Thursday, 2 April 2015

Building a Cold Frame....

I thought I'd use some of our remaining static caravan windows, originally procured as a joblot a few years ago to make solar panels, to construct a cold frame to increase the amount of space we have to grow stuff under glass - as if two greenhouses weren't enough.... 

For the main framework, I'd previously found a source of used 6-foot scaffold boards from a depot a few miles away, who sell them off at a fiver apiece.  They had boards for sale of up to 13 feet long, which would have worked out a little cheaper on a per-unit-length basis, but then I couldn't have carried them home in the car.

So, after knocking up an outline design, I collected fifteen of these boards.  

the raw materials....

The first job was to remove the galvanised steel end capping strips used to prevent the boards from splitting.  This was initially a little fiddly using a hammer, screwdriver and wrecking bar to prise out the nails, but after the first couple I found my own modus operandi and it only took an hour or so to finish the lot.

The design was finalised in AutoCAD, sized to allow the flanges of the three aluminium-framed glass panels to sit on top of the frame structure.  The internal dimensions of the frame are 3,400 mm long by 850 mm wide, so it's just less than 3 metres squared in additional under-glass growing area.  

Then onto the construction, first cutting the boards to length using the cut-off saw - at around 9" wide, they were beyond the saw's slider capacity for a single cut, so they were cut through from one side as far as was possible and then the board was turned over and re-aligned with the blade to complete the cut.

Next stage was to fabricate the front and rear panel sub-assemblies.  Three board widths high at the rear, and two at the front.  Offcuts of boards were used for simple central lap joints, and short lengths of rectangular section timber were set near the edges, to form internal corner posts.

front and rear panel sub-assemblies

The two pairs of lower side panel boards were cut square to length, and then the frame assembly could begin, joining the front and rear panels using the side boards with screws into the corner posts.  

The upper side panel boards needed long angled cuts, which were made using the circular saw.  Then to finish the carcass, the two intermediate rails at the top of the frame were cut and screwed in position.  These divide the open frame top into three equal window bays, allowing the side edges of the glass panels to land onto a solid rail.

frame assembled, and a trial fit of the glass panels

A couple of coats of timber colouring on the external surfaces, and then the frame was ready to lift into position.  I dug a shallow trench in which to sit the frame edges - this was necessary because the site slopes slightly east-west but the frame will be set level.

The assembled frame was very heavy, and required two of us to slowly manoeuvre it into its final position in front of the larger greenhouse.  A narrow flagstone pathway separates the two structures.  This allows us to still get close up to the front of the greenhouse for maintenance and cleaning, and also provides a shadow gap so that the new cold frame doesn't throw shade over the soil in the greenhouse behind.   

lifted into position...the  pigs were totally uninterested

After packing and levelling of the frame, soil was firmed into the base of the trench from both inside and outside, using the heel of a welly.

So, onto the hardware finishing.  After an initial clean of the glass panels, brass hinges were pop-rivetted to the top edge of the glass panel frame.  The hinged panels were then fixed to the top rear edge of the cold frame and checked for clearances and ease of opening, etc.

hinge rivetted to glass panel frame

On eBay, I found three cheap aluminium casement stays, to hold the glass panels fully closed and also to allow partial opening for ventilation in the summer months.

casement stay....

I also bought some spring-loaded elbow catches to fit to the greenhouse mullions.  They were intended to be used to hold the glass panels in the fully-open position when preparing the soil, planting and watering etc.  Even though they're lying back against the greenhouse in this position, and the centres of gravity of the glass panels are behind the hinge centres, it's still not an entirely stable situation since a gust of wind could easily pull them back down with a crash, and so the elbow catches make working underneath much safer.

However, when these particular catches arrived they weren't really suitable and so I'll have to find something better.

Several bucket loads of our rabbit manure and three big bags of fresh compost were mixed into the frame, and job done !  

left-to-right - closed, vented and fully-open positions....

Total project cost including the compost fill, but excluding the glass panels which we already had, was £110.


  1. You're a much neater and better worker than I am, I'm envious ;)

    One major hazard I found with using glass was the wind - over the five years since I constructed mine gusts of wind have caught the glass when the panels were open and smashed every single one of the 8 panels I used, which are now replaced with box-section polycarbonate. Some of that is because we have volunteers opening these at times and they don't always think forwards, but I would imagine the wind is also a serious hazard up where you are. OTOH I never knew about elbow hinges - I used notched lumps of wood to hold the panels open and chains to keep 'em closed.

    1. Sorry for the delay in replying - been sunning myself on a very nice, stress-free, week-long work project in the middle-east...

      The wind is a serious problem here with the open western aspect. It can be dead calm in the town centre just a couple of miles away but blowing a gale here !

      I only ever fully open the frame glass to water inside and then close them immediately afterwards. And wooden stays were my first thought too, until I decided to hunt around for something more stable and permanent.