Building a garden cold frame – Step by step instructions

Building a garden cold frame is a great addition to your garden. Cold frames allow you to extend your garden season all the way through the winter!

Today’s article is a tutorial on how to build a 4 x 8 cold frame with four 2 x 4 plexiglass lids.  The original design for this cold frame came from Eliot Coleman’s book “Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.”

Building a Garden Cold Frame

Over the last 10 years, I have built 9 of these, and each time I tweaked the design a bit.  I’m pretty happy with this final design I’m going to show you today. My favorite improvement was adding a bolt and nut to each of the 4 corners of the box so that when you are not using the cold frames they will easily break down for storage. It’s so handy and is a great option if you don’t have tons of storage space. So let’s get started!

Tools & Materials

My suggestion is to get your hands on a couple of tools that will make your job of building a garden cold frame much easier.  The first is a chop saw.  There are a lot of repetitive cuts in this project and a chop saw makes quick work of those cuts.

Second, a table saw.  Using a table saw allows you to buy 2 x 4 lumber and cut it to the 2 x 2 pieces you need.  2 x 4’s are always much straighter than 2 x 2’s so your finished product will be better.

You will also need an electric skill saw and some type of electric drill.

Here’s a list of materials you will need:

• 4 – 2″ x 4″ eight foot boards
• 2 – 2″ x 12″ eight foot boards
• 1 – 2″ x 8″ eight foot board
• 3 – 1/2′ x 2″ eight-foot trim pieces (these are basically scrap)
• 4 – 2′ x 4′ pieces of plexiglass
• 8 – 3/8″ bolts and nuts. The bolts should be 4 inches long
• 15 – 3″ deck screws
• 40 – 1 1/2″ deck screws
• 8- hooks and eye bolts (if your cold frames are going to be in a high wind area)

I choose to make my cold frames from Douglas fir. So far they have lasted 10 years but are reaching the end of their life. Fir is 1/3 the cost of cedar or redwood so it just depends on how cost-sensitive you are.

Cedar or Redwood would probably last longer. I would never use pressure-treated wood because the chemicals used to treat the wood can leech out of the wood into the soil and contaminate my organic soil.

A few shop safety notes for you when building a garden cold frame.  Please be sure to wear eye and ear protection while you are working and please be sure you understand how to use your power tools, a new cold frame isn’t worth a finger or an eye!!

This project is pretty basic and I think most people with a good understanding of carpentry can pull it off.  For me, the work from start to finish took between 3 to 4 hours.  When my son helps, we can get one done in maybe 2 1/2 hours.  If you are a beginner at building a garden cold frame you may want to plan a whole Saturday.

Step By Step Instructions for Building a Garden Cold Frame

Step 1 – Cutting the Sides of the Frame

Cut one of the 2″ x 12″ boards exactly in half with the skill saw.  Then again using the skill saw cut each of those resulting 4-foot boards at an angle starting at 12 inches at the back and down to 8 inches in front.  For some reason when I did this project I forgot to take a picture of this step.  But you can see the end result in the picture above.

The top of each piece angles down from back to front.  There isn’t a specific angle you are cutting here, instead draw a line from 12 inches in the back to 8 inches in the front and cut along that line to create the angle.

Step 2 – Ripping 2 x 4’s in Half

Using your table saw rip all of the 2 x 4’s into 1 1/2″ wide pieces.  You will end up with eight- 8-foot boards that are 1 1/2 inches wide.

Step 3 – Additional Ripping

On the table saw turn two of these boards on their sides and rip them so that they are 1 inch tall.  The resulting boards will be 1 1/2 inches wide and 1 inch tall and 8 feet long.  This cut also leaves you with a couple of long 1/2-inch strips of scrap.  Hang on to these.

Step 4 – Long Sides of the Lid Frames

Using your chop saw cut four of the remaining 1 1/2 x 8-foot boards exactly in half (48 inches).  This gives you 8 four foot pieces.  These are the long sides of your lids.

Step 5 – Short Sides of the Lid Frames

On your chop saw cut the 1 x 1-1/2 inch boards (created in step 3) into 22 7/8 inch pieces.  The resulting 8 pieces are the short sides of your tops.

Step 6 – Creating the Channel for the Glass

This is the tricky part of building a garden cold frame.  You need to use your table saw to cut a 1/4 inch channel into all 8 of the 4-foot sideboards from Step 4.  The bottom of the channel needs to be exactly 1 inch from the bottom of the board.

This channel is for your plexiglass to sit in.  You will probably need to run each piece through the saw twice moving the saw rail a little on the second cut to widen the channel.  Check that your cut is wide enough by trying to run your plexiglass through the channel.  The cut only needs to be about a half-inch deep.

Step 7 – Predrill

I’m all about pre-drilling!!  There is nothing more frustrating than splitting a piece while your assembling.  So pre-drill two holes at each end of the 4-foot side pieces (Created in Step 4).  I like to use a 3/8 inch countersink bit for this so that the screw heads will be recessed.

