic foot.

Torsten, an engineering friend of our son, helpfully replied to our enquiry:

Green roofs consisting of membrane, protection layer, gravel drainage layer, filter fabric, and soil, have been in use over parking garages and underground structures for decades.  Green roofs are not new, despite the fact that they have only become mainstream in the last few years…  they have been around for centuries, and consisted in a multitude of different configurations, including simple wood and dirt.  The real issue these days is with durability and insurance requirements.  The design criteria being implemented are intended to make the systems more durable and rugged, to avoid short term failures and costly insurance claims. The main concerns from insurance firms are leakage and fire.   Leakage can be attributed to premature failure of the membrane due to low quality materials, poor workmanship, insufficient protection from foot traffic (during installation) sharp components and root penetration, or tears due to excessive deck movement.  Fire can result when plants or grasses become dry and brittle during a drought.  You should confirm what requirements need to be met with your insurance provider, or you may run into problems down the road.

A typical torch on membrane is acceptable, but considered low quality and may only provide 15-20 years of expected service life in a properly designed system.  Obviously you would have to use a self-adhered or mechanically fastened base sheet to encapsulate all flammable/wood elements prior to torching on a cap sheet.  Thicker more durable torch on membranes are available from many of the manufacturers.  A high quality green roof membrane may be worth the material cost increase (eg. Siplast Teranap).  The concern with a wood deck would be load capacity and movement.  Wood decks will move significantly more than concrete decks, and put more stress on membranes at plywood joints and perimeter interfaces. Consideration should be given to using a high density moisture resistant cover board over the plywood deck.

Gravel is an excellent drainage layer, but is quite heavy and therefore not often used on green roofs, especially on wood structures.  Same goes for soil…  average topsoil is quite heavy…   modern assemblies use drainage mats and specially formulated growing media to reduce loads on the structure.  If you are planning to use gravel and soil, make sure your structural engineer is on board.  The membrane should be protected from damage by the gravel above, and from root penetration.  Some root barriers could likely do both.  The gravel would have to be wrapped in filter fabric, to prevent contamination from the soil.  Consideration should be given to irrigation systems or similar, as gravel will have poor water retention capability.

Our engineer could figure out the weight loads, and so, together with what I had learned, he designed the roof with these layers (in order of construction):

1. Into the concrete walls is bolted an 812 laminated beam to support the weight in the centre (see photo in point 3.) then a double 212 joist, 16-on-centre construction. 22 x 24 is the size of the roof.

2. 3/4 tongue-and-groove plywood for the roof deck

3. Inside, 15 batts of R20 insulation covered with vapour barrier. That doesnt sound like much, but the soil above has some insulating quality as well. Besides, as the building inspector has noted, its not a residence, only a studio, so it doesnt matter so much.

4. Back outside, there are two layers of torch-on roofing. One drain hole at the lower corner of the roof deck collects water to send it to our rainwater collection cistern. (Later edit: we discovered that such nutrient-rich drainwater created a huge algae problem in our waterfall and shallow pond, plus it was too muddy for flushing toilets. We have diverted the rooftop drain to a rain barrel for watering the plants only. )

5. A layer of heavy-duty landscape pond liner, skillfully finished with flashing by the roofer so that there are no perforations in the waterproof layer.

The roofer used an S-finish method of joining the flashing so that he didnt  have to screw through the fascia boards. Additional insurance by way of silicon caulking.

The flashing corners are custom-made to shed water.

6. 3 of round 3/4 grade gravel for a drainage layer. (Round so that sharp edges dont perforate the waterproof layer.)

7. Heavy-duty landscape root barrier cloth over the gravel.

8. Our garden boxes and pathways.

Around the perimeter of the garden are placed lengths of cedar 212. This box will be bolted to the outside of the fascia board with custom-welded steel brackets at the corners. We have found used wrought iron fencing to make the railing on top of the 212, which will bring the rooftop railing up to BC Building Code height of 42.

by Gail on April 1, 2011

(Warning. This is a long, detailed report of the requirements for a rooftop garden and the report of our process. I am well aware that blog posts should be short/concise, but this is not an E-How version. I am posting this for people who may wish to design such a garden themselves and need all the detail I wish we had had when we began.)

When our engineer and I were doing research about building a rooftop vegetable/flower garden on the studio building, we found little information. Wikipedia makes an applicable  distinction between a green roof and a rooftop garden:

Roof garden vs. green roof

A roof garden is actually very different from a green roof, although the two terms are often and incorrectly used interchangeably. A roof garden is an area that is generally used for recreation, entertaining, and as an additional outdoor living space for the buildings residents. It may include planters, plants, dining and lounging furniture, outdoor structures such as pergolas and sheds, and automated irrigation and lighting systems. A roof garden reestablishes the relationship between humans and nature that can be lost in urban environments.

It is different from a green roof in that the considerations are primarily of an aesthetic or recreational nature, whereas a green roof is usually constructed to cover a large area in the most economical and efficient means possible with an emphasis towards improving the insulation or improving the overall energy efficiency of cooling and heating costs within a building.

The panels that comprise a green roof are generally no more than a few inches up to a foot in depth, since weight is an important factor when covering an entire roof surface. The plants that go into a green roof are usually sedum or other shallow-rooted plants that will tolerate the hot, dry, windy conditions that prevail on most rooftop gardens. With a green roof, the plants layer can shield off as much as 87% of solar radiation while a bare roof receives 100% direct exposure.

