Room in roof across king post trusses

[jayc89]

I want to make some storage space in our cold loft, which will hopefully also eventually be used for our boiler and UVC.

 

Ideally I'd use the area between 2x king post trusses we have. Distance between the two trusses is approx 2m, so ideally I'd create a 2x2m usable area.

 

Could I run suitably sized joists across the 2x tie beams, secured with joist clips and nogins, and lay Caberdek on top of that? 

[George]

Due to the point load of the UVC it'd be sensible to get an engineer to come and assess the trusses.

[jayc89]

1 minute ago, George said:

Due to the point load of the UVC it'd be sensible to get an engineer to come and assess the trusses.

 

UVC is still TBD, I need a larger one and could possibly go for a horizontal version, which would change the calcs again. Based on most joist span calcs going up to 1.5kN.m I'll at least double up on them and run cross members to spread that load across as many joists as possible. 

[George]

That's fine but the trusses still need to be strong enough.

[jayc89]

12 minutes ago, George said:

That's fine but the trusses still need to be strong enough.

 

Agreed. The tie beams are 8x8" or 200x200mm and span 7m, but my understanding is the king posts deflect most of the load to the external walls (hence not having an internal load bearing walls) meaning the tie beams I want to lay these new joists on are only currently working in tension. 

Edited September 20, 2023 by jayc89

[George]

The ties work in tension as part of the truss but it is the truss that needs to be checked. The compression in the principal rafters and struts, as well as the tension in the king post, would be considered as a whole.

 

It's up to you but you seem decided not to seek professional advice. I don't think anyone else on this forum could or would be able to provide any additional reassurance on your plan.

 

 

[jayc89]

18 minutes ago, George said:

The ties work in tension as part of the truss but it is the truss that needs to be checked. The compression in the principal rafters and struts, as well as the tension in the king post, would be considered as a whole.

 

It's up to you but you seem decided not to seek professional advice. I don't think anyone else on this forum could or would be able to provide any additional reassurance on your plan.

 

 

 

Not decided, otherwise I'd have JFDI and not asked for the advice at all We're all here to learn after all, aren't we?

[George]

Yeah of course. Sorry I didn't mean to come across as too negative but without more information it's not appropriate for forum advice. 

 

If just boarding out for light storage I wouldn't be particularly concerned. But a high point load could cause deflection, cracking and slipping tiles.

[markc]

In a king post truss the post is in compression, keeping the tie (bottom cord) and ridge from coming together. This is why it is very important to assess a king (or queen) post truss before loading the tie as the post could well be in loose tenons top and bottom. Sometimes the tenons are pegged but that was just to prevent it coming apart while being lifted

[George]

The king post is actually in tension. This is why you will see iron bolts or straps, to account for the poor performance of timber connections in tension.

 

 

[Gus Potter]

On 26/09/2023 at 09:02, George said:

The king post is actually in tension. This is why you will see iron bolts or straps, to account for the poor performance of timber connections in tension.

Good point from @George but what does he mean?

 

 

Imagine you have the weight of the tiles and some snow acting down on the king post rafters.. they want to bend inwards between the eaves and the ridge. The rafters and ceiling joist act as a triangle which is strong. The rafters are getting bent inwards (a bending force) and to stop the walls from speading out the ceiling joist is in tension. Goerge calls these the principle members I think. Now you can see on the King post diagram that the ceiling joist is long.. and may need to support a heavy lath and plaster ceiling, a well built plumber going into the attic and your holiday suitcases etc. To reduce the size of the ceiling joist the span is split by the king post so it hold the joist up in the middle.. more efficient and this is where the tension load comes from in the King post.

 

The Queen post truss is at times more efficient at times (depends on roof span and loading) as it splits the rafter span so you can use a smaller bit of rafter wood. Here the rafter bends down in the middle transferring compression into the diagonal struct which pushes the ceiling joist down in the middle, this downwards load is resisted by the vertical King post in tension which is in turn resisted by the main triagulation of the frame.

 

@jayc89 and for all. On a traditional timber cut roof you'll see the that the timbers are checked a little. If you have a 2" thick timber it may be checked by about 1/2 inch and this gives you a direct timber to timber contact area that is good for transferring compression... here the nails just hold the joint together. But as @george says joints in tension need some king of mechanical fixing and they are less efficient and on big roof you need the straps etc. Once you get into how old roofs work you appreciate what is going on and how elegant they can be.

 

@jayc89 Hope this helps a bit, post more info as interesting.

 

Maybe you can convert you existing member arangement into something like this.

 

 

Here you would move the vertical soldiers in so the ceiling joist is split into three equal spans and drop the top tie a bit so you divide the rafter span up also.. then you could maybe get you cylinder in?