You can make large plastic domes outside to cover your plants in the wintertime, protect ponds from freezing or to create a greenhouse. Whatever the reason, plastic domes are easy to make and only require some patience and a trip to your local home improvement store. Large plastic domes are cumbersome to erect so enlist a few helpers to make the job easier.
Determine the size of your dome by measuring the area you are trying to cover. If you are making the dome for a greenhouse then choose an area that receives full sun.
Calculate how many arches you will need. Every 3 feet of area will need one arch.
Create double the number of stakes as arches needed. Cut 5-foot stakes out of 3/4-inch galvanised conduit. Use a pipe cutter to cut the conduit.
Pound the stakes 2 feet into the ground using a rubber mallet. Set a stake every 3 feet around the area. Make sure each stake is aligned with a stake opposite it. Slant the stakes slightly inward.
Decide how tall your arches need to be. Make the arches at least 60 per cent taller than the width of the area. Cut the middle arch out of 1 1/4-inch PVC electrical conduit. Make the middle arch the longest arch. Cut the arches on the sides of the middle arch so they are progressively shorter than the ones before them. This way the arches decrease from the top and create a dome shape.
Set the middle arch in place by sliding one end of the conduit over a stake. Pull the other end of the conduit down over the stake that is opposite it. Create the rest of the arches in the same way.
Create the spine of the dome. Pound a 4-foot piece of rebar into both ends of the area where the arches become shortest. Slide 3/4 inch PVC electrical conduit over the top of a piece of rebar. Pull the other end of the spine under the arches and out the other side. Slide the conduit over the rebar so the spine arches up to touch the high points of each arch. Secure the spine to the arches with duct tape and zip ties.
Lay 6 millimetre plastic sheeting over the dome structure. Center the plastic sheeting so there is equal excess of plastic on the ground surrounding the dome. Lay a few sandbags on top of the excess plastic on one side of the dome. Pull the plastic from the opposite side so the plastic is tight against the dome structure. Lay sandbags on top of the excess plastic all around the base of the dome to weigh it down.
Excess snow around the bottom of the dome can pull the plastic too much and cause it to tear. Remove snow from around the dome after a snowstorm. The dome's plastic may need readjusting periodically. After a storm, rain can collect between the arches. Make sure the plastic is tight against the dome structure to prevent this.