Two breather membranes on roof?

[Hilldes]

Hopefully, will be tiling the roof in the next few weeks. I produced this detailing a couple of months back based on searches on here. I'm still wondering if I have the right detailing for the membrane(s).

 

Doe this look right - having two breather membranes on the roof:

 

• the lower one (yellow) fitted by the timber frame supplier.  This sits on top of rafters. There is no sarking board. Insulation is fitted between the rafters immediately below this membrane.
• the upper one (green) is fitted between counter battens and battens. This one is lapped over the fascia vent as per roofing felt

 

Do I really need two membranes? 

 

Can't locate it now, but sure I read one of the purposes of the counter battens is to lift the breather membrane off the insulation between rafters. The lower yellow membrane will rest on the insulation ?.

 

Not shown on the diagram but there will be a VCL on the onside of the roof make up.

[Temp]

Are they definitely providing a membrane on the roof?  I don't believe you need two.

 

2 hours ago, Hilldes said:

Can't locate it now, but sure I read one of the purposes of the counter battens is to lift the breather membrane off the insulation between rafters.

 

Yes and no.

 

There are two reasons for the counter battens when you have insulation that fully fills the rafters...

 

1) Some manufacturers (Kingspan?) allow their membrane to be in contact with the insulation but others don't. Presumably your frame manufacturer uses a membrane made by a company that does.  I've never quite understood why this is the case. Perhaps they simply haven't tested it in that situation?

 

2) Another reason for the counter battens is to raise the tile battens so water blown under the slates/tiles can run down to the gutter. Without this gap a "mud" of dirt and pollen can form above the batten and cause water to pool rotting the batten.  So you still need counter battens even if you don't fit the green membrane.

 

Aside: In the past (when rafters weren't fully filled with insulation) a 25mm void was formed below the membrane to allow it to drape and form a drainage channel to solve 2). This avoids the need for counter battens. This is different to the 50mm ventilated void needed when the membrane is not vapour permeable, although frequently one void did both. 

Edited March 13, 2021 by Temp

[Hilldes]

Thanks @Temp, yes pretty sure they are providing a membrane on the roof - a comment that it will be losely tacked down and a suggestion to have the roofing done ASAP after frame erection completes.

 

The insulation between the rafters will be mineral wool. I'm not sure how to stop it moving up between the counter battens - I guess the lower membrane if pulled tight would do that?

[dpmiller]

I think they're saying the roof is ready to be battened and to get a move on as there's nothing much holding the felt down?

[Hilldes]

12 hours ago, dpmiller said:

I think they're saying the roof is ready to be battened and to get a move on as there's nothing much holding the felt down?

Yes, that is the message. I'll check what type of membrane it is and then maybe move it up above the counter battens and just use the one breather membrane. 

 

Still deciding whether to get a pro to tile the roof or DIY if pro costs are too high or they cannot schedule when we need them.

[Hilldes]

A little more about the roof build up.

 

Eaves ventilation: this is the eaves ventilation specified by Marley.

 

I'm assuming the eaves rafter roll goes under the breather membrane as shown in the pic. My roof has no sophits so it would have seemed better to have a continuous breather membrane from the external walls and up onto the rafters. If I now move the single breather membrane up onto the counter battens and then lap over the eaves rafter roll, then that creates a gap in the breather membrane at the eaves - is this normal?

 

For the felt support tay, I'm guessing this is needed more for roofs that have no habitable space and the loft insulation extends into the eaves. In my case with insulation filling the rafters, it just the counter battens that ensure air flow over the insulation (but I'm still unsure what is stopping the mineral wool insulation expanding into the void created by the counter battens) .

 

I'm sure this will all get sorted if I use a professional, but still not fully sure how to detail if I do it myself.