Old Roof & Building Regs - Will it take the strain?

[RobertG]

Hi,

 

I'm restoring an old mid 1800's cottage and adding an extension. Because the cottage needs major work the roof will need to be insulated to modern levels. My problem is that my architect thinks that the existing roof structure wont take the extra weight. The cottage has been re-roofed in the past but using only relatively small rafters and joists. Will Building Control demand that the roof is replaced? It'll make a big hole in my budget if this is the case. 

 

I'll attach a few photo's. 

 

Thanks,

 

Robert

[Gus Potter]

9 minutes ago, RobertG said:

My problem is that my architect thinks that the existing roof structure wont take the extra weight.

To provide a bit of context to become an Architect takes about 7 - 8 years, a degree coupled with professional development and on the job training. As an SE / Designer I agree with your Architect. Your Architect is giving you good advice.

 

If BC pick this up they may ask. But at the end of the day do you want a safe house or not? BC are not there to make sure your house is structrally safe or be SE's.

 

There may be some way to strengthen the roof in a cost effective way to make it compliant.. don't despair.

 

 

[RobertG]

6 minutes ago, Gus Potter said:

To provide a bit of context to become an Architect takes about 7 - 8 years, a degree coupled with professional development and on the job training. As an SE / Designer I agree with your Architect. Your Architect is giving you good advice.

 

If BC pick this up they may ask. But at the end of the day do you want a safe house or not? BC are not there to make sure your house is structrally safe or be SE's.

 

There may be some way to strengthen the roof in a cost effective way to make it compliant.. don't despair.

 

 

Thanks Gus,

 

I agree, the roof needs strengthened even just to support the new insulation. Hopefully it can be fixed as apposed to replacing. I'll talk to a roofer initially I think. He'd maybe be able to suggest a solution.

 

BTW, would you commission a SE to design the upgrade? Thanks.

 

Robert

Edited November 17 by RobertG
add my text

[nod]

Normal practice would be to install a 100 mil insulated plasterboard on the slope and 400 mil above the ceiling 

Decent insulation which will satisfy BC and won’t break the bank 

[RobertG]

5 minutes ago, nod said:

Normal practice would be to install a 100 mil insulated plasterboard on the slope and 400 mil above the ceiling 

Decent insulation which will satisfy BC and won’t break the bank 

Yep, I'm not concerned about the insulation. It's the effect on the roof structure that I'm worried about. If it needs strengthening or to be replaced. 

 

Hopefully not replaced.

 

Robert

[nod]

1 hour ago, RobertG said:

Yep, I'm not concerned about the insulation. It's the effect on the roof structure that I'm worried about. If it needs strengthening or to be replaced. 

 

Hopefully not replaced.

 

Robert

Then your next call is to a SE

Architects can only give an opinion 

Like the rest of us

Get something down on paper 

 

[saveasteading]

1 hour ago, RobertG said:

roof will need to be insulated to modern levels.

The Architect would be wrong not to advise of the possibility of the roof work.

Can you give us more info? what sort of insulation are you considering.

If it and any plasterboard etc are not too heavy then there is a decent chance of the roof you show us being fairly readily reinforced from within. 

 

Don't despair but work at a resolution.

[Iceverge]

That roof looks fine and straight. 

 

As a side note though, it's probably only in such good condition because of the lack of insulation which kept the timbers warmer and less lightly to suffer from condensation. 

 

If you do insulate, you will elevate the risk of condensation significantly. To counteract that you'll need to install an appropriate vapour control membrane. 

 

What's your aim for the house? 

 

Do you want to flip it or live in it? 

 

 

[RobertG]

5 hours ago, Iceverge said:

What's your aim for the house? 

 

Do you want to flip it or live in it? 

 

 

Thanks Iceverge,

 

I'm hoping to live here for the foreseeable so I'm committed to having a home that's as warm and dry as possible. In an ideal world I'd replace the roof but I don't have a huge budget and I hadn't allowed for it. 

 

 

[RobertG]

Thanks for the input everyone. 

 

Saveasteading: here's the proposed insulation for the roof. 

[Redbeard]

Ah,

 

That's why your archo has misgivings. The flexi WF is about 20kg/m3 heavier than PIR. However, get a SE to appraise the plans.  I have similar plans and my SE has just OK'd my roof for the additional load. Structure is not wildly dissimilar to yours but significantly steeper pitch and bigger purlins. (but mine will have a bit more flexi WF and  circa 5mm lime plaster, not 50mm Diathonite)

 

But hang on, that lay-up looks unusual. From the pic I had taken the rafters to be 75mm. However the spec says 100 between rafters, so I guess they must be 100! However no ventilation gap, and I cannot quite understand the breather membrane as an 'interstitial' layer.

[saveasteading]

2 hours ago, RobertG said:

Saveasteading: here's the proposed insulation

Aaah!

It is admirable to go for the sheep's wool  and the lime.

However, if you were to use modern materials you wouldn't have all that weight and nobody would be questioning the structure.

If you have to rebuild or severely reinforce the roof then that counteracts the sustainability intent of the design. 

 

Sorry to put a downer on the green ideas, but I think my suggestion could be argued ( shown in numbers even) as more sustainable, if that helps. 

 

Pragmatic sustainability. Try multiple methods, test the design, decide if it is appropriate.

[RobertG]

11 hours ago, Redbeard said:

But hang on, that lay-up looks unusual. From the pic I had taken the rafters to be 75mm. However the spec says 100 between rafters, so I guess they must be 100! However no ventilation gap, and I cannot quite understand the breather membrane as an 'interstitial' layer.

I see what you mean but what is proposed is 80mm battens attached to the 70mm existing rafters to make the 150mm then allowing for the 100mm FB with the 50mm gap (I think:)). 

