Log Cabin Roof Advice

[john_pullen]

Hi. I have ordered a log cabin (Finnforest Valko) that arrives next week. It comes with a pitched T&G roof and roofing shingles (which from the instructions look like strips of felt that are overlapped and designed to look like tiles). I have two questions that are hopefully easy and quick to answer:

1. Can I put a membrane on the outside of the T&G roof under the roofing shingles to help avoid leaks. I already have a role of the blue plastic DPM. Can I use this or does it have to be a breathable membrane?

2. On the inside when you look up you see the pitched roof. There is no flat ceiling and loft space. I want to insulate the ceiling and was planning on doing this like the walls using a solution recommended in the Bulding forums:

Cabin wall
Breathable Membrane
Insulation
Vapour barrier (i.e. blue plastic DPM)
platerboard

Firstly, is this the best way to insulate the ceiling. Secondly, do I need the breathable membrane (especially if one is on the outside)?

Thanks in advance for any help offered.

 

[masona]

Hi. I have ordered a log cabin (Finnforest Valko) that arrives next week. It comes with a pitched T&G roof and roofing shingles (which from the instructions look like strips of felt that are overlapped and designed to look like tiles). I have two questions that are hopefully easy and quick to answer.

Just a thought, have you seen the metal roof sheet tiles effect, fit them once for maintenance free

//www.diynot.com/forums/viewtopic.php?t=4236&highlight=summerhouse

 

[Deluks]

Principles of ceiling are same as walls. You only need 1 layer of breathable membrane on the walls, behind the outer sheathing (but leave a gap) DPM on the roof is not a great idea, the shingles will be fine if fitted correctly and you use mastic.
Remember to fit vents in the walls, and if you put a ceiling in, you will need to ventilate the space between this and the roof.
Are you planning on putting the plasterboard directly under the roof, keeping the room height, or a flat ceiling, like the upstairs of a house.

 

[john_pullen]

Thanks for all the help so far.

No, I don't plan on having a flat ceiling with log space. I will keep the pitched roof and was planning on boarding it like a wall so there will be no space between the boards and the T&G roof boards other than the cavities field with insulation. This will mean I can see the joists or eaves (whatever they are called) and I don't know at this stage as I have not erected the cabin what exists in terms of ventilation in the roof area. It's not necessary to board this cabin which is fairly solid so I would not expect any or much ventilation in the roof area.

I have been looking long and hard on the web and this forum on how best board this cabin and had some great advice. However, come of the info is complicting. I have seen pictures of people boarding cabins and sheds on the web and it's does not look like they use a breathable membrane on the inside of the walls or roof. Is this really necesary? The outside (and initially the inside) will be treated every year with wood preservative and this should prevent and moisture getting inside the cabin.

Thanks again for any help.

 

[Deluks]

Well it's not strictly necessary to use membrane, but think of it as a belt and braces system. If you have lined the walls and water gets in behind the cladding, how will you know it's there? A small leak over time could lead to dampness in the cavity, rotting away at the structure of the building.

If you are doing the ceiling as planned then you will need a cavity in between insulation and t&G boards, minimum of 50mm airspace is recommended. The alternative is a 'warm roof' where the insulation goes on the outside, but this gets more complex.

What's the thickness of the rafters that you want to leave exposed.

 

[john_pullen]

Thanks Deluks. This is a great explanation. Can you confirm a couple of things. I can see that if I don't have a breathable membrane on the inside cabin walls any moisture or leaks will end up in the cabin walls and this is bad. However, if I have a breathable membrane and a plastic vapour barrier behind the plasterboard the only place for leaks to go is through the floor or collect in the cavity. Is this not just as bad?

The only other thing I am a little confused about is the need for the space between the 50mm gap between the insulation and roof. If I do this there won't be any anywhere for the air to escape so surely this cavities will collect condensation. I don't think the cabin has any natural air vents. What if I nailed polystyrene backed plasterboard directly to the T&G roof or did not leave a gap?

I was thinking an alternative was to insulate the ceiling and use plasterboard but have no vapour barrier or membrane. Any leaks (and hopefully there won't be any) will be visible early. I am not trying to cut costs here. Just keep things simple and keep as much roof space as possible.

It does make you laugh, in the FAQ's for the cabin web sites when people ask about insulation all it says to do is use fibre glass type insulation and plasterboard. Nothing about membranes and ventilation. They make it sound easy even for someone like me!

Again, I really appreciate the help you have given and I am sure this must help other non-DIYer's like myself reading this thread.

 

[Deluks]

I can see that if I don't have a breathable membrane on the inside cabin walls any moisture or leaks will end up in the cabin walls and this is bad. However, if I have a breathable membrane and a plastic vapour barrier behind the plasterboard the only place for leaks to go is through the floor or collect in the cavity. Is this not just as bad?

I've had a look at the installation instructions. I'd be inclined not to put floorboards down in that cavity, or fit the floor with vents to the space beneath the structure.

The only other thing I am a little confused about is the need for the space between the 50mm gap between the insulation and roof. If I do this there won't be any anywhere for the air to escape so surely this cavities will collect condensation. I don't think the cabin has any natural air vents. What if I nailed polystyrene backed plasterboard directly to the T&G roof or did not leave a gap?

Yes you will need to fit vents into this roof space as well. If you fitted the polystyrene with no gap, then any moisture getting in will soak the t&G boards and it will have no chance to dry out, not good.

Best face the fact that you're gonna have to hack bits off and modify this spanking new cabin of yours to achieve 'house like' insulation properties.

Maybe you should use it for a few months installed as per instructions. It may be warm enough as it is, wood is a very good insulator anyway. While you are using it and looking at the structure you'll be able to figure out the best way to proceed. Just do a bit more research and everything will fit into place. Being timber it will be very easy to retrofit a solution once you've worked out what's supposed to go where.