Does it have to be 18mm ?

[jamesrandom]

I need to put a new roof on a large outbuilding in our garden - we call it "the garage" but it isn't actually big enough for cars: it is about 4.5m x 8.5m. The roof has a pitch of about 10 degrees. It is currently tiled and leaks terribly (not surprisingly for such a shallow slope).

I am planning to cover the roof with WBP plywood and a couple of layers of felt. The question is: can I get away with 12mm ply? The main reason to avoid 18mm is just the difficulty of getting the panels up there and moving them around.

If I have to use 18mm ply, is there any disadvantage in using smaller panels so I can handle them?

 

[JohnD]

the existing rafters are probably too thin to support 18mm ply, which is quite heavy.

Tell us the size, spacing and span.

Some photos of the underside would be nice.

I applaud your use of WBP rather than chipboard or other inferior boards that are not strong or water-resistant

 

[^woody^]

12mm may be OK on joists at 400 centres. But it depends on what felt system you are having - some layer systems may be relatively heavy per m2.

What about a 12mm board and then a 6mm board with staggered joints, if 18mm will be too heavy?

You should use large boards as this ties to joists together nicely and maintains a more flatish deck

 

[justlead1]

Hi. Have you considered giving the delivery driver a good drink so he can helps to load on roof. or if access good order from a firm with a hiab. Failing that flat roofer would fit the 18mm ply and felt and still be finished by lunch time.

 

[jamesrandom]

Thanks for the suggestions.

The joists are 50x100mm (2x4) spaced at about 530mm (21") and about 4m long. There is also a supporting beam going across but I am not convinced it add a lot of value.

Here are a couple of pics:

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• jamesrandom
• 27 Aug 2008
• 1

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• jamesrandom
• 27 Aug 2008
• 1

The joists are pretty marked by water but seem to be sound still - but maybe this is a good reason not to put too much weight on them...

For the felt, I was thinking of Wickes "high performance" polyester underlay and capsheet or similar - the "traditional" fibre-based stuff just feels to fragile. I was planning to ue adhesive for both layers (i.e. no nails - I'm not sure how the joists and the old stone wall would take all that banging )

 

[justlead1]

Hi The weight of the existing roof covering far exceeds the new proposal. 18 mm ply has a mass of approx 10kg/M sq. The tiles you have at present are nearer 8 times that.

 

[Jaffa72]

...............just feels to fragile.

Are we on about the roof here or you?
Im not meaning to sound rude or offensive here but are there any disabilities or ailments that hinder you from heavy(ish) lifting?
On a daily basis i would load a roof up with around 20-30+ sheets a day,but then im just a young lad

IMHO 18mm plywood will be the best to use and on a roof that size and height should not pose too many problems,unless access is limited.

If you are worried about too much BANGING then id recommend SCREWING the plywood sheets down.

As Justlead1 has said,a skilled roofer would have this roof stripped and covered just in time for a liquid lunch.

When you say about using no nails to stick the covering down,do you mean the adhesive in a tube or are you refering to NOT using nails to stick the covering down?
Ive not had the pleasure( ) of using anything from the sheds so cannot comment on the quality of felt from these places.

 

[jamesrandom]

> hinder you from heavy(ish) lifting

just my natural skinny wimpiness . I am not a regular diy-er ... in fact I don't normally do anything much more energetic than walking the dogs!

Getting the professionals in is tempting - I am assuming I will need a couple of days to do this myself. The problem is, as always, money.

Yep: I was planning on screwing the ply down. And yes, not "no nails" (TM) but not nailed (i.e. using the (cold) adhesive for both layers). Putting nails through a waterproof layer just feels wrong somehow...

 

[Jaffa72]

Good call on the nails scenario

A couple of days sounds enough time on this for a beginner(+1).Im sure you'd be able to manage lifting the sheets with the help of an accomplice

Id be tempted to recommend a professional for to do the job BUT i do admire anyone who is willing to have a go themselves.Just make sure you read up thoroughly about various aspects of finishing the roof i.e flashings,drips,upstands etc.

Another option is to strip and board the roof yourself to reduce costs and get a professional in to lay the felt.Should only take them a couple of hours to do a roof that size

 

[jamesrandom]

Just make sure you read up thoroughly about various aspects of finishing the roof i.e flashings,drips,upstands etc.

I have been giving some thought to this. This is a free-standing building with a single pitch. So I assume no need for flashing/upstands as the roof doesn't meet any adjacent walls. The drip edge sounds fiddly but doable. I am not sure about the other three edges. I suppose I can't assume that the rain will run neatly down the slope into the guttering. So I guess I need a raised edge (kerb?) on the two sides. And what about the "back" edge (the highest one)? I was planning to leave this hanging out a few inches and take the felt round underneath.

Another option is to strip and board the roof yourself to reduce costs and get a professional in to lay the felt.Should only take them a couple of hours to do a roof that size

Now that sounds like an idea...

 

[^woody^]

Board it and do it with a nice single piece of EPDM, which is not only easier to fit, but will last longer than felt too.

 

[Xenon]

Board it and do it with a nice single piece of EPDM, which is not only easier to fit, but will last longer than felt too.

Depending on the felt.

 

[jamesrandom]

Board it and do it with a nice single piece of EPDM, which is not only easier to fit, but will last longer than felt too.

A single piece sounds good. I don't like the idea of joins/overlaps in the felt: it just feels like a leak waiting to happen.

Manhandling one great big rubber sheet verus cutting and laying a dozen rolls of felt... Hmmm... What could possibly go wrong

Now I need to ask the wife how she feels about rubber...

 

[jamesrandom]

It's taken a while, but thanks to everyone who provided advice:

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• jamesrandom
• 26 Sep 2010
• 1

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