Redoing an attic

[Garald]

Many of you probably remember my renovation saga. At any rate, what I have right now is an attic that, while pretty nice, (a) has an insufficiently insulated ceiling/roof, (b) has a low ceiling and beams - it's easy to hit one's head often.

 

(Also - (c) I'm going to look into the possibility of installing double-flux ventilation - there may be a way; two technicians will visit in mid-January; (d) I may want to insulate some walls from the outside - all of these are projects that need to be coordinating.)

 

What people understand by "lifting a roof" in France seems to amount not to lifting a roof, but to destroying everything and redoing the top level from scratch - a bit like a caterpiller doesn't really become a butterfly - its bodily functions cease, it dissolves, and a butterfly develops from its embryonic cells. Needless to say, this is expensive and destructive. So, I am trying to see what alternatives there could be.

 

Here is the top floor as it is nowadays. I have a few solar panels on the south side. Warning: the measurements in the floor-plan are not particularly precise.

 

 

Left is NWN, right is SES.

 

 

 

 

 

 

(Note: the rectangle on the right does not mean anything.)

 

What I can imagine as plausible changes, given my still very limited understanding of structures, labor costs, etc.:

 

a) Something like this, facing the SWS (street) side. Downsides:

- not that much gained space (defined as: the area where a 1.8m tall person can stand)

- changes visible from the street side may be harder to get approved

- the street is noisy - I would need the most noise reduction I could humanly get

- I could not get many more solar panels (not that I currently have plans to).

 

(image source: Zion visitors center, courtesy of DOE/NREL)

 

(b) Something more like this, with the changes on the northern half (not visible from the street):

 

Downsides:

- the structure of the northern half would have to be changed, and that of the southern half possibly reinforced - well, beam calculations are hardly rocket science, but I'll have to see whether I'd be required to hire an engineer (or whether it would be wise to do so);

- what is the cost likely to be?

- would I be able to reuse the skylights? I have about ten new skylights in good condition. It would be a good idea to reuse them, both for cross-ventilation and for solar gain in mid-season, no? (But then again, there is the issue of noise...)

 

I imagine people here will be able to propose several other possibilities.

 

What I imagine would not make any sense is any of the three options proposed by the fake architect, in that they would require scrapping everything (or so she said):

 

 

 

 

 

What do you think?

[SteamyTea]

Ah, Paris, the city of love.

Ménage à trois, though I think I spot a 4^th body there.

 

 

I don't know about the French regulations, but the UK has size limits.

[MikeSharp01]

So if you choose a lighter but better insulated roof a meter or so higher you won't be adding much weight on the structure below and maybe none at all if done with care and engineered well. I like the idea of the half lift but think that might be a structure too far - worth the discussion though.

[joe90]

So being France is it not up to the Mayor to approve?

[Garald]

4 hours ago, MikeSharp01 said:

So if you choose a lighter but better insulated roof a meter or so higher you won't be adding much weight on the structure below and maybe none at all if done with care and engineered well. I like the idea of the half lift but think that might be a structure too far - worth the discussion though.

The issue is whether simply lifting the roof 1 meter is something doable at all without changing the entire structure (note the lengthwise beams in the photos). You will notice that the pseudo architect proposed that in her sketches - but also said, verbally, that that would require scrapping the structure, and would take mire than 100k. So, what is the reality?

[Garald]

1 hour ago, joe90 said:

So being France is it not up to the Mayor to approve?

Yes, townhall would have to approve it. This is a suburb in the couronne rouge adjoining Paris (what Londoners would call zone 2 or perhaps the outer reaches of zone 1), and the people at the local town hall are pretty reasonable, but that’s why it’s a plus if things are not too visible from the street - easier approval.

[Garald]

8 hours ago, SteamyTea said:

Ah, Paris, the city of love.

Ménage à trois, though I think I spot a 4^th body there.

 

 

I don't know about the French regulations, but the UK has size limits.

Limits on size or on numbers?

