wallplate position

srf

Traditionally the wall-plate is placed on the inner leaf of a cavity wall construction to take the roof timbers, but my question is why not fit it on the outer leaf. Does it have to be on the inner one only, or is that just the tradition.

Bonzo Delaney

Depends on the roof construction really
Can't discuss structural specifics but usually the wall plate was kept to the inside to allow expanding metal or a slab to be fixed on to make up the gap between the joist and blockwork internally
Wherever the wall plate is fixed the roof must retain and acceptable level of triangulation to maintain its structural integrity. And it's load transferred sufficiently to the buildings foundation.

Johnnyhpipe

Inner leaf is a structural wall panel restrained by roof/floor.

Outer leaf, from a structural perspective, is essentially a rainscreen and nothing more. I have designed load bearing outer leaf in the past for some very bespoke projects, so of course anything's possible, but needs careful consideration. Why do you ask? I dont see the point??

Metric Tensor

Like johnnyhpipe - I've had to design it once or twice for specific reasons.

There's nothing to stop it being done but traditional cavity wall construction has evolved to have the inner leaf as the main load bearing leaf so you have to be careful to rethink/design the wall from top to bottom - lintels, wall ties, etc can all be affected.

Like I said though - not impossible but unusual and requires careful thought and design.

Calahonda52

Interesting.

Just to shoot the breeze a bit here for normal domestic houses

The cavity wall ( min 100mm leaves), with proper ties, or a 215 wide cavity block is all that is allowed in Part A for external walls, so I wonder what the positional design criteria are for a 215 wall for the wall plate.

I know it being on the inside face makes life a lot easier detail wise but is there any guidance/design criteria out there for wall plate position

sydthebeat

my standard eaves detail now has 2 wall plates, one on either leaf with both strapped back to the inner leaf (to prevent twisting), rafters birds mouthed over outer leaf, ceiling joists fixed to both wall plates.

same detail for truss where the loading is designed on the outer leaf wall plate.

i work with 3 different structural engineers and all are happy to sign off on the structural integrity of the design.

Metric Tensor

I've done exactly that before too syd - no issues if installed correctly and wall below is sufficiently kitted out.

The extra wide cavities happening on many builds these days can cause problems with projecting rafters from an inner leaf wall plate.

Calahonda poses an interesting point. In every case I've seen with a 215mm inner leaf the wall plate was kept to the inside of the leaf. In theory this would cause a slightly uneven "torquey" load on the wall - you'd have to give EN 1996 an in-depth read to see how you officially calculate for that!

In practice it might be the uplift that would be more problematic.

Tom Hagen

if you sit the wall plate on outside leaf, how do you strap this down onto the walls? do you fix the restraining strap onto the outer leaf?