First Joint – Scarf Joint, Rear Top Plate – the green oak barn

Saturday 21/11/20

Today my friend Ben arrived who knows a thing or two about building things with green oak. We will be working on the build together. It will look like he’s doing all the work as I’ll be taking the photos!

The first task was to join the two timbers that make up the rear wall plate (or top plate). This is the horizontal beam that that runs across the top of the posts along the rear (East) wall. Luke and I laid these out on Thursday (19/11/20).

In my O Level woodwork classes, one of the first things I learnt was how to make a piece of wood square in cross section. You started with one of the wider sides and went to work with a jack plane until it was flat in all directions. You labelled this side with the a special sign to indicate that this was the ‘face side’. Then you worked on an adjoining edge and made that flat AND at right angles to the face side and this became the ‘face edge’. With a face side and a face edge in place you can mark cuts and joints from these two faces with confidence that everything will end up nice and square.

Dealing with wonky timber

That process is fine for a small piece of timber but you can’t do this with a 4 metre piece of green oak that weighs a couple of hundred kilos, is already a bit wonky and likely to get wonkier over time. Another approach is required.

Instead of working from the wonky edges to make nice right-angled joints, you need to work from a centre line along the length of the timber and right angled reference points you create half way along the beam. We created the centre line by measuring the centre of the beam at both ends. We pulled a chalk line between these two centre lines and pinged it to get a perfect straight line the length of the timber. A hint from Ben was to use a square to help you raise the taught chalk line perfectly vertically before letting it go (pinging it). This side will become your ‘face side’. We then rolled the timber over on its side and marked centre line on that – this is our face edge. We will be doing all our marking from these straight centre lines, not from the wonky edges. These centre lines will also allow us to line up the timber end to end (see below).

We will be building the walls on their sides and then we will pull them up through 90 degrees when we put them in place. In order for the walls to end up perfectly vertical, they will need to be perfectly level when on their sides during the build. So how will we achieve this? It is not possible to get the top plates level along their whole length because they are wonky, but you can get it level at some point along its length and if we mark this point then we will be able to return the beam to this exact point of ‘levelness’ next time. We put the level at the centre point of the beam and levelled it by sliding wedges in between the timber and the trestles. We marked this point as shown below. We then checked the edge and by coincidence it was perfectly vertical. We marked this position too.

Face side/Face edge marks

When we went through the same process on the other beam, when the top was level the side was not vertical so we got a rebate plane and planed out a rebate that was perfectly vertical.

Face side/Face edge - rebate plane

These reference points will enable us to return these timbers to the same ‘near level position’ when ever we want to. We will need to do this when joining the posts and braces to this top plate while in a horizontal position.

Face side / Face edge marks

Making the scarf joint tenon

Before going anywhere near the timber itself I made drawings of all the the joints. This wood was expensive and replacement bits are not available form B&Q! There is no room for mistakes. The drawings also meant Ben and I could discuss and modify them, cooly and calmly over a cup of coffee. This was much more pleasurable than trying to have these discussions in the ‘heat of the build’. Doing detailed drawings like this was a wise move. I made many mistakes drawing these joints before getting a final version of each one.

Scarf joint WP2 and WP3

With the reference marks on the timber, it was time to mark out the tenon of the scarf joint. We did this by measuring out from the chalk centre line, double-checking each mark as we went.

We did all the cuts we could with a small handheld circular saw but this had a limited cutting depth and some cleaning up was needed using hand tools.

Tidying the shoulder for the first scarf joint tenon - 1

Completing the scarf joint tenon by hand

Tidying the shoulder for the first scarf joint tenon - 2

Tidying the shoulder for the first scarf joint tenon - 3

Lining up the timber to make the scarf joint mortice

As well as providing a straight line to measure from, the centre lines allow us to line up our two timbers precisely. To do this we placed our timbers end to end and cut a small notch at the midpoint of the leftmost and right most ends to take nylon lines and pulled these very tight.

Alignment lines - 1

We then moved the end of one of the timbers from side to side until the line laid exactly on top of the chalk centre lines on both bits of timber

Alignment lines - 2

Marking out the scarf joint mortice - top

With the nylon line lined up with the chalk centre line, we were able to mark out the mortice. We took as many of the measurements as possible from the tenon itself and the others from the above drawing.

Marking out the scarf joint mortice - side

Then it was time to set up the morticer. We set the depth by lining it up with the tenon and then bolted it in place on the timber that was to take the mortice.

These mortices are the only ones in the whole frame where the depth needs to be accurate. All the other mortices need to be ‘not too shallow’ rather than a precise depth. This meant that after the morticer had done its work we needed to finish the mortice to the correct dept with a chisel. We measured the required depth using an adjustable square:

Depth check - tenon

And transferred this to the mortice:

Depth check - mortice

The joint went together nearly perfectly. We just had to run the handsaw gently around joint shoulder with the joint in place to get a near perfect fit. This was the end result

Scarf joint from side

The final task was to mark the peg holes…

Marking for peg holes for the scarf joint

…and drill them.

Scarf joint - peg holes

The holes in the tenon were drilled slightly off centre to ensure that the pegs pulled the tenon in and down.

Pinned scarf joint

Job done.

Key posts

How it started
The plan
The site folder
Dealing with wonky timber
Moving into the third dimension
Raising the Barn
Putting up the softwood roof
Fixing the wall panel frames: Day 1
Roof complete


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