Minimum Slate Size - Building Regulations / NHBC / BS 5534

[Beagle2]

Hello, I’m converting a small barn to a dwelling which is listed by curtilage. 

 

Planning and listed building consent was done by me. I’m now on the build phase and am finding the Building Control side tricky. 

 

Roof was originally slated, a tin roof was put on in the 1960s. We still have a lot of the old slate onsite and want to reuse it. 

 

However, pitch is fairly low (~26 deg). According to BS 5534, minimum slate size for my roof is 500 mm x 250 mm (double lap) with a headlap of 91 mm, i.e. way larger the the original slates. 

 

NHBC and Building Regs (Part C) don’t seem to mention slate size. 

 

What can LABC actually enforce? Just Building Regulations Approved Documents? Or any recognised best practice guidelines?

 

Apologies if this is a basic question. 

[Temp]

Got to wonder why it was removed and replaced with steel? Perhaps it leaked because the slates are too small.

 

I think only the headlap is critical because ultimately that's how far up the roof water has to be blown before it gets in.

 

The batten spacing is determined by the headlap required and tile size.

 

Google found..

https://www.designingbuildings.co.uk/wiki/Roof_slates

 

Quote

 

The roof pitch typically relates to the required head lap as follows:

• 20 degrees: 115 mm minimum head lap.
• 25 degrees: 85 mm.
• 35 degrees: 75 mm.
• 45 degrees: 65 mm.

Sometimes contractors can try cutting corners with the head lap to use less slate and reduces the weight of the roof. However, it is far better to be cautious and install a roof with extra head lap rather than risking problems which may require that the slates are removed and replaced.

 

I would do a mockup with 85 to 91mm headlap (instead of the 3" shown) and see what it looks like. Does it work with the  holes?

 

Check you have plenty of slate in good enough condition.  Can you get more of the same size if needed?

 

American site but..

https://www.slateassociation.org/slate-roof-condition-assessment-surveys-part-1/

 

Quote

Sounding a slate is an excellent method by which to gauge its integrity and confirm one’s visual observations.The method is simple: grasp the slate in one hand and then tap the exposed face with a knuckle or metal object, such as a slating hammer. Sound slates will emit a distinctive ring when tapped in such a manner. A dull thud, similar to that which might be emitted by a piece of wood, is usually indicative of a very weathered slate that is delaminating. A rattling sound typically indicates a cracked slate, or a slate containing a loose fragment separated from the main body of the shingle along its cleavage plane.

 

[nod]

The lower pitches mean that you smaller 500 x250 should read maximum 

[Temp]

Are there enough wide slates for the verges etc.

 

[nod]

You will find that if the slates are pre holed like most are The headlamp is already determined for you You will also need slate and a half’s 

[mr rusty]

On 18/07/2023 at 21:50, nod said:

You will find that if the slates are pre holed like most are The headlamp is already determined for you You will also need slate and a half’s 

...unless you use hooks

[Beagle2]

Hello,

 

Thanks for all the quick replies, sorry for the delay. 
 

I should add that I have no doubt the slates I have are fine and with a suitable headlap will not leak. I may even move to a triple lap as described in this guide: https://www.spab.org.uk/sites/default/files/SPAB Technical advice note-Slating in South West England.pdf

 

(The tin roof was put on by my father when most of the wooden pegs holding the slates rotted)

 

My question is what is actually enforceable by LABC? 
 

Just the approved reg documents or any relevant BS standard etc?

 

Say I install a lovely high standard roof, completely in keeping with the surrounding buildings. Only to find LABC state the slates do not meet the minimum size (regardless of headlap) as described by BS 5534. 
 

Here is an extract from BS 5534 displaying minimum slate size dependent on pitch:
 

http://www.sigaslate.co.uk/knowledgebase/design-guidance/headlap-tables-for-natural-slate-bs-5534/

 

Why don’t we just ask LABC? Well, to date everything we have asked in advance has ended in lengthy debate often costing 3rd party advice to convince our point. 

[Beagle2]

Hello, any thoughts what’s enforceable by LABC?

[kandgmitchell]

Right, what is enforceable is the Requirements of Part C - Site Preparation and Resistance to Contaminants and Moisture. In particular C2 Resistance to Moisture.

 

C2 states: The walls floors and roof of the building shall adequately protect the building and people who use the building from harmful effects caused by;

a) ground moisture,

b) precipitation including wind driven spray,

c) interstitial and surface condensation; and 

d) spillage of water from or associated with sanitary fittings or fixed appliances.

 

OK so it is only paragraph b) that is an issue here. Section 6 of the Approved Document applies to roofs.

It gives three concerns : precipitation from the outside, interstitial condensation, risk of condensation on the underside of the roof covering. Here we are considering slates so interstitial condesation is not an issue and if the roof is a traditional cold one with adequate ventilation then there is little risk of condensation on the underside.

 

What is left is keeping the rain and snow out.

Paragraph 6.3 states: Roofing can be designed can be designed to protect a building from precipitation either by holding the precipitation at the face of the roof or by stopping it from penetrating beyond the back of the roofing system.

 

Paragraph 6.4 states: Any roof will meet the requirement if: (b) it has overlapping dry joints, is impervious or weather resisting, and is backed by a material which will direct precipitation which enters the roof towards the outer face (as with roofing felt).

 

Finally Paragraph 6.6  states: Roofing systems may be:  (b) weather resisting including natural stone, or slate, cement based products, fired clay or wood.

 

Sorry it's long winded but that is what the LABC should consider. 

 

So they cannot insist on BS 5534 being used but they are using it as a reference to base their judgement on whether you satisfy the above. However, the Approved Document right at the start states:

"Approved Documents are intended to provide guidance on some of the more common building situations. However, there may well be alternative ways of achieving compliance with the requirements. Thus there is no obligation to adopt any particular solution contained within an Approved Document if you prefer to meet the relevant requirement in some other way." (Bold in document).

 

Consequently you can refer to other guidance particularly specialist historic building documentation to support your arguement. Certainly a SPAB document is appropriate in the circumstances and they ought to listen to reason.

 

I would also pay particular attention to the underlay or consider a product such as Isoline by Onduline which is used for slates/tiles below the recommended pitch. It's a polite but firm approach challenging their position, explaining the age and appearance of the building and how you have reached out for further advice. It's all about appearing to concede whilst not!

[Beagle2]

Thank you Kandgmitchell, this is just what I was looking for.

 

I'm going to follow the SPAB guide combined with a modern breathable membrane, copper fixings and BS 3455 batten spec (rather than chestnut lath and oak pegs :)). I may wet lay the final courses depending on slate size left at this point.

 

It might be just me, from (limited) experience I find BC deep dive into some details but others are just 'yeah whatever, great'.

 

Probably should have gone Full Plans rather than Building Notice.