how to join timber gallows advice needed please

[happyhero]

Hi I am making an Oak gallows construction and would like some advice how to join the diagonal part for the best strength.

I want to mount a beam on the ceiling with an upright each end and a gallow construction both ends.

There is not much room for movement and the beams that I am using are very heavy so it is not something I can make on the ground and lift into place. I did weigh the beams, the horizontal piece is 3.5m long just over and it is 250mm x 150mm and weighs about 18 stone, the uprights are 200mm x 150mm and about 2.25m long and weigh about 8 stone each.

The diagonals are 150mm x 150mm.

What I am planning to do is to make holes in the walls each end of the room for the horizontal beam to go into as it the beam is about 7cm longer than the space, ie I will end up with 3.5cm in the wall each end. I will hold the beam up with support acros and put slate, brick and cement etc in each end to support the horizontal beam partially but I realise this is not enough so I will then fit in the 2 uprights tightly (they will be standing on a concrete floor) so that they act as the main supports.

I am not a professional carpenter but I am fairly good with timber and have done stuff like this before.

Last time I did something similar I made a mortice and tenon affair to join the diagonal and paired it away until I could actually slot in the diagonal into the already fixed in place horizontal and vertical parts (obviously the diagonal can not just slot in if the horizontal and verticals are already fixed in place).

I used home made oak 14mm pegs to lock it all into place with offset holes for tightness. It worked pretty well but left small gaps at the diagonal to upright joints, not a disaster but a little annoying. I also wondered about how strong this would be. That was for an outside structure, this time it is for an inside structure and so I want it to be even neater and stronger as there is a chance kids may grab the diagonals and hang off them while playing.

Doing it the way I did with the tenon paired away to slot in obviously it was floppy in the join until the pegs were driven in and the faces touched. I would have liked the tenon and mortice to be much bigger but could not keep it all big and still slot everything in, in place. I also used a bit of Gorilla glue as this fills the gaps a bit as well as sticks.

The ones I did before are very strong but I would still like the advice of others, ie

Is there a special way to do this?

How would you fix the parts together?

Would you cut the tenon any special way?

Here is not the one I did but a very similar one, so that you know what I am talking about.

Untitled

• happyhero
• 8 Dec 2010
• 1

Any advice/help appreciated.

 

[foxhole]

Is this structural or decorative, if the former then the brickwork should be cut back much further to allow penetration into each wall.The diagonals should be mortised for full strength, it's quite possible to lift the whole unit with the correct kit.

 

[happyhero]

Hi foxhole thanks for your reply, it is just decorative and just about goes into the 2 end walls (3.5cm) each end so that’s why I am using the uprights to give a lot of support as I realise 3.5cm is not enough on its own. It was the longest Oak beam I could get at the time, otherwise I would have gone for another 150mm plus in length.

Although I am pretty good with carpentry I think trying to lift 40stone of Oak upright, even with help, would be difficult for me and probably costly, I appreciate that you mean with special equipment but funds are a bit limited for me. Plus if the horizontal beam is about 10inches wide wont it hit the ceiling before it gets to the fully upright position, ie the diagonal will be longer than the height when upright fully, hope I am making sense.

So because of this I am stuck with making mortise and tenon’s that can be fitted with the horizontal beam and vertical posts in place, i.e. they will have to be loose as far as I can see and then tightened with pegs or similar, I was just thinking that there may be a better way with the mortise and tenon’s if they have to be fitted with the beam and posts in place, i.e. carpenters must have to deal with this problem sometimes.

Am I doing it the best way or is there any improvement on my method that I could use?

 

[ladylola]

If you cut your mortises square then lift up your beam (maybe supporting on acrows if necessary) it's straight forward. Cut the post and brace as per normal but ensure the tenon on the top end of the brace is cut square to the beam i.e. parallel to the post. Then join your post and brace together and you will be able to lift these components vertically upwards into the mortises in the beam. You may need to excavate a small recess in the floor to do this if your post goes all the way to the ground.
An alternative if you don't want to damage any of the exsisting structure is to use slip tenons on the top two mortises. Cut the beam as above but cut corresponding mortises into the top of the post and the brace. For cosmetic considerations these mortices don't need to go through to the visible face. Then with the brace and post assembled slide it over the slip tenons protruding from your beam and fix.

 

[happyhero]

Hi ladylola, thanks for that, yes the posts do go all the way to the ground and I can see what you are saying if excavated the floor a little but for various reasons I prefer not to damage the floor so I prefer your second method to use slip tenons, this sounds good but i am struggling to visualise how this looks or works. I have googled it under images but cant quite see from the pictures what is different to and ordinary mortise and tenon so that it would work in my case. Do you know of any good pictures of this on the net?

Apologies if I am being a bit thick here but you mention “top 2 mortises”, what does that mean surely there is only one at each end of the brace?

You also say cut mortises into the top of the post, why the top of the post, is that a typo?

You say the mortise does not have to go through the visible face, do you mean don’t cut it as wide as the brace so that it is hidden? When I did this once before I cut the tenon 1cm back from the edges all round so that you could only see flat face to face contact and actually had a big chunky tenon and obviously a mortise to match, but as I said I had to pair the bottom tenon away at its bottom so that I could actually get it in, this left it tight to the right and left but loose when it came to upwards and downwards movement, until I put the pegs in place.

 

[ladylola]

No typo.
The mortises are placed into the beam AND the tops of both the brace and the post. The tenons are a seperate piece of timber. If for example the mortises are 3" deep then the tenon is 6" long and therefore fits into both timbers.
By the mortises not going through to the visible face I mean-
If for example your post is 200 x 150mm (and here I assume that one of the 150mm faces sits against the wall) then the mortise is cut from the back i.e. open at the back face to a depth of say 175mm to leave it 25mm from the face. When it's all put together then the tenon or mortise won't be seen. Same thing on the brace.
Hope this is clear but I'll try to upload a sketch.

 

[ladylola]

Sorry for the crudeness of the sketch but hopefully it should be understandable

 

[happyhero]

Hi ladylola, thank you, thank you, thank you, that is very helpful, you are a diamond, I thought my replies might be getting too long and things too complicated to sort this out on here but you have done a wonderful job, of explaining this.

I not sure if I need to mortice the upright posts at each end into the horizontal beam as they are from floor to beam and going to be squeezed in and have their backs against the walls, so I had not thought of mortising them but perhaps it will keep things tighter, and stop future movement, do you think it would be wise to do this?

Also last time I did one of these gallows I was told to offset the holes so that everything pulls in tight afterwards, but although I did this and made some 14mm pegs to push in I never knew how much I should offset the holes and was worried if I did it too much my pegs would break, how many millimeters would you say is right, i think i did about 2 or 3mm last time but it seems I should have offset more than this?

Also would you advise glue with this as well or do you normally rely on the pegs alone?

Thanks for the explanation and diagram you got the point across beautifully.

 

[ladylola]

I'd always stick with the mortise affair as it as you say stops any future movement.
As to the offsetting of the holes , or draw dowelling as it is also called, yes offset a little more but not really more than 5mm.Tallow your pegs when you knock them in .
In this , and most situations really, you can do without glue although it does add that "belt and braces" approach and it can be useful for filling small discrepancies. For this I normally use Cascamite powdered glue with a little burnt umber powder in it to remove the whiteness and bring its colour closer to the timbers.