[Garald]

6 hours ago, MikeSharp01 said:

So if you choose a lighter but better insulated roof a meter or so higher you won't be adding much weight on the structure below and maybe none at all if done with care and engineered well. I like the idea of the half lift but think that might be a structure too far - worth the discussion though.

Also, what should I read to prepare myself for a serious discussion? The last and only time I did a beam calculation was at 14 or so, when I tutored a neighbourhood kid one afternoon and got a dish towel as a reward.

[Mike]

1 hour ago, Garald said:

The issue is whether simply lifting the roof 1 meter is something doable at all without changing the entire structure (note the lengthwise beams in the photos). You will notice that the pseudo architect proposed that in her sketches - but also said, verbally, that that would require scrapping the structure, and would take mire than 100k

This would probably be the better option in terms of added value, as it would change the Loi Carrez floor area (the area higher than 1.8m) - worth verifying that with an estate agent?

 

It won't impose much additional load on the walls, but there's a good chance that all the timbers would be under-sized compared to modern standards and would need to be replaced, rather than reused. Of course you'd need a Structural Engineer to evaluate that.

 

I wouldn't want to guess the cost, but >100k wouldn't be a big surprise. Labour is expensive in France thanks to all the social costs.

[Garald]

13 minutes ago, Mike said:

This would probably be the better option in terms of added value, as it would change the Loi Carrez floor area (the area higher than 1.8m) - worth verifying that with an estate agent?

 

Sure, resale price and taxes depend mainly on Loi Carrez area. That’s one reason why I was suggesting the second option, with half the roof being lifted, rather than the first one: all of the northern half would become loi Carrez.  The resulting space would already be more than large enough given my foreseeable usage (and it would arguably be nicer than something very standard). The big question is whether I could do that at about half or not much more than half what it would cost to scrap the entire attic and build a new structure.

 

Also, in either case, can new skylights be reused?

 

 

13 minutes ago, Mike said:

 

It won't impose much additional load on the walls, but there's a good chance that all the timbers would be under-sized compared to modern standards and would need to be replaced, rather than reused. Of course you'd need a Structural Engineer to evaluate that.

 

I wouldn't want to guess the cost, but >100k wouldn't be a big surprise. Labour is expensive in France thanks to all the social costs.

 

[Garald]

I should add that I rather like my attic - it would likely be my favourite place in the house if it were not for the mediocre roof insulation and the constant risk of hitting one’s head against the beams. It was recently renovated to current standards (except of course for the roof insulation, which was only improved in the spaces around the new skylights). Hence my reluctance to scrap it entirely 

[Mike]

49 minutes ago, Garald said:

that’s one reason why I was suggesting the second option, with half the roof being lifted, rather than the first one: all of the northern half would become loi Carrez.  The resulting space would already be more than large enough given my foreseeable usage (and it would arguably be nicer than something very standard). The big question is whether I could do that at about half or not much more than half what it would cost to scrap the entire attic and build a new structure.

Your roof is already fairly shallow, so a key question is whether or not the tiles can be laid at an even lower pitch (I'd guess not), or if there's another tile that can. Otherwise you could go to zinc roofing, which can be laid at a very shallow angle & would be in keeping, but which would cost more than tile (and tend to absorb more heat from the summer sun - so add plenty of insulation). I'd guess that it may be possible to reinforce the other half of the roof, rather than replace it, so subject to the zinc cost, overall I'd guess that this may be a cheaper option.

 

1 hour ago, Garald said:

in either case, can new skylights be reused?

Yes, I would expect so.

[Garald]

33 minutes ago, Mike said:

Your roof is already fairly shallow, so a key question is whether or not the tiles can be laid at an even lower pitch (I'd guess not), or if there's another tile that can. Otherwise you could go to zinc roofing, which can be laid at a very shallow angle & would be in keeping, but which would cost more than tile (and tend to absorb more heat from the summer sun - so add plenty of insulation). I'd guess that it may be possible to reinforce the other half of the roof, rather than replace it, so subject to the zinc cost, overall I'd guess that this may be a cheaper option.

