The size of the frame’s box is set by the size of the window frames, and the angle at which these will sit. It’s time to get the calculator out.
The first thing I realised is that my initial sketches were a bad idea. I had drawn the vertical pillars to go right to the top of the frame, with horizontal and sloping sections attached onto these pillars. This would mean that the end-grain of the wood is exposed at the top, where rain could more easily weather it. It would be better to have the end grain exposed on the sides of the frame, where it would be covered up by the feather board panels.
So, I went through a very quick redesign in PowerPoint. I created a rectangular form to sit below the windows and which in turn would sit on the vertical pillars. This design had the added advantage of requiring half the number of 15 degree cuts. Measuring an angled cut is a little more tricky than cutting at right angles, so this would save some effort.
The rectangular form would have two long wood strips across the front and back, with three shorter strips running up the sides and up the line between the two windows. I decided that the windows should overhang the cold frame box at the sides and the front, to accommodate the thickness of the feather board panels and to make sure rain didn’t drip from the windows into the box. The back edge of the box would need to line up with the windows so that the hinges could sit properly.
In the end I went with a 2cm overhang along the front, and a 1cm overhang at each side. This meant my long wood strips would be 120cm, and the shorter strips would be 57cm.
At the beginning of the project, I had bought a pack of four 2.4m long strips of 4x3cm plain sawn pine. I had so far used two of those strips to make the window frames, leaving two spare. There would be plenty of these to make this rectangular form. The rest of the box would be made up of 2x4cm and 4x4cm strips I had salvaged from the old cold frame.
The decision for the overhang, and the size of the rectangular form, set the pattern for the dimensions of the rest of the box. This requires some trigonometry…
A 57cm piece of wood that joins two 4cm wide pieces creates a section 65cm long. This section will sit at 15 degrees to the horizontal. The angle means that the section extends a distance of 65 x Cos(15) = 62.8cm horizontally, and it extends a height of 65 x Sin(15) = 16.8cm vertically.
So now I know that my frame will be 62.8cm deep, 120cm wide, and with a front height of 25cm, the back would be 41.8cm tall. With vertical pillars 4cm wide, the internal depth of the box, and hence the width of the table, becomes 54.8cm. This figure shows how all these numbers come together.
Now to saw up the wood for the frame. I need to cut at 15 degrees (or rather, at 75 degrees, to create a 15-degree slope in the top of a vertical pillar.) This would ordinarily be quite tricky, but I had decided earlier to upgrade a piece of kit that would make life much easier. You may remember a post where I talked about The Chap’s Frame – a picture frame I got my son making to keep him busy during lockdown. I mentioned in there that I had decided to order a new Mitre Saw to replace the wobbly old mitre block I had been using for many years. This mitre saw arrived just in time for his picture frame and my cold frame.
The Mitre Saw is great! It has a saw blade that is held in guides that keep it at a set angle. The blade slides smoothly back and forth and up and down to cut the wood, but it can’t twist horizontally or vertically so it keeps exactly to the line you want for it. It also contains a simple mechanism that lets you fix the blade to a precise angle, so setting a 75-degree cut is really simple. The six vertical pillars needed for the cold frame were cut in no time.
I had one more thing to think about for assembling the frame. I was using Decking Screws to join the pieces together, and it took some thought to make sure I could properly assemble everything. I would have places where three pieces of wood would come together, requiring two sets of screws to go through the same area at angles to each other. I had to plan out the screw positions so that they passed each other and didn’t clash.
So now the basic frame is assembled, and given some wood preserve. It looks ok on the ground – all the legs look about the right lengths, so my maths seems to have been good.
Attached to the table, the frame looks square enough, so I’m happy!
Time to start fitting the feather board panels. I have lots of these salvaged from the old cold frame, and some that were kept from the original shed, so should have enough to choose from. The tricky part is that I want to keep the sections whole if possible, and want to avoid having to join two short bits end-to-end to make up a longer piece. Some of the boards are split and weathered at their ends so it’s a little touch and go that I can do this. Fortunately, it all played out, and I had enough pieces to go round.
Before I begin on the hinges, I can now see I have some fairly fundamental errors to correct. I have made the unit so that the table is at a normal, comfortable height. Comfortable for me, that is, though I’m 2 metres tall. MakeWalkRead is not quite so tall. Also, by making the table this height, the top of the cold frame has ended up being super high, and is hard to reach into.
So, before I begin with the hinges, I have the unit on its side to saw off 20cm from each leg. This makes the table a fair bit lower, but it does end up much more in proportion. I moved the bottom shelf up too, to keep the space below and above the shelf the same.
While I’m at it, I correct a visual problem with the alignment of the legs. I had initially fitted the legs to the middle of the base, with equal overhang on either side. However, because the frame was offset to one side, this gave the appearance that the unit might topple over. I moved the legs over to the side to be more directly underneath the box. This gave a much nicer impression of balance, with the box getting the main support and the table sticking out to the side.
Now onto the window frames.
I don’t have a great track record with hinges. What gets me about them is that you can’t line them up easily to measure and mark up where you need to fit them. The alignment is best done at that point when the hinge is closed, so the thing you need to mark up is obscured by the other thing that you’re marking up against. At least here, one of the surfaces I’m attaching hinges to is relatively horizontal. So, I can begin by placing the hinges across the back of the box to mark up where they will sit.
I’ve chosen heavy duty, stainless steel hinges, which should survive the weather, but are quite thick. I will need to recess them into the wood to get the window frames to sit nicely without too much of a gap. Out with the chisel then…
I can repeat the process with recesses in the window frames, measuring carefully between the hinge positions to ensure the windows will be aligned with the frames once attached. I think I have everything lined up, so I can begin screwing them together. So far so good…
For some reason, the first attempt causes the two windows to rub together. I trace the problem to one of the hinges being attached incorrectly. The position was ok left-to-right, but it had been screwed in ever so slightly too far forward on one side, causing a slight twist in the position of the window frame. You can just about see this mistake in the picture above.
I need to reseat the hinge on the window frame. The trouble is, the new screw holes will overlap with the incorrect holes I’ve created. Time to execute a standard repair technique, known as ‘gluing a matchstick into a misplaced screw hole’. Once this has dried, I can trim the sticks down to give firm, filled surfaces into which I can make new screw holes.
That is much better! I now have even gaps between the two window frames and between the frames and the box. I can open and close the window lids and they don’t rub together.
And so, I get to fit the glass. I return to the angled edge mouldings I had cut to shape earlier. These would be nailed into place using galvanised pins. I begin at my workbench by hammering the nails through the moulding. Hammering near glass is bound to be risky, so I get the pins in place and lined up before going near the windows themselves.
With the glass in place, I can secure each piece of moulding in in turn. I covered the glass with a piece of hardboard, then I could drive each pin home by running the hammer head safely across the hardboard.
Pretty soon I had finished the windows. Everything sits comfortably, and they open and close nice and smoothly.
Something else I decided to make at this point was a selection of wooden shapes that could be used to prop the windows open for ventilation. I fashioned some scraps together to make props of different sizes that wouldn’t fall out or be easily knocked out.
Lastly, I sorted out the cold frame’s location and base. The old cold frame had stood on a selection of paving slabs that had been left over from the shed’s foundation. I decided to use these to form a floor underneath the unit that could be kept weed-free as a place to store compost bags and similar. I then sunk four old house bricks into the ground around this floor, to provide bases for the legs to stand. These let the unit stand sturdy and stable, and they kept the feet out of the soil. Hopefully this will keep the frame clear of things that will bring rot.
And that is the job done! We have a place to cultivate and propagate plants, a table to work at, a shelf to store pots on and a place to stash the compost. It’s made mostly from reclaimed wood – some of it has been reclaimed twice! It sits between the shed and the greenhouse, and next to the veg plot, and is all ready for use.