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How to replace sluice gates

Updated: Aug 15

How to replace the sluice gates in a water mill

This is a departure from my normal ramblings about wilding, coppicing and patching up Le Moulin. This time we get technical. I’ve been searching for six months for information on replacing the sluice gates here at Le Moulin, but could find nothing and no one of any help. As usual, we take on the task ourselves. Both the replacing of gates and the writing up of the process in the hopes that we can save you making the same mistakes we did!

I knew the main project for our July stay here had to be replacing the sluice gates. The old ones might not last the winter, they have been patched up with tin sheet and you can push your fingers through the gaps in the planking. The purpose of the gates is to hold water in the Leet, which is the man-made stretch of river, like a canal that runs into the mill race. Water levels are critical if you have a water wheel to turn, which we don’t anymore, but maintaining the rivers and Leet are important parts of maintaining the Mill itself.

There are two main parts to the job. Removing the old gates and installing the new ones. Unlike most jobs, you can’t just do phase one then move on to phase two at your leisure. The time between the old gates coming out and new ones lowing into place has to be minutes, or you lose all your water in the Leet. If you have a few dozen sand bags to hand you could use those to stop the water, but we didn’t have any sand bags.

Sluice gates for a water mill being replaced
New sluice gate installed over old

I thought about planking off the sluice somehow, but that too was overly complicated. During my 6am creative dreaming time, I had the answer. We could take out the old sluice plank by plank and install the new one on top, plank by plank. With every complex project, there are things that are unknown, a moment in the plan of action, where you shrug and say, ‘we’ll make it work.’ It is the leap of faith moment, or the moment when a little magic is required. The final removal of one sluice and the lowering of another, simultaneously, was my leap of faith moment...

I hope it goes without saying that the winter, with swollen rivers, is not the time for any work on your sluice. The rivers in France struggle in July and August, there is a lot of illicit demand, agriculture, ponds, pools, lakes. So the water levels are always low, this month it’s down to about two feet, so a good time to take the plunge, literally.

Our existing sluice is made of planks that are 6cm wide, the metal structure supporting them can accommodate wider, so I decided on 7cm. I didn’t want to have to repeat this job in my lifetime, so I wanted them to last. There would be several reasons to regret this particular decision...

Measuring the old water mill sluice gates before removal
The wooden upright makes the seal to the metal structure

How it all works

The wooden sluice gate structure is simple. A vertical upright either side, with planks running horizontally that are bolted to the uprights. The uprights sit at the back with the main planks bolted to their face. The pressure of the water presses the uprights against the metal supporting structure and seals the gates.

The gates are raised and lowered with a geared approach. You attach a metal handle and wind a small ten toothed cog, this in turn winds a large sixty tooth cog, so your power is not increased by the gearing, but your torque is. The large wheel engages a smaller cog whose teeth interface with the teeth of the large lifting bar which has a 16mm bolt at the bottom which attaches to the swing bars bolted to the gate. The gate hangs from this single bolt so it must be evenly balanced. The system allows the gate to tilt forward and back, so the water pressure seals the verticals against the metal support structure.

sluice gate winding mechanism
winding mechanism

The large sluices control the level of water in the leet, and the smaller sluice controls the water running under the house that would have driven the water wheel.


Measuring is crucial and should be done at least ten times. The metal supporting structure of our sluice was made at time when men measured with their thumbs and hands, so each of the two main sluices are actually a different size, one is 143cm across and the other is 146cm. I decided on a two cm clearance, a cm either side. This is about right as the lip of the metal girders are 4cm. You want your gates to run freely, but you want as much overlap as you can, so the wooded uprights seal well against the structure. Too little clearance and the gates will catch at the sides as you try to close them. Too much clearance and they won’t seal with the water pressure against the structure.

