Building a green roof on your shed is a way to add interest to the shed and produce a new area to cultivate. There are some things you need to check before building but the job is not very difficult.
Step 1 - Check If it Is Load Bearing
You are going to be adding significant extra weight onto the shed roof so you need to know that the structure is strong enough to support it. It isn’t only the roof itself that will be supporting the weight but also the shed walls.
Step 2 - Waterproof the Shed
Make sure that the shed roof is fully waterproof.
Step 3 - Allow for Drainage
Make sure that the roof has a pitch that will enable drainage. If the pitch is more than 20 degrees from horizontal, you will need to take precautions so the soil in your garden does not slip.
Step 4 - Build a Retaining Frame
Using rot proof timber, build a frame around the roof to contain the garden. You will need two pieces of 2X4 timber the same length as the width of the roof and two pieces the same length as the depth of the roof minus 4 inches. The frame can be made by simply nailing the corners together or by using L brackets to form the corners.
Step 5 - Fit the Frame to the Roof
Often it is sufficient to place the frame on the roof and fasten it by a simple metal strap screwed to the frame and the shed near each corner. Unless the roof is steeply pitched the weight of the roof will hold it in place and the straps will prevent any tendency to start slipping.
Step 6 - Fit Root Barrier
Inside the frame, fit a waterproof root barrier. This can be a pool liner or heavy-duty plastic sheeting. This barrier will prevent plant roots from growing into the roof and add a second waterproof layer. This barrier can be fitted with waterproof glue.
Step 7 - Add Filter Sheet
The filter sheet helps retain the fine particles in the soil while allowing the free passage of water. You next must fit a filter sheet over the root barrier. These can be bought in hardware shops and garden centers.
Step 8 - Add Moisture Trap
The moisture trap helps to slow down the drainage of water from the garden. You can buy this or use old towels or blankets.
Step 9 - Fill the Frame with the Soil
Put a layer of clay pellets in the frame to aid drainage and then cover these with a mixture of soil and sharp sand to a depth of about 3 inches.
Step 10 - Add Plants
You can use grasses, mosses, and alpines to start your roof garden off and then work out which plants you prefer and which plants grow better.
Your new green roof will add to your garden and replace the area taken up when the shed was built.