Step 8 – Assemble the Lid Frames

Assemble the lids by attaching the long pieces to the shorter ones (from Step 5).  Remember that the shorter pieces are assembled so that the glass will sit on top of them in the channel.

Step 9 – Cut the Plexiglass

Cut the plexiglass to length and width.  Your plexiglass pieces will be a little too wide and tall to fit in the frames.  Take measurements of each frame and then cut the pieces using your table saw.  An alternative to this would be to build your frames before you buy the plexiglass and then have the plexiglass cut to the proper length and width by the store where you purchase the glass.

Both Lowe’s and Home Depot will do it for you.  I cut mine to 46 1/2″ x 22 3/4″ but I would really suggest you wait to cut until you have the lids assembled.  That way you don’t mess it up! Plexiglass is expensive so you don’t want to cut a $28 piece of plexiglass too short!!

Step 10 Add a Stop Block

The next step is to add a stop block to hold the glass in.  This can be as fancy or simple as you like.  I usually run a piece of scrap through the table saw to create a small notch for the glass.  Then I cut that piece into 8 smaller pieces and attach it with a screw.

Or if you don’t want to go to that trouble you could just add a screw to hold the glass in place.  Do not put the screw in the glass. The important part here is to keep the glass from sliding out either end.

Step 11 – Slide the Glass into the Lids

Slide the glass into the lids.  The glass should slide easily into the channel you created in Step 6.  The glass should overlap about 3/4 of an inch at each end.  Add the second stop block and your tops are finished.

Step 12 – Assemble the Frame

This step is the change I made that allows the frame to come apart easily. Cut 4 pieces of scrap 2 x 4.  Two pieces are 8″ long and two are 12″ long.  Set the 2 x 4 pieces flush in the corner and then drill 2 holes, one towards the top and the other towards the bottom.  The hole should be wide enough for the bolts you bought and should go through both the frame piece and the scrap.

Once the holes are drilled install the bolts.

Then put three 3″ deck screws in from the side.  The screws go into the scrap piece.  Repeat at all 4 corners.  Now when you are ready to take the frame apart at the end of the season all you need to do is undo and remove the bolts at each corner and the frame will come apart!!

Step 13 – Cut a Notch for the Stretcher

Next cut a notch in the center of the front and back of the frame.

Install your last 1 1/2″ x 4-foot piece as a stretcher and stiffener for the frame.  This piece is also handy for lifting and carrying the frame.

Step 14 – Add Scrap to the Bottoms

This step is optional but I like to take some of the 1/2 inch scrap pieces that are leftover and attach them to the bottom edges of the frame.  This is where the wood comes in contact with the ground the most.  This piece of scrap adds some separation between the frame and the ground and helps to keep the wood of the frame from rotting as fast.  You can even replace this scrap piece every few years.

Step 15 – Add a Fastener

This is also an optional step.  You can add some type of hook or fastener to each of the cold frame lids.  Do this if your frames will be in a windy area and you are worried about them blowing off in the wind.  Only one of my frames has this option added and I have really never had a problem.  The frames are very low profile and usually don’t get caught in the wind.

Well, there you have it!  Building a garden cold frame is as easy as one, two, three . . . fifteen!  Okay so it’s not easy, but it really isn’t that difficult either.  For around $130.00 you can have your own 4 x 8 cold frame!

You can learn how to use your new cold frame by purchasing a copy of my Year-Round Gardening Course:

What can I grow in a Cold Frame?

The list of crops you can grow in a cold frame includes spinach, lettuce, kale, arugula (rocket), bok choy, carrots, turnips, beets, radishes, mache, and any other cold-hardy leafy greens.  You can learn more about what to grow in a cold frame here.

How can I make a cheap cold frame?

Using recycled lumber will help bring down the cost of a cold frame.  A smaller-sized cold frame will also cost you less.  You might also want to consider using plastic instead of plexiglass, this will not be nearly as durable, but it will save you a lot of money.

Many people will use an old storm door or window on top of a wooden frame.  I have even seen people have success with the box being formed by straw bales and the top just being 6 mil plastic.  The point is to protect your plants from the cold wind and freezing temps.  My design works well and looks good but it is not the only way to protect your crops.

How deep does a cold frame need to be?

Having an angle on the top of a cold frame is important as this helps let in light.  So I would say a cold frame should be a minimum of 4 inches deep in front and 8 inches deep in the back.  But if you can go deeper  (more like 8 inches in front and 12 inches in back) that will give you more room for taller plants.

Does a cold frame need to be airtight?

No, it doesn’t need to be perfectly airtight, there is no need to seal seams or go to that extent.  But the less cold air that gets into a cold frame during the winter months the better.  So make sure your cold frame lids fit tightly and that you fill in any gaps at the bottoms of the frame so the wind can get inside.