Riverside Cottage 5

[Tim Lamb]

One for the structural engineers...

The architect has specified manufactured dormer trusses. When I asked him to supply a drawing for the manufacturers, showing the two studwork partition walls which are below the new timber (90deg. roughly midway), he said it was not necessary as they are not load bearing.

Now I was only ever electrical but they still made us do applied mechanics. A simple beam carrying a load has to deflect whether the load is distributed or not. Any deflection must cause some of the load to be carried by the walls below?

[harry]

Unless the roof is tiled before the studding is installed.

The trusses are made to standard size/spans, ie no allowance is made for any additional support. If the roof is heavier, they put them closer together.

[Tim Lamb]

This is domestic loading (bedroom/bathroom). I vaguely assumed they might reduce the floor joist thickness because of the additional support.

[GB]

That must be correct, unless the walls below are not fully in contact with the truss, or at least not taking much weight. As Harry says, one way to achieve that is to fully load the truss before building the studwork partition walls. I suppose the real question is what's under the partition walls, and can that take part of the roof load?

Houses are generally built with so much reserve strength that you can get away with all sorts of mess-ups like this.

[Tim Lamb]

Surely the roof load is carried by the outer walls and simply stretches/spreads the floor component.

It is a bit academic anyway as the thickness turns out to be the same as was used when the rest of the house was build in 1995.

[Tim Lamb]

This is chalet bungalow, Phil. The roof load goes on the outside walls no question. I wondered why they ignored existing internal partition walls when considering sizing the tie/joist supporting the bedroom floor.

[newshound]

Because that would really complicate the calc, you would need to look at what doors there were through the stud wall, for example. Easier to assume that it isn't there. Obviously, the studs may take a bit of load as the truss deflects, but a little bit of support mid-truss will be very good at reducing the truss deflection. You don't need to worry about the studs buckling. You are right that, in theory, you might get a bit of bowing in any studs which are tight up against the bottom of the truss. But it won't be significant (unless the build is really cocked up).

[GB]

I'd completely misunderstood before. I assumed these were first floor partition walls, and you were worried about them taking part of the roof load and transmitting that down on whatever the partition walls were resting on.

If the tie/joist was undersized because it was resting on a partition wall, what would happen when somebody decided to remove the wall? After all, partition walls aren't supposed to be load bearing, so your successor might decide to open things up a bit and go open plan.

[Weatherlawyer]

Your biggest problem will be if they lift the stud walls from the floors. I don't fully understand why that happens but I have heard of it happening.

[Tim Lamb]

OK All. I was just curious. Future proofing and ease of calculation seems the most likely scenario.

>

[newshound]

Perhaps from the studs shrinking after installation, particularly if ordinary sawn timber rather than kiln dried?

Interesting to see in "Building Alaska" how allowance is made for settlement / shrinkage of main wall logs by having "floating" window frames.

[Weatherlawyer]

I should imagine such shrinkage is confined to summer when a fresh slap of mud and moss will compensate, or ant cracks in winter are self fullfilling filling when it snows.