The planters on a roof garden, on the other hand, can generally range anywhere from 6 in up to 3 ft (0.15 to 0.9 m) in depth, depending on the weight-bearing capacity of the roof, and would be placed more for aesthetic purposes. These planters can hold a range of ornamental plants, anything from trees, shrubs, vines, or an assortment of flowers. Since the planters on a roof garden are placed in random fashion, it would be much less likely to provide the environmental and energy benefits of a green roof.

119) EngineeringDesignConstruction of a Rooftop Garden

I went to the Light House Sustainable Building Centre, but there was little help there. They did have some models of green roofs from BCITs Centre for Architectural Ecology. but the models didnt include a full garden on top of a building nor was there an indication of supporting substructure. Communication with the BC Landscape and Nursery Association (BCLNA) revealed a list of landscapers who have qualified to design green roofs. I ordered their publication, BC Standards for Green Roofs. The information therein did not address the design of the building to support a rooftop garden. From a qualified BCLNA rooftop landscaper, I learned:

The best way to proceed with designing your studio is to first determine the live weight of your roof. This is determined by your requirements for soil or growing medium on the roof. If you plan on growing vegetables and shrub then you will need at least 12 inches of soil but 18 would be better. The weight depends on the type of soil used, the rule of thumb is for a light weight growing medium (not soil) is 60 pounds per cubic foot.

Torsten, an engineering friend of our son, helpfully replied to our enquiry:

Green roofs consisting of membrane, protection layer, gravel drainage layer, filter fabric, and soil, have been in use over parking garages and underground structures for decades.  Green roofs are not new, despite the fact that they have only become mainstream in the last few years…  they have been around for centuries, and consisted in a multitude of different configurations, including simple wood and dirt.  The real issue these days is with durability and insurance requirements.  The design criteria being implemented are intended to make the systems more durable and rugged, to avoid short term failures and costly insurance claims. The main concerns from insurance firms are leakage and fire.   Leakage can be attributed to premature failure of the membrane due to low quality materials, poor workmanship, insufficient protection from foot traffic (during installation) sharp components and root penetration, or tears due to excessive deck movement.  Fire can result when plants or grasses become dry and brittle during a drought.  You should confirm what requirements need to be met with your insurance provider, or you may run into problems down the road.

A typical torch on membrane is acceptable, but considered low quality and may only provide 15-20 years of expected service life in a properly designed system.  Obviously you would have to use a self-adhered or mechanically fastened base sheet to encapsulate all flammable/wood elements prior to torching on a cap sheet.  Thicker more durable torch on membranes are available from many of the manufacturers.  A high quality green roof membrane may be worth the material cost increase (eg. Siplast Teranap).  The concern with a wood deck would be load capacity and movement.  Wood decks will move significantly more than concrete decks, and put more stress on membranes at plywood joints and perimeter interfaces. Consideration should be given to using a high density moisture resistant cover board over the plywood deck.

Gravel is an excellent drainage layer, but is quite heavy and therefore not often used on green roofs, especially on wood structures.  Same goes for soil…  average topsoil is quite heavy…   modern assemblies use drainage mats and specially formulated growing media to reduce loads on the structure.  If you are planning to use gravel and soil, make sure your structural engineer is on board.  The membrane should be protected from damage by the gravel above, and from root penetration.  Some root barriers could likely do both.  The gravel would have to be wrapped in filter fabric, to prevent contamination from the soil.  Consideration should be given to irrigation systems or similar, as gravel will have poor water retention capability.

Our engineer could figure out the weight loads, and so, together with what I had learned, he designed the roof with these layers (in order of construction):

1. Into the concrete walls is bolted an 812 laminated beam to support the weight in the centre (see photo in point 3.) then a double 212 joist, 16-on-centre construction. 22 x 24 is the size of the roof.

2. 3/4 tongue-and-groove plywood for the roof deck

3. Inside, 15 batts of R20 insulation covered with vapour barrier. That doesnt sound like much, but the soil above has some insulating quality as well. Besides, as the building inspector has noted, its not a residence, only a studio, so it doesnt matter so much.

4. Back outside, there are two layers of torch-on roofing. One drain hole at the lower corner of the roof deck collects water to send it to our rainwater collection cistern. (Later edit: we discovered that such nutrient-rich drainwater created a huge algae problem in our waterfall and shallow pond, plus it was too muddy for flushing toilets. We have diverted the rooftop drain to a rain barrel for watering the plants only. )

5. A layer of heavy-duty landscape pond liner, skillfully finished with flashing by the roofer so that there are no perforations in the waterproof layer.

The roofer used an S-finish method of joining the flashing so that he didnt  have to screw through the fascia boards. Additional insurance by way of silicon caulking.

The flashing corners are custom-made to shed water.

6. 3 of round 3/4 grade gravel for a drainage layer. (Round so that sharp edges dont perforate the waterproof layer.)

7. Heavy-duty landscape root barrier cloth over the gravel.

8. Our garden boxes and pathways.

Around the perimeter of the garden are placed lengths of cedar 212. This box will be bolted to the outside of the fascia board with custom-welded steel brackets at the corners. We have found used wrought iron fencing to make the railing on top of the 212, which will bring the rooftop railing up to BC Building Code height of 42.


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