 

I think the insulation proposals will need some tweaking as the initial proposal is going to lower the existing sloped ceilings by 200mm which will impact the room size significantly. Work to do I think. 

[RobertG]

10 hours ago, saveasteading said:

Aaah!

It is admirable to go for the sheep's wool  and the lime.

However, if you were to use modern materials you wouldn't have all that weight and nobody would be questioning the structure.

If you have to rebuild or severely reinforce the roof then that counteracts the sustainability intent of the design. 

 

Sorry to put a downer on the green ideas, but I think my suggestion could be argued ( shown in numbers even) as more sustainable, if that helps. 

 

Pragmatic sustainability. Try multiple methods, test the design, decide if it is appropriate.

Thanks for the reply. 

 

Yep, I'm starting to think the same. I'm committed to using breathable products on the walls as they were cement rendered outside and non breathable materials inside and the walls were sopping wet. Drying out nicely now but the ceiling is problematic with the materials suggested. 

 

Would it be possible to use modern materials and make sure the roof space is well ventilated instead?

 

 

[saveasteading]

2 hours ago, RobertG said:

use modern materials and make sure the roof space is well ventilated instead?

Yes.

[Iceverge]

Sheeps wool is ace for clothing. I love my 100% wool jumpers. I wear them every day until they are full of holes.  However I'm not a big fan of it for insulation. 

 

It's very dear and can attract wee beasties more than some other alternatives. 

 

I would not use it if you are budget constrained.  Cellulose is a great option, as in mineral wool. I'm usually not mega keen on PIR boards but I think @nod is spot on here. 

 

My suggestion for the roof is this. 

 

On the slant,

 

Slates

Bituminous Felt 

75mm ventilated gap 

2x50mm layers of PIR with joints staggered and foamed.

Inner face of PIR taped and returned to adjacent surfaces. 

Plasterboard 

Skim. 

 

Horizontal section. 

 

Ventilated attic

400mm of Mineral wool or celluloseabive and between joists.  (better!)Membrane returned to all adjacent surfaces. 

20mm battened service cavity. 

Plasterboard and skim.

 

 

 

[Iceverge]

Sorry I should have said to put a layer of 20 x 70 strapping under the PIR on slope to create a service cavity too. 

[Roger440]

22 hours ago, RobertG said:

Thanks for the reply. 

 

Yep, I'm starting to think the same. I'm committed to using breathable products on the walls as they were cement rendered outside and non breathable materials inside and the walls were sopping wet. Drying out nicely now but the ceiling is problematic with the materials suggested. 

 

Would it be possible to use modern materials and make sure the roof space is well ventilated instead?

 

 

 

When you say the walls were sopping wet, do you mean downstairs, or was it wet upstairs?

 

If so, that, surely must be an ingress issue. I see no reason to use "breathable" materials up here. Its no different in contruction to a modern house. They are all fine.

 

Id be doing, and in fact am, what nod and saveasteading are suggesting.

 

Out of interest, there appears to be an error on your drawing for the sloped part. It says to batten to leave a 50mm air gap, then 100mm insulation betweebn the rafters. That suggests they are 150mm deep. The pic, unless its very decieving suggest 75 or 100mm.

Edited Tuesday at 20:00 by Roger440

[RobertG]

18 minutes ago, Roger440 said:

 

When you say the walls were sopping wet, do you mean downstairs, or was it wet upstairs?

 

If so, that, surely must be an ingress issue. I see no reason to use "breathable" materials up here. Its no different in contruction to a modern house. They are all fine.

 

Id be doing, and in fact am, what nod and saveasteading are suggesting.

 

Out of interest, there appears to be an error on your drawing for the sloped part. It says to batten to leave a 50mm air gap, then 100mm insulation betweebn the rafters. That suggests they are 150mm deep. The pic, unless its very decieving suggest 75 or 100mm.

Hi Roger,

 

As far as I can see the walls downstairs were very wet. There were some damp spots on the walls upstairs but I haven't removed the plaster yet. When the cement render was removed on the outside the walls were wet top to bottom - cracks in the render and no way of drying for 50+ years didn't help but there are definitely some ingress issues around windows in the upper floor though. I've all the cement render removed, old crumbly mortar removed, back pointed and repointed. I'll probably need some sort of shelter coat as well as the exposed stones aren't in great condition.

 

Thanks for the input. I'm going to have to use a lighter method of insulation for the roof to negate stressing the roof structure I think. Good info here as usual. 

 

There does seem an error there. I'll need that looked at before anything is submitted. Cheers.

 

 

[Roger440]

16 hours ago, RobertG said:

Hi Roger,

 

As far as I can see the walls downstairs were very wet. There were some damp spots on the walls upstairs but I haven't removed the plaster yet. When the cement render was removed on the outside the walls were wet top to bottom - cracks in the render and no way of drying for 50+ years didn't help but there are definitely some ingress issues around windows in the upper floor though. I've all the cement render removed, old crumbly mortar removed, back pointed and repointed. I'll probably need some sort of shelter coat as well as the exposed stones aren't in great condition.

 

Thanks for the input. I'm going to have to use a lighter method of insulation for the roof to negate stressing the roof structure I think. Good info here as usual. 

 

There does seem an error there. I'll need that looked at before anything is submitted. Cheers.

 

 

 

Understand the wall issues, and with render, then all the more important. There are, afterall, only types of render, render thats cracked, and render thats going to crack! Which is why i cringe everytime i see EWI covered in render. The long term outcome can surely only go one way.

 

But your roof build up is exactly the same as a modern house, so breathable products here are just spending money for no benefit.