DIY Dormer Framing Questions

[scott08]

I'm currently working on my dormer loft conversion, doing it diy so bear with if any of the questions are a bit obvious!

I have plans through from the engineer but there isn't too much on the actual frame. I've attached the drawings - as part of this we have had to lower the FF ceiling, which is around 30cm lower now. Currently we have gotten the two steels in for the floor and all the new joists. We'll be taking the rear of the roof off shortly to frame it out, but i'm getting a little confused by a few of the details:

1. The loft had been converted many years ago (previous owner), with dwarf walls added to replace the purlins (pic attached). The SE said this is fine to replace like for like, down to the new lowered floor joists, but i'm not sure this is the best thing to do. It would mean a bottom plate directly onto the joists, then uprights screwed into each rafter. Is this acceptable or should it be constructed differently?

2. On the rear section of roof, where the joists change direction, there is no steel specced. I would have assumed the dormer frame therefore needs to sit directly on the existing wall, however it's a solid wall and the eaves are minimal so this almost definitely would not leave the required 200mm from edge of eaves. Is there a simple way around this?

3. The new wall plate at the front of the house (where the pitched roof is remaining) is now lower than the rafters, with original joists being removed. Do the new joists need to be tied back into the rafters, or does the dwarf wall achieve this?

4. I'm looking to get the outside clad in greencoat plx (will get someone to do this though). Does metal cladding typically require 50mm gap (battened and counter battened)?

 

[scott08]

Found this online... Would something like this be fine for the dwarf wall sitting directly on the joists, or would I need more substantial support/ birds mouth the timber in?

 

[^woody^]

The load bearing eaves walls should be designed. The drawing says they are assumed adequate, so who is confirming that they actually are?

You say that the SE says they are OK to replace like for like. yet the drawing says the existing are adequate, so that's confusing. Why are they being replaced?

Anyway, supporting timber walls should have a top and bottom plate, not just propped above joists and below rafters. These plates may be OK as 4x2 flat or perhaps they may need to be doubled up and vertical or thicker sections it all depends on the design. And if you are using CLS as the framework, then these top and bottom plates may need to be different sections to the uprights. You need to get a better specification from your designer.

The eaves set-back is crucial for PD, so if the dormer can't fit on the wall, then you will need a beam across to take it. Again, poor design and you need the designer to think about this.

Nr 3 is not clear, but sounds like more poor design information which you should clarify with the designer rather than guess

Any cladding on a timber frame would normally require a drained and vented cavity. You need to refer to the manufacturer's guidance. However you mention the dormer set back which implies the work is being done under PD, and yet unless you have a metal roof already your choice of cladding would not seem to conform to the PD criteria. You'd also be wise to check this with building control in terms of fire spread across the boundaries.

 

[scott08]

The load bearing eaves walls should be designed. The drawing says they are assumed adequate, so who is confirming that they actually are?

You say that the SE says they are OK to replace like for like. yet the drawing says the existing are adequate, so that's confusing. Why are they being replaced?

Anyway, supporting timber walls should have a top and bottom plate, not just propped above joists and below rafters. These plates may be OK as 4x2 flat or perhaps they may need to be doubled up and vertical or thicker sections it all depends on the design. And if you are using CLS as the framework, then these top and bottom plates may need to be different sections to the uprights. You need to get a better specification from your designer.

The eaves set-back is crucial for PD, so if the dormer can't fit on the wall, then you will need a beam across to take it. Again, poor design and you need the designer to think about this.

Nr 3 is not clear, but sounds like more poor design information which you should clarify with the designer rather than guess

Any cladding on a timber frame would normally require a drained and vented cavity. You need to refer to the manufacturer's guidance. However you mention the dormer set back which implies the work is being done under PD, and yet unless you have a metal roof already your choice of cladding would not seem to conform to the PD criteria. You'd also be wise to check this with building control in terms of fire spread across the boundaries.

Thanks @^woody^ , sounds like i'll have to go back with all these questions... Been so much back and forth already, i'm dreading it!

The dwarf wall is being replaced as it is currently sitting on top of boards on the original joists. As the floor has been lowered and there are new joists, all of the original will be coming out, so this dwarf wall will have to be extended to reach the new joists. I've drawn a section which should hopefully make more sense (and make point 3 clearer):

The cladding is fine here as it was specifically noted on the PD application, and we got the certificate of lawfulness before starting. Good shout on BC however, so I will check with the manufacture/ installer on the fire safety aspect when we get to it.

 

[^woody^]

Why can't the wall just be packed off the new floor and the existing ceiling joists cut flush with the wall?

What is bracing the rafters to prevent roof spread once you remove the existing ceiling joists?

 

[scott08]

Why can't the wall just be packed off the new floor and the existing ceiling joists cut flush with the wall?

What is bracing the rafters to prevent roof spread once you remove the existing ceiling joists?

i've got confirmation I can run the dormer wall off the new joists, no need for another steel, so happy days there.

For your second question, I think that's essentially what i was asking about in a roundabout way. Still unsure what that detail is supposed to look like, as it feels like I would need to connect the rafters back to the new (lower) joists somehow

 

[scott08]

I've got confirmation for the first two points - not super detailed, but i'm guessing it's because it's more clear cut than i'm making it out to be.

Still unsure on the 3rd point though, preventing roof spread, as I can't seem to get a proper answer (or rather the answer is 'nothing else is needed'). Anyone know the standard way of bracing the front section of a roof in this kind of scenario, where original joists are removed and floor is lowered?

 

[^woody^]

It may be that the ridge beam and the eaves wall will be sufficient to prevent spread. If the engineer is happy then just get it confirmed by the inspector and that's fine.

 

[scott08]

I've run into a new issue whilst putting in this new stud wall. The measurements were getting smaller as the studs were going long, which I initially assumed was sag in the rafters. After some more investigation, I can see that on one side the brick supporting the ridge board has cracked in half, making that side dip by 34mm:

.

It looks like the original stud wall (done roughly 20 years ago by the previous owner) must have been put in after this crack has happened which makes the situation a little trickier. Ideally I would prop up the ridge with an acrow, replace the brick, then rebuild the stud wall as I was doing. The issue with this is the current stud wall will be holding it down, and removing it to repair this will mean the rafters are unsupported in the middle - any ideas?

Current structure - some of the original studs remain (behind) and some of the new studs are in place:

 

[scott08]

Probably also worth mentioning that removing the tiles to reduce the weight on the roof isn't possible as I don't have scaffolding up...