You should make your gates the same height as the ones that are in place already. Their height should be the same as the overflow system that is in place where the concrete next to the sluice is at a lower level to allow the Leet to spill over into the river if the gates are not opened. Equally you can set the gate height ten or twenty cm lower than the overflow if you want to allow water to flow both under and over the gates when they are open a little. Most people think that the water should only flow under the gates, but many systems allow for under and overflow of the gate.

sluice gates for water mill
Sluice gates

The gates should be made of green oak. This is strong and will not overly expand when wet. It’s a pleasant surprise that green oak is cheaper than dried wood, as there is no drying process. It’s also twice as heavy as it’s wet. We bought ours from Balatieu, our local wood yard here at Nanteuil. 18 planks in all, 7 for the horizontals on the short gate, 7 for the wider gate and 4 uprights, 2 for each gate. The wood came to about 500 Euros, which was a good price. A local wood yard means no transport costs. It would have been cheaper if I’d have asked what their standard sizing was. I could have gone for 6cm depth instead of 7cm and got 1.5m and cut it down myself. This would have saved me about 120 Euros in thicknessing and cutting, as I had to go for 8cm depth planed down to 7cm. The advantage of getting the wood planned and cut to the right lengths is that you don’t have to do it yourself and importantly it is all square. Every surface must be square or your gates will be out and won’t hang straight or ride up and down properly. We tried to work to about 2mm tolerances throughout. This is quite a challenge when you’re dealing with such huge bits of wood, but its important to be accurate, so take your time, sharpen your pencils and polish your reading glasses!

· 18 planks

· 56x 15cm size 10 bolts with nuts and washers to secure the planks to the uprights

· 2x 8 cm 16mm bolts to secure the whole gate to the winding mechanism (these can carry about 9000kg so are plenty strong enough.

· 8x 10cm 12mm bolts to secure the lifting plates to the planking.

A deceptively simple shopping list.

Now comes the hard bit.

The hard bit

I was planning to do the whole thing on my own, but as luck would have it we had friends staying and one of them, Nicolò Della Chiesa, was keen to help. Every insane project needs heaps of luck, and Nicolò was mine. There is no way I could have finished the project without him, it is definitely at least a two man job, and if you’re lucky like me, you will find a second team mate who is happy to question methodology, quietly suggest alternatives, and provide a second brain and strong pair of willing hands. This is a difficult project involving very heavy wooden structures and tolerances of a few millimetres, it can’t be done alone.

The first thing we did was assemble our gates in the dry of the garage. It’s important to ensure everything fits as you expect before you get to the crucial stage of putting the gates in. It’s much harder to make changes when your waist deep in water trying to manoeuvre a half ton sluice gate.

pre-assembly of sluice gates for water mill
Pre-assembly is vital

It’s essential that all the holes are drilled dead straight. I’ll say this again, It’s essential that all the holes are drilled dead straight. Great if you’ve got an accurate bench drill, but we didn’t, so we used a cheap hand drill guide you can pick up online. Something like this is fine. You will need a very good set of drill bits and more than one for the 10mm holes, as the wood is tough, a blunt drill bit will not do it and you will get exhausted trying to drill the 56 holes.

It's vital that the whole structure is square, and a couple of large carpenters ninety degree squares would have been useful, but we didn’t have them, so we had to work of the wood itself, and we were lucky that Balatieu had cut the wood true.

First we placed the uprights down on the ground for the first gate. Then we took our first plank and put it in place on top of the uprights at what would be the bottom of the gate. Then we took the next plank and placed it on top of the uprights at the what would be the top of the gate, forming a square, the two uprights underneath and the planks on top forming the top and bottom. It’s very simple to do this as close as possible to the plane of the actual structure, so you can easily imagine lifting your gate up and putting it in place. We managed to do the opposite, and tied ourselves in knots trying to work out which would be left and right and up and down and front and back, when we put the gates in place. We should have stood facing the river and the gates, then left and right, up and down would have corresponded. Instead we got half way through before thinking properly about how our gate on the ground would end up in place on the river behind us. Either way, it’s vital to label all your pieces as you put them in place. Once you’re out in the river, you need to know for certain which plank goes where, so label, left and right, top and bottom and the open and closed faces of all your planks. We had 7 planks on each upright so numbered 1 to 7 with 1 being the bottom.

We spent a long time getting our four planks perfectly square. This is crucial if your whole structure is going to be square when you finish. We then marked a line 10cm in from the end of the top and bottom planks and marked these 5 cm in from the edges, these would be our drill holes. We did this with a template not a tape measure to assist in accuracy. As all our planks were 20cm wide including our uprights our bolts would run down the centre of the uprights and be evenly placed at the ends of our planks. Structurally this makes sense and it also looks better aesthetically to have a neat line of bolts. We drilled all the way through the horizontal planks through to half way into the uprights and put bolts in to hold the structure square and firmly in place. We put the rest of the planks in place and drilled them out, putting bolts through to hold them all firm.

