Bit of advice needed for garage roof.

[Park_monkey]

Hello all.
I am in the process of doing a few improvements to my garage and need a bit of advice on what to do with the roof.
It is a prefab concrete panel job with an angle iron frame and corrugated tin sheet roof (not asbestos). It is all a bit battered to be honest as it was second hand when I bought it but it will have to do.
My main issues with it have always been the fact that the roof is too low, the terrible condensation and the lack of security, so I have started making it a bit more useable.
I have already extended the uprights by 800mm at the front sloping down to 500mm at the back by cutting them in half and welding a piece in the middle. These are now refitted and the roof support crossmembers bolted to them, I have put the roof sheets back on but not fully secured yet. I have a load of insulation and used plastic coated lining sheets that I plan to line the inside walls with, which should tidy it up and make it a bit more secure, as well as a bit warmer, but I'm not sure what to do with the roof.
The tin sheets are a bit scruffy and full of holes where they have been drilled before but I can't really afford to replace them and don't want to end up with the same condensation problem. I did consider fixing a thin layer of ply on top of the sheets then felting but wasn't sure if the cavity left between the ply and tin would cause problems. I was thinking about taking the tin off and fixing timbers the length of the garage then putting ply and felt but I'm a bit concerned about the weight. So would it be possible to felt straight on to the tin sheets? Or can anyone suggest another option? I can't afford to throw a load of cash at it but at the same time i want to get it sorted while I'm doing the work anyway.
I was going to insulate the ceiling against the tin and plaster board the ceiling to try and stop the condensation.
So any suggestion folks? Cheaper the better, I'm all ears.
I do have a few pics of the garage if it helps, I've probably not described what I'm doing very well.
Cheers.

 

[candor]

Throw up a few pics till we see, I'm in a similar boat myself.

 

[RonMig]

It's not exactly the same I know, but when we had our garage roof changed from flat to pitch the builders used a sheet called VapR-Free which is breathable and waterproof so any condensation (or hard-driven rain) under the tiles runs off without dripping inside. Something similar should work on a lower pitch like yours? I'm thinking of doing the same thing on my shed/workshop, whic does have the same issue as yours.
Doesn't need a layer of ply underneath, it just stretches across the beams, so what you save on the ply (expensive stuff!) you could lash out on the barrier layer...

 

[Wendelspanswick]

^^^^^^ Is just a breathable roofing membrane, there are lots of manufacturer's. It's a modern version of underfelt with the added benefit of being breathable. Toolstation and Screwfix sell their own versions of it, as well as the cheaper non-breathable versions.
If you are going to insulate and plasterboard you could get away with the non-breathable version as the thermal break should stop the condensation.
Chances are if you have a concrete floor it won't have a damp proof membrane. If you can vapour seal your floor with either plastic sheeting and screed or even an old piece of lino, that would help stop condensation.

 

[gaz1]

as long as your pitch of roof is over 31/35% its fine to use breathable felt with no plyboards any less its advised to use plyboard breathable felt latted down the roof on the joists and then cross latted this allows water to run off between the latts

plyboard 6/9mm is fine to use

then id put the metal roofing on top of this as the breathable membrane would keep it dry and get rid of condensation under the steel roof

 

[Park_monkey]

Some pics-
http://s296.photobucket.com/user/park_monkey/slideshow/Garage

 

[Park_monkey]

I haven't seen that breathable membrane and will look into it, only issue with that is the lack of security. The metal sheets are too easy to prise up and get through, that's why I was originally thinking of leaving the metal sheets on then fixing ply on top of that an felting it, the weight may be a problem then though. The angle iron h frame roof supports are already a bit twisted but I can't afford to replace them.
The slab is rubbish, I did it myself but had to do it in two halves because of lack of space, where the two halves meet it isn't level. I am hoping to screed it at some stage and will put a dpm under it but that will have to wait.

 

[Park_monkey]

I think the easiest and cheapest thing is going to be leave the tin sheets on, lay some thin osb on top and screw it down then felt. I will then insulate between the metal joists and plasterboard. Will this work or not?
Cheers guys

 

[gaz1]

to be honest its not a house the only reason you use breathable felt is to stop the wood rotting away from the central heating systems warmth as the damp rots the wood thats why you create a cold roof

this is what id do take the tim roof off felt it and reinstall the tin roof again in the garage use your insulation inside put it as thick as you like but i would advise a different insulation near the walls the highest part use standared insulation thats to stop heat loss

the felt under the tin roof will run off any water that goes through the holes the insulation inside will keep it warm add a wood burner and its nice and warm in there

 

[Park_monkey]

The problem with that is the tin looks crap, it's tatty and looks horrible from upstairs. It's quite a big garage and I'm trying to make it look as presentable as possible to keep the mrs happy. I'm only keeping the tin as I already have it, I can use thinner therefore cheaper osb over the top, and it should give a little added security.
I have bought a load of reclaimed timber cladding so am planning to clad the whole side of the garage we can see from the house to make it look a bit better, there should be enough to clad round the top of the other sides too where I have raised the roof.

 

[Park_monkey]

Hello all.
I am still plodding on with my garage improvement but money and time constraints are making it difficult.
I would just like someone's opinion on my plans, just to confirm it will work.
I am trying to achieve a few things-
Insulate the garage.
Reduce condensation.
Improve appearance.
I am planning on fixing a thin layer of treated osb straight to the tin sheets currently on the roof, then felting over the top of that. The. I will slide timber batons into the angle iron, h-frame joists which I can fix plaster board to. I will use a layer of glass fibre insulation between the tin sheets and plaster board.
Do I need to leave an air gap between the insulation and tin sheets? From what I have read, if I leave a gap, I may still get condensation forming on the underside of the tin which would then be absorbed by the insulation and eventually cause a bigger problem.
I am using a vapour barrier on the walls, then they will have a layer of insulation with plastic coated sheets over the top so hopefully the walls should be ok. As the slab is rubbish I will put a dpm down, then screed seal and paint the floor.
I am hoping this will solve the condensation problem and make it pretty cosy in there, but I welcome opinions from those of you in the know if there are any major problems with my plans.
Thanks in advance.

 

[captain-destructo]

you could also try putting a layer of polystyrene between the steel and osb that will help
also try to make the void above the plasterboard airtight that way no additional moisture can get in there to create condensation if the void is 200mm then use 200mm of insulation

also the pics dont work!!!!!

 

[Rig Pig]

I have just built a workshop on the back of my garage from a second hand sectional garage, I did the roof the other way about - osb on 6 x 2 joists then felt then the corrugated tin on top, attached with 4" tec screws and painted with tar varnish - should last a few years and looks a lot better than the exposed galvanised steel.

 

[Cris B]

Get yourself onto the garagejournal.com - plenty of advice on there...