Building Regulations: 'substantially non-combustible'

[space.dandy]

With respect to permitted developments and building a shed of between 15-30 sqm. floor space within 1m of a boundary, from what I have read this is allowed providing it is constructed of 'substantially non-combustible material'. What does this mean in practical terms? What options are there for the design of a timber framed shed that would be substantially non-combustible? One local authority site suggests this means tiles rather than OSB for the roof, but makes no mention of the walls.

 

[MikeG.]

This is interpreted differently by different authorities. Some read it as "the outer covering of those parts of the walls that are within 1m of the boundary should be non-combustible". Others, unfortunately, treat it as "if any part of the building is within 1m of the boundary then all of it needs to be non-combustible". Others yet only consider the elevation facing the boundary. Some want the entire build up of the wall to be non-combustible (not many, in my experience) where most consider the outermost layer only. I've done any number of sand & cement rendered timber frame buildings hard up to boundaries, for instance. The cementitious fake feather-edge boards have been a godsend in these circumstances, because the authorities around here are quite happy for those to be on the boundary elevation and timber feather edge boarding to be on the others. I have never had anyone even look at the roof. Windows are the most problematic of all, however, and are universally disallowed on the boundary facing elevation ion those circumstances.

So, I'm afraid I can't give you an answer other than to suggest you ring your local Building Control and ask them how they interpret the rules.

 

[space.dandy]

Thanks, MikeG, that's all really helpful. This isn't my local area yet, we're moving to W. Sussex (Bognor) and I'm considering my shed options. I'll look into their specific requirements.

On a practical note, how do you render a wall that's hard up against a boundary? Do you build and render that wall in isolation and erect it in one piece?

 

[Lazurus]

Try the online planning portal which can be a great help.

https://www.planningportal.co.uk/

 

[mindthatwhatouch]

I’m just down the road (worthing)
As mike stated you need to speak to the authority.
For mine they insisted on wholly non combustible, so no timber frame allowed. Roof structure wasn’t mentioned so that’s timber.

 

[space.dandy]

I’m just down the road (worthing)
As mike stated you need to speak to the authority.
For mine they insisted on wholly non combustible, so no timber frame allowed. Roof structure wasn’t mentioned so that’s timber.

So what is the construction of your shed to make it wholly non-combustible?

 

[Sheptonphil]

My current build has to meet the ‘predominantly non combustable’ ruling. Here, In the West Country, I have timber frame, Fibre cement Plank cladding externally and had to incorporate fireline plasterboard inside with fireproof rockwool insulation. This met with their criteria. Each authority is different though.

 

[space.dandy]

I contacted Building Control and got this response:

Class 6 (Small detached buildings) of Schedule 2 (Exempt buildings and work) of the Building Regulations 2010 only describes the building (taken as the whole building) as being substantially (a considerable amount of it) non-combustible (not flammable). The principal being not to put a high fire load close to the boundary (it does not differentiate between a boundary to a house or a garden). Copy attached.

When designing controlled buildings in terms of unprotected area (windows, doors, combustible construction) in accordance with the guidance in Approved Document B, then the focus is the relationship between a specific elevation and the relevant/notional boundary. In controlled buildings the walls close to the boundary have to be fire resisting from each side as well.

The timber frame walls would need to be clad internally and externally with a non-combustible material to qualify as being substantially non-combustible (calcium silicate board, or plasterboard). Treatments are also available for exposed timber linings/cladding (Envirograf products, or similar).

If you provide confirmation of the construction, then we can then confirm that it is exempt.

So it sounds like they are only concerned with the boundary facing wall, but it has to be fire resistant on both sides. Obviously I'll check back with them when I have a better idea of what I want.

 

[Sheptonphil]

So it sounds like they are only concerned with the boundary facing wall, but it has to be fire resistant on both sides. Obviously I'll check back with them when I have a better idea of what I want.

That is where our authority stands, and the need for the fireline plasterboard internally as well as cement cladding externally to at least one metre from the boundary. So I’ve gone fireline cladding on all walls and Hardie Plank all round. It’s added £1200-£1400 to the materials cost, but at least enabled the build to happen.

 

[space.dandy]

So it sounds like they are only concerned with the boundary facing wall, but it has to be fire resistant on both sides. Obviously I'll check back with them when I have a better idea of what I want.

That is where our authority stands, and the need for the fireline plasterboard internally as well as cement cladding externally. It’s added £1200 to the materials cost, but at least enabled the build to happen.

£1200 sounds like quite a lot. How big is your shed? What does that £1200 represent in materials and area?

 

[Sheptonphil]

£1200 sounds like quite a lot. How big is your shed? What does that £1200 represent in materials and area?

The workshop externally is 9.2 metres long on gable wall, two ends at 3.9m each, 2.4m high on ends, gable is 4.4m to ridge, nothing on house wall.

I bought 100 lengths of 3.6m Hardie Plank, with trims and vents came to £2055
20 sheets of fireline plasterboard was £220.

Area covered is circa 54sqm cladding externally and 70sqm plasterboard internally.

If I could have used timber cladding it would have cost £500-600 and I wouldn’t have used plasterboard at all and left it at the OSB lining which I had to use for structural integrity. So instead of £600 I’m in to £2200+.

It is 29.8sqm internally, and I had to even deduct the thickness of the plasterboard to get under 30sqm internal measurement.