Here's where the labelling comes in. It’s vital that once drilled, the order and rotation of the planks is not altered. We drilled through into the uprights about an inch, so the bolts went through the cross planks into the vertical. Even with accurate drilling, our holes were likely to be a mm or two different to each other, and this could be enough to prevent the bolt from going through, so – same plank – same place – same hole – throughout the process.

When we put the last plank in place we expected it to be a tight wasn’t. We’d forgotton that our upright was 141cm long so our 7x 20cm planks put in place, would leave a cm gap. Interestingly the whole thing fitted well to the eye, it was only our concern that the planks should be tighter, that made us realise we had 1cm of slack in the system. With our top and bottom planks braced and positioned, this looked like a problem. In fact it was an asset. We were able to drive 1cm wedges in between the top plank and the one under it, to force the lower planks in tight. We then simply repositioned the top plank 1cm down and redrilled the verticle holes for the top plank. Had we got the whole system tight from the start, I’m not sure we would have been able to fit the final plank.

Gaps and tightness

It's an interesting debate on how tight the planks need to be. We went for pretty tight. The top planks will dry out in summer, so we need to keep the gaps small. However, even 2 or 3 mm gaps will quickly fill with sediment. We left a 12mm hole in our gate for a few days as we were missing a securing bolt for the supporting plate. It filled itself in a couple of days. I was told by someone online that ‘in the old days’ a bullrush leaf was placed between each plank to ensure a good seal.

On bolts

I spent a long time wondering about marine bolts and the type of steel I should use. In the end any option other than standard zinc plated steel was too expensive. I concluded that the bolts would rust up pretty quickly and hopefully stay tight, I could probably tighten them each year for a bit, but they would soon rust up solid. If there were problems down the line, I would just grind out the rusty bolt and replace it with a new one. One decision I did regret was getting 15cm bolts for the main structure. I figured the width of the planks together would be 14cm, allowing 1cm for the nut. I thought the washer would sink into the wood a bit, and it did, but not much. This left the end of the bolt flush with the nut, which was not really enough, I should have gone for an extra cm on the bolt length to have more protruding. Or have gone for 6cm planks not 7 cm...

I picked these bolts with a round head and a square notch underneath. The square part sinks into the wood and prevents the bolt from turning as you tighten. This is essential and proved a good decision. When you’re waist deep in water one side of the sluice, you don’t want to have another person in even deeper water holding onto the bolt to stop it turning.

On holes

We then took all the bolts out in order to complete the drilling of the uprights. We wanted the holes tight to prevent movement, but a lesson here. The drill I used was slightly smaller than 10mm, not by design, so the bolts were tight, they needed wacking in with a mallet. This meant they had to be screwed out to remove them, which was a real effort with 56 of them. Thank you to Claire for her help on this one. As you’ll see later, we had to remove them again with the first gate. With the second gate, I drilled them out with a sharp 10mm drill then again with a blunter 10mm drill. The older, blunter drill was slightly bigger than 10mm meaning the bolts could be put in an out easily, this was very useful and labour saving.

I had planned to buy an 11mm drill, but Nicolò woke up in the night, shouting, ‘Don’t drill out the holes, there will be too much play!!”

The wrong removing methodology for the old sluice gate.

I was aware we were approaching the ‘moment of magic’ and I still had no idea how we were going to do it. Nicolò was increasingly sceptical that any of my methods would work. We dragged over a big old bit of chip board, with the idea of placing it infront of the sluice to stop the water. Nicolò gazed at it sadly “I don’t think this is the magic we need.” He was right. It rained that night and the chipboard dissolved.

We removed the lifting plate from the old sluice. This involved grinding out the 16mm supporting bolt and all the plate retaining bolts which were 12mm. The lifting plates were bent and Nicolò applied his superior intellect to straighten them, by standing on them. We then had the problem of the tin plate screwed to the gate as a make-shift repair years before. I tried a metal chisel on the screw heads and it just bent them over, I tried grinding them out, which was tough. We tried levering the plate off, which half worked. I then used an angle grinder to cut the plate in half. We folded over the tin, which left large slices of tin plate sticking out. Not great.

