Slates: sarking board vs OSB3 etc

[WWilts]

Slate roof low pitch, located nr boundary of moderate and severe exposure zones. Mostly 19 degrees, one elevation 22.5 degrees

Lots of hips.

Roof trusses.

 

Question:

A) Fix slates to sarking board (over breathable watertight membrane)

OR

 

B) OSB3 + membrane + counter battens + battens?

Also, hook fixing OR nail fixing. 

 

Single layer of membrane or double layer?
 

Edited April 5, 2021 by WWilts

[PeterW]

2 hours ago, WWilts said:

Mostly 19 degrees, one elevation 22.5 degrees

Lots of hips.

Slate in a moderate to severe area has a minimum pitch of 22.5 degrees so check with your BCO that they will accept it. 
 

siga have tested to lower than that but it’s very specific on the design. Slate hooks, 150mm headlap etc

 

 

[Mr Punter]

Can you have interlocking tiles, or are natural slates a must have?

 

The imitation interlocking ones will be a lot cheaper and lighter and I doubt most people would ever notice.

[WWilts]

Natural slate required.

Can try to have planner approve natural for the 22.5 degrees and interlocking for the 19.

Does Building Control in England allow slate hooks to be driven directly into sarking boards (covered by breathable membrane)? Or is it necessary to have counterbattens & battens?

[PeterW]

2 hours ago, WWilts said:

Does Building Control in England allow slate hooks to be driven directly into sarking boards (covered by breathable membrane)? Or is it necessary to have counterbattens & battens?

 

Sarking south of Carlisle is unheard of ..!

 

Have you engaged with BCO yet..?

[WWilts]

6 minutes ago, PeterW said:

Sarking south of Carlisle is unheard of ..!

Low pitch hence sarking board advised by architect in order to allow natural slate

Question is whether to go for OSB3 sheets (+counterbattens & battens) or 
traditional sarking boards with 2-3 mm gaps between.
 

PS Builder once appointed will deal with BCO

Edited April 5, 2021 by WWilts

[PeterW]

Ok but you’re not defining a sarking board then. You’re defining an underlay of OSB. What has the architect drawn and specified .?? He can’t “advise” then wander off ..!! Who’s doing the Building Regs section drawings ..??

 

8 hours ago, WWilts said:

Builder once appointed will deal with BCO

I would get myself into that relationship before you end up with “BCO says ....” conversations that cost you £££££. Also, I would never price a job on unapproved drawings or spec - who is adjudicating when BRegs comes back with X  and your builder says he priced Y..??

 

Private BCO or Local Authority..?

[WWilts]

Underlay boards: either
a) OSB3 or

b) tongue and groove sarking board OR
c) traditional Scottish timber sarking boards (150mm wide)

 

All would have breathable watertight membrane over, then slate. Only b) or c) might work without battens & counter-battens.

Which would you choose out of a) b) c) and why? Cost is a consideratioin.

Would you use weathering slate or non-fading slate? Slates will not be visible from ground, except from some distance.

- - - - -

 

PS Cold roof, ventilated attic space. Central attic space will allow some storage and perhaps a bookshelf/chair for the summer (roof light available).

Private BCO is preferred route of architect who did the BR drawings and will do the CML certificate.

Presumably approved drawings will not prevent BCO from requiring tweaks to details, and we will have to agree a schedule of rates + a formula for variations/unforeseen work

 

Intention is to incentivise builder to control costs, by sharing half of any savings as a result of design/specification tweaks in the course of the work. Hence initial notice and final certificate preferred.

Edited April 6, 2021 by WWilts

[PeterW]

29 minutes ago, WWilts said:

Only b) or c) might work without battens & counter-battens.

This isn’t a recognised design detail in England & Wales so your BCO needs to sign off 

 

30 minutes ago, WWilts said:

Private BCO is preferred route of architect who did the BR drawings and will do the CML certificate.

Why is the Architect telling you who to use ..? Make your own choices as you’re paying the bills. 
 

