Lateral Support

23 Mar 2008
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United Kingdom
I am replacing the ceiling/floor joists in my loft for a future loft conversion. And I am planning to lower the bedroom ceilings by 2 courses of brick.

At the rear of my house, Mid victorian terrace, the wall plate is on the outer skin, so the SE drew up a ply box structure that tied my new lower joists in with my rafters.

this was going to be used on the front and rear of the house. I have since found out that the front wall wall plate is located on the inner wall.
So instead of taking out 2 courses of brick, I have a wall plate in my way.

Now am i right in thinking that i just use joist hangers off the wall plate, and then I need to use twist wall straps to tie in the joists with the wall/wall plate?

But I thought the leg on a normal wall strap had to hook over the first skin, into the cavity to make the lateral support? But I have a wall plate in my way?

Sorry if i haven't explained this very well
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So, you are going to remove all existing loft joists and replace them with a new set lower down, then build some kind of ply and timber 'box' structure that ties the wall plate down to the new joists thus countering spread issues?

And where the wall plate sits on the outer skin you are somehow going to link the rafters rather than the wall plate via this ply box thingy?

Are you going to be doing all this with the roof tiles still on? You must have a purlin each side also?

By the way, what did your SE say when you asked him the question?

Any chance of seeing a drawing and how all this is going to work?

There's a typo error with the 19mm ply, should be 12mm.

The ply box structure is used because the wall plate is on the outer skin, as in the diagram.

But the front of the house, unknown to us at time of visit by SE, the wall plate is on the inner skin. Which i think makes this ply box structure incorrect.

So how does the new lower joists tie into the front wall to achieve the lateral support?
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The ply box structure is used because the wall plate is on the outer skin, as in the diagram.
Isn't the ply used because the joists are lower than the rafters? Usually the joists act as ties by connecting directly to the rafters and preventing the roof from spreading.
The ply does the same thing, as long as your SE has done his calcs right and taken all of the rotation force into account.
If the rafters and joists are the same distance apart then the same detail can be used, regardless of whether the rafters sit on a wall plate near the inside or outside of the wall.

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