We then had to remove the old planks from the top of the old sluice, to make room for the new gate on top. The old planks looked like they’d fall out if we looked at them hard enough. I grabbed a hand saw, the first cm was easy, then the saw stopped. The inside of the planks was black and hard as iron. I took a small chain saw, not advisable when waist deep in water, and used that to saw through more of the top planks. It blunted quickly and I had to resharpen it twice. But it eventually got the job done. I than had to use a mallet and chisel to attack the remaining upright. Thankfully the previous sluice maker had use 10cm wide uprights, not the sturdy 20cm we had used. It took A LOT of work, to get the uprights cut through. We then levered out the top planks half covered in razor sharp slices of tin. It was...unpleasant.

When we finally had the first gate installed above, of which more later, I then had to cut out the lower planks so we could lower our new gate into place. Most of it was under water, and even I am not stupid enough to attempt this with a chain saw, so I got out the hand saw and began. It took about a day. Sawing underwater is as you’d expect it to be, really hard. The last bottom plank took me very close to having to hold my breath and go under to finish it off, but eventually with inches to spare, I got through it. By this point, there were only a few inches different in the water levels either side of the gate, as the old gate structure finally gave way, I was sat in a surge of water holding a saw, and trying to hold onto two pieces of heaving wood stuck with bits of tin. This was the magic. Or rather it wasn’t the magic. It was the getting it wrong bit you have to go through first in order to get to the magic.

The wrong way of installing the new sluice gate

Wind back in time, the old sluice is half out, just three old planks remaining in place. The new planks are stacked up ready to go in. The lifting mechanism is wound out of the way. I’m ready below to receive the planks and Nicolò is above, getting the bottom plank ready for lowering on a strop. I’m chiselling out a few shards of wood to keep our installing surface on top of the old sluice as flat as possible, I give the chisel a wack with my mallet, and suddenly the lifting mechanism bar comes crashing down, it smacks into the wood about a foot from my head, its 20kg weight driving the end a cm into the wood. That was a shock. We had no idea the cogs of the mechanism wouldn’t hold the bar in place. We would not make that mistake again. We secured the bar with a bolt between the teeth of the big lifting cogs. And for good measure Nicolò roped it all in place. This is a big lesson, do secure anything that can move, as it will move and you might be under it!

Studying the fit of sluice gate for water mill
Our perfect gate doesn't fit!

We then got the bottom plank down an in place, next we lowered the right hand vertical plank and tried to align the holes, they wouldn’t align, it was impossible. We studied the fledgling structure, frustrated that our very first planks didn’t seem to fit. I then realised that the vertical had ‘open face’ written on the side facing me, and ‘closed face’ written on the far side, this was the wrong way round. This is a good lesson as well. There’s no point in labelling something if you don’t explain what the labelling means. When I wrote ‘closed’ on the verticle plank, I meant it would be the side to covered by the horizontal planks, Nicolò thought it meant ‘facing away from the river,’ which might have been more logical, but hey. Took a while to unscrew the bolts from their too tight holes and start again. Soon we had the two verticals in place and the bottom plank secured. As we’d got all our angles right in the garage, the rest should be easy. We tightened up the bottom bolts fully and lowered the next plank into place, it fitted perfectly, at last! In went the bolts, fully tightened, and so on until we got to the fourth plank, which didn’t fit. It took a lot of manoeuvring, when we finally got the bolts through the horizontal plank stuck out past the verticle by half a cm. There was something very wrong, so we stopped, and we stared, and we stared some more. We examined all the labelling; we must have got something wrong. We even went back into the garage and imagined where the sluice had been constructed, then moved, then put into place. Our labelling must have gone wrong somewhere, but we just couldn’t work it out. I was all for getting a bigger hammer and forcing the gate to fit together. Nicolò suggested we loosen off the bolts a bit. We took off the nuts but couldn’t get the bolts out, we had to drive them out with another bolt from the other side. When the bolts finally came out, they were bent! We had no idea how this had happened, but we knew we had to start again, we began removing all the bolts, with some difficulty, to start again, as we loosened them off, the structure creaked and settled. The planks were moving back into alignment. ‘It’s the oak,’ said Nicolò, ‘It’s alive.’ Tightening up the first planks had twisted the green oak out of true, so it no longer fitted together. Nicolò remembered being taught this years ago about hot metal. You never fully tighten until everything is cool and in place. We were amazed at the amount of movement in the green oak created by tightening the system too early.