30 minutes ago, WWilts said:

Presumably approved drawings will not prevent BCO from requiring tweaks to details

Approved by whom..?? Approved BRegs drawings are those approved by Building Control so unless they are “approved” then they aren’t ...

 

32 minutes ago, WWilts said:

Intention is to incentivise builder to control costs, by sharing half of any savings as a result of design/specification tweaks in the course of the work. Hence initial notice and final certificate preferred.

So the builder will include the potential loss from not making any savings and then increase the price accordingly.
 

You’re not making any money on this - you’re just reducing your cost as you’re not developing for profit. 

[WWilts]

6 hours ago, PeterW said:

This isn’t a recognised design detail in England & Wales

Looked at the approved docs for Structure, they do not prescribe. Is there another official BR doc that contains required design details? Would be good to see it. Considerable flexibility to optimise the design for good cost/function ratio. Not tied to anything other than what the BCO might say.
 

6 hours ago, PeterW said:

Approved BRegs drawings

These would not prevent a BCO from requiring tweaks to details, since the BCO is the final judge for completion OR final certificate
 

6 hours ago, PeterW said:

then increase the price accordingly

Quote already in. Sharing in cost savings would merely incentivise careful cost control. Together with an incentive payable at the end if the build is brought in under the quote price.
Disincentive for cost overruns will be co-pay of overruns (eg 25% carried by builder).

Interested in all points of view on incentives too, although the topic is whether or not to nail/hook directly into sarking board(s) through the breathable membrane. Cost/function is the criterion

Also very interested in hearing about people's experience with different kinds of slate. Weathering/non weathering etc. 
 

Edited April 6, 2021 by WWilts

[PeterW]

3 minutes ago, WWilts said:

Disincentive for cost overruns will be co-pay of overruns (eg 25% carried by builder).

What type of contract are you going to try and use to impose that ..?? Who says what caused an overrun ..? What about bad weather ..? Failure of one of your suppliers to deliver by a week..? Can the contractor take costs from you if your windows are a week late ..? Can they claim for overruns from you ..??

 

All things to consider in a fair contract, and if they have priced without seeing the contract terms don’t be surprised if they don’t want the job. 
 

9 minutes ago, WWilts said:

Quote already in.

Quote..?  Fixed Price..? Firm Price ..? All different. 
 

4 minutes ago, WWilts said:

These would not prevent a BCO from requiring tweaks to details, since the BCO is the final judge for completion OR final certificate

Nope - if you get drawings done and approved by BCO, build to the drawings then you get approval. That’s the rules. 
 

5 minutes ago, WWilts said:

Considerable flexibility to optimise the design for good cost/function ratio.

Cost / function ..?? Improve your roof slope to 22.5° and then use slate on battens. Quick, cheap and understood by roofers. 
 

I don’t think this has been value engineered by design, I think you’re trying to build something that isn’t really viable at the cost you want to pay, and you’re trying to pass your risk to a contractor. 

[WWilts]

26 minutes ago, PeterW said:

use slate on battens.

Thanks, that is the opinion I sought

Noted all the other points, thanks

Good point about disincentive for overruns, the incentive payment at the end needs to be paid whether or not there are overruns, but be reduced by the amount of the overruns.

It is for the builder to say what suits them. In essence, they are being offered an opportunity to make more money than their quote by exercising reasonable cost control. They can always say no to more money.

Edited April 6, 2021 by WWilts

[WWilts]

56 minutes ago, PeterW said:

build to the drawings then you get approval.

Building better than the drawings is the option being considered (scottish style waterproofing for England)
Interested in hearing about what people might choose and why

a) OSB3 / breathable watertight membrane / counterbattens / battens / slate on hooks (or nails)
b) tongue and groove sarking board + membrane (no counterbattens/battens) / slate on hooks (or nails)
c) 150mm wide sarking boards + rest as in b)

Relative cost of a) vs c) per sqm is what I'm trying to work out

Edited April 6, 2021 by WWilts