 

Are you referring to the half that would be left relatively untouched (except for having its insulation redone)?

1. If yes, why would the pitch need to change? I'd imagine one can leave the ceiling at its current height and add insulation by putting it above, and then laying the current clay tile on top again, no?  Or is this naïve? Note the solar panels are on top of the clay tile - they weren't installed in replacement of tile.

2. If you are referring to the other half, that would be greatly changed in plan b - right, if we were to have a lower pitch on that half as in my sketch, one would need a different roofing material (wouldn't clay pantile do?).  I sketched things that way in order not to create a large unnecessary volume (4.50m high on the north end) and to allow for one more degree of freedom ( = we could choose the pitch that makes most sense structurally, whatever that is), but you are right that there is a natural limit to the pitch; I doubt town-hall would approve a roof that is half clay and half tile.

It may be simpler to just choose one pitch, if structurally possible:

 

 

 

Edited Friday at 16:19 by Garald

[Mike]

53 minutes ago, Garald said:

Are you referring to the half that would be left relatively untouched (except for having its insulation redone)?

 

This part - it's now shallower than before.

 

54 minutes ago, Garald said:

wouldn't clay pantile do?

That depends on their minimum pitch - you need to research that. My guess is that they're already close to the limit.

[Garald]

9 minutes ago, Mike said:

 

This part - it's now shallower than before.

 

That depends on their minimum pitch - you need to research that. My guess is that they're already close to the limit.

 

Right. Well, then one could do a straight line, if structurally possible; perhaps one can even add a mezzanine. If the roof insulation is brought up to standards (or beyond; I think I should use wood fibre rather than rock wool - better for the summer) and one uses a mixture of double- or triple-glazed windows and well-insulated wall on the new vertical edge, then the place should be easier to heat (let alone keep cool) than now.

 

What the co-op and townhall would think of so much added height on the courtyard side - that's an open question and a different matter.

 

Is there some obvious structural reason why a straight line would be bad?

Edited Friday at 17:26 by Garald

[Mike]

4 minutes ago, Garald said:

I think I should use wood fibre rather than rock wool - better for the summer

+1

 

4 minutes ago, Garald said:

 

Is there some obvious structural reason why a straight line would be bad?

Not in principle.

[Garald]

So the main issue would be to learn enough structural engineering to (a) know what are the possibilities (b) know whether I'm hiring a good structural engineering (and hopefully get a reasonable rate in return for having already done much of their work for them). What sort of quick in-depth introduction would you recommend for a maths person (who knows a bit of physics, taught ODEs to civil engineering students twenty years ago, etc.)?

Other things to keep in mind: cross-ventilation (it gets hot in the summer up there, though that's no doubt mainly due to the substandard roof insulation), solar gain (... though given that it's generally a bit pitiful in winter and a nuisance in summer, I think it would be best to have large windows on the north side and not go much wilder with the skylights than I already have, no?).

[joe90]

39 minutes ago, Garald said:

So the main issue would be to learn enough structural engineering

Or, what the Mayor would allow before you invest too much time and effort into a project that’s going nowhere 🤷‍♂️

[SteamyTea]

1 hour ago, Garald said:

Is there some obvious structural reason why a straight line would be bad

It is how the forces vary with the angle.

 

 

When PO = PQ = 0° all the forces are downwards and the 'beams' are in compression.

When PO = PQ = 90° all the forces are spread along the beam, plus any point load, which puts part of the beam in tension, and part in compression.

 

But I am sure you know this better than most.

[Garald]

1 hour ago, SteamyTea said:

It is how the forces vary with the angle.

 

 

When PO = PQ = 0° all the forces are downwards and the 'beams' are in compression.

When PO = PQ = 90° all the forces are spread along the beam, plus any point load, which puts part of the beam in tension, and part in compression.

