at 90 degrees
3. 7ft square frame built around top pair of boards using (2 × 6). Two 6 inch coach screws to each corner
Beams measuring 4 × 3 inches (10 × 7.5 cm) are immensely strong – probably far too strong for the job. Given that the trunk is likely to be uneven, they will almost certainly have to be different lengths. First cut them roughly to size, being generous. The hard part is cutting the join where the top of the beam meets the platform.
The strength comes from the fact that the platform sits on a flat surface at all four corners. The joint for this looks a little like a bird’s open mouth. Cut it by hand, marking it out carefully. The first task is cutting a ninety-degree triangle with two saw lines.
Mark a point 4 inches (10 cm) from the end on both sides, then draw a line to it from the opposite edge. Repeat to give you two diagonal lines. Where they meet is a neat centre point. Measure it all twice. Cut from the edge inwards.
The second and spatially trickier cut is straight down on one of the cut edges. Again, measure carefully and cut. It might be worth practising on a bit of scrap wood first. You should end up with four ends that fit neatly inside the corner of the main platform and support it as well.
Eight-inch (20 cm) coach screws might seem excessive to attach the four diagonals to the trunk, but everything rests on them. Drill through the four-inch length of the diagonal beam, so another four inches of steel goes into the tree. Don’t worry, you won’t kill it. Trees are very resilient and a good park keeper’s pruning does more damage than this.
When the four diagonals are in place, the platform cannot tip without actually crushing one of them. This is practically impossible. We tested the strength by putting six adults up in the finished treehouse, with a combined total of more than sixty stone (380 kg).
We used offcuts of 2 × 6 to add bracers to any spare gap in the platform. Rather than our usual overkill, this was to support the decking. Make sure you leave a gap for the trap door. We used standard pine decking available from any DIY store. It has the advantage of being treated against damp – as were all the timbers here. Getting them treated is a little more expensive but makes the difference between a treehouse lasting ten years and twenty. We screwed the decking straight into the bracers and main beams of the platform, using a jigsaw to shape it around the actual trunk. Leave a little gap to allow for tree movement and somewhere to sweep dust and dead leaves.
THE WALLS
It is easiest to assemble these rectangles on the ground, then hoist them into place. That said, they are extremely heavy, so use ropes and two people at least. Do not attempt to lift the section without it being held by a strong rope.
For each wall, 4 × 2 inch (10 × 5 cm) beams were used, with four-inch coach screws holding them together. We planned to cover the lower half of each wall with overlapping shed planking, except for one left open with just wire to stop the children falling through. It was absolutely crucial to have a drill powerful enough to send screws straight into the wood without pre-drilling. If we’d had to drill every hole first, we’d probably still be there now.
The shape was a simple rectangle with a ledge and a couple of support uprights. When you are deciding how tall it should be, remember that it is a treehouse for children. We went with five foot six inches, which was probably generous.
Each wall just sat on top of the decking and was screwed into it from above. Please note that it is going to feel wobbly at this stage. The four walls all support each other and when the last one is put in place, it becomes extremely solid. The roof will also add stability.
Also note that two of the walls will be shorter than the other two, so plan and measure these carefully or you’ll have an awful time. You may also have trouble with the heads of the coach screws getting in the way. Although it’s time-consuming, you may have to countersink these with a ¾ inch (16/18 mm) wood drill bit. As well as the four-inch coach screws, we used four six-inch bolts and nuts to bring the sides together.
THE ROOF
Once the four walls are in place and solid, you can think about the roof. We used eight joists of 2 × 4. The length will depend on the angles involved, but allow at least four feet for each one.
Cut them roughly to size, then take out a triangle near the end so that they will fit neatly over the top corner of the walls. In theory, this is the exact opposite of the lower diagonals, but we didn’t think it was worth cutting more ‘birdmouth’ joints.
Measure and cut very carefully here as one end will be in contact with an uneven trunk. Use six-inch coach screws (8) to anchor them to the tree. The roof supports only its own weight.
After placing the four diagonal joists for the corners, add four more between them, one to a side. Use a spirit level to be certain they are all at the same height, or your roof will be uneven.
There are various ways of finishing a roof, of course. We used a plastic roof membrane tacked to the eight joists with clout nails. Over that, we nailed strips of overlapping shed planking, known as ‘shiplap’. It looked very natural, but each piece had to be cut to size and then taken up the tree. We also nailed very thin battens on the diagonals for cosmetic effect.
The roof was probably the most time-consuming part of the whole process – and a good safety rope at that height was absolutely crucial. In fact, to reach the highest point of the roof, we had to stand on the window ledges, make a loop out of the rope and sit on the loop as we leaned out. To say the very least, this is extremely dangerous and for adults only.
Finally, we used the same overlapping planking to cover the lower half of the walls, then made a ladder out of decking planking. We attached the top of the ladder with loose bolts on the basis that it could be pulled up at some point in the future. It probably never will be though – far too heavy.
We made the trap door from offcuts of decking and some pine planking, screwing it all together. To pull the trap door closed behind you, a piece of rope hanging from an eyebolt is perfect.
To prevent the trap door dropping on small fingers, it’s worthwhile counterweighting it. To do this, get yourself a cloth bag of the sort you sometimes get shoes in. Run a rope through the trapdoor, with the knot on the underside. The other end should go through an eyebolt higher up the trunk and a third one out on the wall. Tie the bag of stones to the end and leave it dangling where the children can reach it. To open the trap door from below, they can pull on the bag. To close it, they pull on the knotted rope hanging down from the trap door. You’ll have to adjust the weight of the bag to suit the child, of course, and it means the trap door has to be pressed shut with a foot when you’re up there, but it’s much safer.
The important thing when it’s all done is to wait for a nice summer evening, take some cushions, blankets and a torch and spend the night up there under the stars. Take snacks – all that fresh air will give you an appetite.