Box guttering with warm roof, is it possible?

[Tuppers]

Hi all,

 

The person doing our plans is on holiday at the moment and I wanted to try and get some advice on something I noticed following a comment on my intro post (https://forum.buildhub.org.uk/topic/42202-extension-slightly-large-and-renovation-in-cambridgeshire/). We're on the path to getting planning permission so don't have all of the structural/full technical documents created at this time.

 

I'm very keen to have a warm roof on our large single storey rear extension. I'd not noticed that as part of the plans, there is what I understand to be box guttering placed on top of the external skin wall on the attached neighbours side, thus allowing for the extension to go 100% to the boundary. Would you think this would be possible? I've searched to try and see if anyone has mentioned this before and looked online but not found anything, apologies if there is.

 

I'd appreciate any advice anyone can give!

 

Tuppers

[DevilDamo]

Yes, it is possible subject to notifying the neighbour via Certificate B on the Planning application and then again via a Party Wall Notice/Agreement.

[Tuppers]

Thanks for the response and I'm glad to hear it! I've tried drawing it out to help me, but I can't seem to work out how you fit it all the requirements in for a warm roof with the insulation sitting on the rafters, for example where the stop battens would go, how the rafters would sit, etc.

 

 

 

 

[EdHat]

I'm not sure if i understand your question. Do you want to build right up to the boundary and therefore don't have room for a normal gutter?

 

With a warm roof you can form a gutter in the insulation, you only need to achieve a 1:80 fall. Roof manufacturers will have a standard detail for this.

Alternatively is it possible for the roof to fall a different direction and then have a kerb along the boundary side so no need for gutter there?

[DevilDamo]

9 hours ago, Tuppers said:

Thanks for the response and I'm glad to hear it! I've tried drawing it out to help me, but I can't seem to work out how you fit it all the requirements in for a warm roof with the insulation sitting on the rafters, for example where the stop battens would go, how the rafters would sit, etc.

 

 

 

 

Why is a warm pitched roof being proposed? A cold (vented) roof would be more common but you can still achieve a similar detail with a warm roof.

[Tuppers]

22 hours ago, EdHat said:

I'm not sure if i understand your question. Do you want to build right up to the boundary and therefore don't have room for a normal gutter?

 

With a warm roof you can form a gutter in the insulation, you only need to achieve a 1:80 fall. Roof manufacturers will have a standard detail for this.

Alternatively is it possible for the roof to fall a different direction and then have a kerb along the boundary side so no need for gutter there?

To your first question, yes, exactly that.

 

I'd looked at forming of gutters in the insulation but I'd assumed that this wouldn't be acceptable to BC as it would lower the u-value in a given location.

 

On the fall direction, it's an ~8m protruding rear extension so the most logical method (and in my mind asthetically pleasing) for a pitched roof is to have the pitch perpendicular to the rear of the house, which is where the issue lies!

[Tuppers]

13 hours ago, DevilDamo said:

Why is a warm pitched roof being proposed? A cold (vented) roof would be more common but you can still achieve a similar detail with a warm roof.

We were hoping for a warm roof design for a few reasons:

1. Due to the size of the open plan area and the amount of glass we're expecting to be involved, we were hoping to have it be more efficient to heat which typically warm roof designs are
2. With the propensity for change of use in the space, we wanted (where possible) to have ceiling services run between rafters in the void that a warm roof design can afford. To achieve this using a ventilated cold roof design invariably means lowering the ceiling height by adding battens so if you want to change the position of a light you're not having to dig out insulation
3. Ease of install was also part of the consideration

I'd rather not over complicate things however so we'll likely proceed with a vented cold roof design and see about raising the ceiling height slightly before the final plans are submitted in order to allow for the additional battening.

[DevilDamo]

@Tuppers On the assumption you are working to a maximum roof/ridge height, then adopting a cold roof isn’t going to make a huge difference to the internal height. Overall, a the build up of a warm roof is quite a lot thicker than a cold roof hence having that space to play with.

[Mr Punter]

With a pitched roof, most people either have a cold roof with insulation between and under rafters or a cold roof with insulation at ceiling level.  It is unusual to have a warm pitched roof, with all the insulation above the rafters.