We put the whole thing in place, with the bolts loose, and it worked perfectly. We gently tightened them up from the top and bottom, and it worked. Now we had the saga of removing the rest of the old gate, see above!

With the old gate finally gone, we began lowering the new gate down into position. Nothing moved. The winding handle was stuck. Then suddenly it wasn’t. The half ton gate crashed down. The teeth on the cogs of the old mechanism were so worn that it couldn’t hold the new heavier gates. I tried not to think about this happening when I’d been underneath it. You can see that it took us a few goes to actually remember the lesson to secure everything above you, always.

We tried winding the gate back up, worried that the new weight was too much. But it worked! A bit clunky and the cogs stuck a bit, but it worked. Winding back down is harder with the worn mechanism, but it worked.

The gate was in place...and it leaked. Water gushed through on the right had side up to about a foot. It was somehow not sealing properly on that side against the upright. Our conclusion was that the new gate was so heavy and the new planks so wide, that it couldn’t rock properly at the bottom so the water pressure could not make the seal. We decided we could solve this potential problem with the second gate by chamfering the bottom of the uprights and the front of the bottom plank to properly allow for the rocking movement.

The right way to remove old sluice gates.

Lessons have been learned, and luckily no injuries. Though there could have been.

This time I used a screw driver to turn the heads of the rusty screws securing the tin, breaking them off, this was soooo much easier than the other methods I tried before. I used the angle grinder to cut through the tin, removing it in sections, so no jagged edges sticking out.

We then hoisted up the old gate as high as it would go, giving plenty of clearance above the water level. I used a bigger chain saw to saw up through the bottom planks, along the centre line, to a point that would be above the water line when the gate was lowered. This would mean no underwater sawing later. We then removed the planks above, leaving one plank intact. This method was A LOT easier. When we came to remove it completely, once the new gate was installed above it, I could easily saw through the top single plank left, and the whole thing came away easily.

A side note on partnerships.

I was all for charging ahead, I have an ability with these things to stop at nothing to be phased by nothing and get things done with force of will. Nicolò is much more considered, preferring to examine and think, he finds evidence, works things out fully beforehand. If we had employed just one person’s approach, we would not have got the project done at all, it would have failed. Nicolò would not have even started, because he would have foreseen problems that he couldn’t see a way of solving. I would have started and charged ahead, compounding each mistake, until I’d used my biggest hammer and got completely stuck, or worse. Between the two of us we were the perfect combination, each one balancing the other.

Post installation

Both our gates were in...and both different ways. The first gate at the bottom at the side, which we thought we’d solve with the second gate by chamfering the bottom, but that one now leaked underneath. The chamfer on the front of the bottom plank let more water through not less. I had to screw a backing plank onto the bottom of the second gate, against the water pressure and two feet underwater. Not easy, not pleasant, but I did it. This seemed to do the job.

The leak on the first gate remains a mystery. The concrete base the gate sits on is clearly not square or smooth which means the gate does not sit cleanly or square, I’m not sure what to do about this and welcome any ideas!?

I also discovered that the water comes through a number of holes in the concrete sluice structure. Over the years it has found its way through in about a dozen places. The idea that the new sluice gates would completely stop the flow through the sluice turned out to be false.

This is an important final lesson. The man-made leet keeps the water roughly where it is needed, though the water is constantly trying to find its way back to the original course of the river. Any structure you put in its way, over time, will only slow and direct the flow of the water it will not stop it. Water finds its way through the tiniest gap and over the years it makes that gap bigger and bigger.

We now have new sluice gates that slow and direct the water much better than the old gates, but in the end, the river does what it wants and all we can do is persuade it to behave itself for a while. When we are gone the man-made banks and structures we so lovingly maintain and replace will eventually also be gone. The river will go back to being a river, running wherever it wants, whenever it wants, pushing through fields, and eroding banks, our leets, sluices, gates and locks will all rot into the river, and eventually there will be no sign left of our work.

Until then, I continue to take great pleasure in the winding of the big iron handle, the turning of the rusty cogs, and the change in pressure and sound as the water is freed to thunder through the gates on its way along the river.

Finished and installed sluice gates for water mill
Finished - job done

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