Subservient dormers? Offsetting walls on steel?

[ChrisKingston]

We're working with an architect and structural engineer to design a loft conversion. Suburban semi and pretty standard - keep front roof and dormer to rear. We are going for full planning application rather than permitted development.

Surprise was that architect says new loft walls need to be set back 200mm from roof edge so dormer is "subservient" to roof shape (this is part of permitted development regs). Structural engineer now wants 3 or 4 big steels under loft conversion to support new walls just 200mm from existing brick walls up to top of first floor.

This seems a bit crazy, seeing as all the new walls are basically in same place as existing, solid brick walls. In London area so a builder is going to want many thousands to put four big steels in at second floor height.

Has anyone run into this before? Do you just have to shell out for the steel? Or are there other cheaper ways round it?

One idea I had was to put new walls over the existing walls and save the steel then make a little 200mm wide timber and tile construction off them to mimic being in a roof. Could that work?

 

[ChrisKingston]

Here are some rough sketches to try to show the difference.

What the architect suggests:

But could we do something like this instead?

That would save all the steels, and get us about another 2sqm as well.

Architect said if our existing roof eaves stuck out by 200mm or more then we could build over existing walls. But as they don't we have this set the new walls 200mm back into building on new nest of steel supports. Wouldn't it be easier just to build a new artificial bit of 200mm roof instead?

 

[^woody^]

200mm is for PD.

If you are applying for permission, you apply for what you want.

However, check for any local supplementary planning guidance for dormers - I've not seen any for the council's around me, but you never know.

 

[wessex101]

If you are submitting a planning application then the face of the dormer does not not need to be set back from the roof eaves as Woody said. However if your house has flush eaves then running the face of the dormer straight up from the wall below can look really naff. Installing a false projecting eaves might work if it does not look too incongruous, might be difficult though as it is a semi?.
I do not understand why if you are still working on the planning application you already have a structural engineer designing the supporting steelwork. If your planning application is refused the money spent on the structural design is wasted. Big dormers can be very difficult to get through planning which is why most people go down the permitted development route.
As regards alternatives, depending on the size of the dormer and other factors such as the roof and floor span, as the face of the dormer is so close to the external loadbearing wall and the bearing of the floor joists it might be possible to increase the size of the floor joists and reduce the centres to take the weight of the dormer?
The design process and input from the "architect" and structural engineer do not fill me with confidence.

 

[ChrisKingston]

Yes thanks for those. I agree that having the dormer appear to be a bit recessed does look better, at least on the more visible sections of roof.

I think with our setup could probably do false eaves, and there would have to be an equivalent structure over the top of the existing walls anyway on the current design. I wonder if they could be completely cosmetic then we wouldn't need to worry about making such a small section of roof watertight. Has anyone ever done/seen anything like this?

I would also be happier if the architect and engineer were looking at ways to build the walls with cheaper intervention than just putting in loads of steel. The architect has told me steel RSJs are only about 50 quid a metre but this is a tiny fraction of what a builder round here will want to install them in a new second floor structure. So yes just beefing up the floor joists would be better.

Yes we have paid for a proper architect not just a technician to draw so I was hoping for quite a bit more design input too.

 

[noseall]

The architect has told me steel RSJs are only about 50 quid a metre

Lol.

 

[ChrisKingston]

Lol.

Exactly

 

[canofcoke]

If you are submitting a planning application then the face of the dormer does not not need to be set back from the roof eaves as Woody said. However if your house has flush eaves then running the face of the dormer straight up from the wall below can look really naff. Installing a false projecting eaves might work if it does not look too incongruous, might be difficult though as it is a semi?.
I do not understand why if you are still working on the planning application you already have a structural engineer designing the supporting steelwork. If your planning application is refused the money spent on the structural design is wasted. Big dormers can be very difficult to get through planning which is why most people go down the permitted development route.
As regards alternatives, depending on the size of the dormer and other factors such as the roof and floor span, as the face of the dormer is so close to the external loadbearing wall and the bearing of the floor joists it might be possible to increase the size of the floor joists and reduce the centres to take the weight of the dormer?
The design process and input from the "architect" and structural engineer do not fill me with confidence.

in a lot of projects it makes sense to appoint a good structural engineer prior to any design work, this way you can work around structural constrainsts and design something that is easier and cheaper to build that fulfils the brief.

 

[freddiemercurystwin]

A good designer will know their way round dormer roof structures, certainly enough to know a set-back is more expensive, it would be very unusual to consult an SE at the planning stage unless the viabilty of the project depended on it. So that can only mean one thing - the designer in this case is not very good.

 

[canofcoke]

A good designer will know their way round dormer roof structures, certainly enough to know a set-back is more expensive, it would be very unusual to consult an SE at the planning stage unless the viabilty of the project depended on it. So that can only mean one thing - the designer in this case is not very good.

I disagree, in my experience the input of a structural engineer at early design stages can be invaluable.

 

[freddiemercurystwin]

It can be but not here, the designer is a plonka.

 

[ChrisKingston]

The sequencing of getting a structural engineer in at this stage is probably more my fault. But it has flushed out design decisions the architect took without consulting us, and which now look quite expensive.

He is still trying to tell me that offsetting walls is cheaper and easier. I'm not convinced.

 

[canofcoke]

The sequencing of getting a structural engineer in at this stage is probably more my fault. But it has flushed out design decisions the architect took without consulting us, and which now look quite expensive.

He is still trying to tell me that offsetting walls is cheaper and easier. I'm not convinced.

cheaper and easier than extending your roof by 200mm? This would require planning, and be an enormous pain to do. Why can't you build it all in timber? Why do you need steels at all?

 

[freddiemercurystwin]

cheaper and easier than extending your roof by 200mm? This would require planning, and be an enormous pain to do. Why can't you build it all in timber? Why do you need steels at all?

He's applying for planning anyway.

 

[canofcoke]

Has your engineer or architect suggested a structure that either uses the existing loft floor structure if it's usable or replaces it with a new timber one that is strong enough, this means all structure is built from timber. This can be disruptive to a home but is a better way of building lofts in my opinion. This would leave you free to put your rear dormer wall where you want and probably allow you an easier and simpler way of constructing a loft.

You might even be able to avoid party wall issues that way. Assume your architect or engineer has discussed this withy you?