 

But I am sure you know this better than most.

Oh, what I’m lacking is any real-world experience and common sense about materials: isn’t wood (as opposed to masonry) ok in tension and not just in compression?

 

The existing beams are wood, except for a bit that was replaced by metal back when the attic was made inhabitable (this is visible in one of the photographs, I think).

[Garald]

1 hour ago, joe90 said:

Or, what the Mayor would allow before you invest too much time and effort into a project that’s going nowhere 🤷‍♂️

Sure - the people from town hall visited a few months ago, and now I have to engage in some back and forth over trivialities (a door was not as drawn in the pseudo architect’s sketches, we didn’t add shutters and instead added a second layer of windows, etc.) so I can ask. I’d certainly have to be pretty sure I’d get approval before hiring anybody.

 

This entire house project has ended up being about learning things that I hope will never be useful to me in later life (i.e., I hope never to renovate a place again) but learning a bit of engineering would be neither time consuming not useless; for one thing, it could be a source of interesting examples if I ever teach undergrads again (something one should also not necessarily hope for).

[Mike]

19 minutes ago, Garald said:

So the main issue would be to learn enough structural engineering to (a) know what are the possibilities

One thing that would help is to identify are the location of load bearing walls that can be used to reduce the spans, which will make timbers smaller & cheaper, and to think about how using them as supports them affects your layout possibilities. If you want a clear span then you're looking at steel (thinner) or (better for thermal resistance) glulam beams.

 

4 minutes ago, Garald said:

isn’t wood (as opposed to masonry) ok in tension and not just in compression?

Yes, it is.

 

25 minutes ago, Garald said:

What sort of quick in-depth introduction would you recommend for a maths person

It's too long since I studied the the topic. @Gus Potter may have some recommendations?

 

26 minutes ago, Garald said:

Other things to keep in mind: cross-ventilation, solar gain

And having a good summer bypass on the MVHR unit intake air - one that detects when summer external air is cooler and automatically passes all (or most) of the air around the heat exchanger, rather than through it, when the outside air is cooler.

 

1 hour ago, Garald said:

it would be best to have large windows on the north side and not go much wilder with the skylights than I already have, no?

Yes, sounds like a good plan, if that fits your room layout

[joe90]

27 minutes ago, Garald said:

This entire house project has ended up being about learning things that I hope will never be useful to me in later life

I am sure there are a few of us here that thought the same.

28 minutes ago, Garald said:

but learning a bit of engineering would be neither time consuming not useless; for one thing, it could be a source of interesting examples if I ever teach undergrads again (something one should also not necessarily hope for).

Oh I agree, i wish I had studied to be an SE, I love that stuff.[too old now).

[Garald]

1 hour ago, Mike said:

One thing that would help is to identify are the location of load bearing walls that can be used to reduce the spans, which will make timbers smaller & cheaper, and to think about how using them as supports them affects your layout possibilities. If you want a clear span then you're looking at steel (thinner) or (better for thermal resistance) glulam beams.

 

I don’t think there are any load-bearing walls in the attic other than the (short) side walls; the whole rest of the structure consists of wooden beams (and one metal plate and bolt). In the floors under the attic, there’s a long load-bearing wall down the middle of the house, and that’s it, other than outside walls (also masonry).

 

 

 

1 hour ago, Mike said:

 

Yes, it is.

 

It's too long since I studied the the topic. @Gus Potter may have some recommendations?

 

And having a good summer bypass on the MVHR unit intake air - one that detects when summer external air is cooler and automatically passes all (or most) of the air around the heat exchanger, rather than through it, when the outside air is cooler.

 

Right now I just have PIV. Two MVHR companies will visit in January.

1 hour ago, Mike said:

 

Yes, sounds like a good plan, if that fits your room layout

The room layout might improve, if anything (it would still be open, but the fundamental group would become non-trivial).

Edited Friday at 21:33 by Garald

[Garald]

Maybe these photographs give a clearer idea: