Twinwall on a garage roof?

[BubbleBlower]

Hi all and thanks in advance for your advice.

My garage currently has a flat felt roof that is completely knackered (more holes than felt), so I am looking to replace it. I have to decide what option is best out of the following:-

1. Twinwall sheets
2. Corrugated plastic (clear or bitumened)
3. Replace felt and boards
4. Make a pitched roof (felt or tile).
5. AN Other

Bearing in mind that I am extremely ropey at DIY so simple is much preferred but value for money and longevity are equally as important what would you guys suggest?

 

[masona]

What size is the garage?

 

[BubbleBlower]

Garage is 5700 by 2720.

Roof joist centres are 18" apart in case that make any difference.

 

[masona]

Bearing in mind that I am extremely ropey

I'm not sure what you're capable of doing, but I go through your lists.

1. Twinwall sheets
Most average diy'er is capable of doing this but it's not cheap depending on quality & thickness of the twinwall polycarbonate sheets, Mr Plastic is good on these price. Don't know if you have a slope on your roof for the rain, if not, put a timber firring (wedge shape) on top of joists then glazing bar to the joists, then clip-in with the sheets. This idea will bring in a lot of light into your garage.
2. Corrugated plastic (clear or bitumened)
Clear plastic, forget about it, they tend to get brittles over the years but the bitumen sheets in Wickes are okay,
3. Replace felt and boards
Best to have it done by the pro for the hot pour, get a few quotes.
4. Make a pitched roof (felt or tile).
This is the best option for long term, again I don't know what you're capable of doing. Check with building regs (should be no higher than 4m for pitch or 3m high for flat roof)
5. AN Other
I think you've covered it and depending on appearance of the roof. Now you have to work the cost & materials

 

[BubbleBlower]

1. Twinwall sheets
Most average diy'er is capable of doing this but it's not cheap depending on quality & thickness of the twinwall polycarbonate sheets, Mr Plastic is good on these price. Don't know if you have a slope on your roof for the rain, if not, put a timber firring (wedge shape) on top of joists then glazing bar to the joists, then clip-in with the sheets. This idea will bring in a lot of light into your garage.

This was my preferred option but am not sure whether it would work with 450mm joist spacing as the 3m sheets that I have seen at B&Q, Wickes etc are all 700mm. There isn't a Mr Plastic in my area.

There are wedges on top of the joists but with only a slight drop (35mm over 3m) so is it possible to purchase these as I don't fancy my chances of cutting 2 identical ones, let alone the number I require?

 

[masona]

There isn't a Mr Plastic in my area.

It's a shame because Mr Plastic will cut the width as they do a 2m wide and will cut it at the width you need. You'll get 4 sheets at 450mm wide out of 2m sheet. You could put in an extra joists to allow for the standard width of the twinwall sheet. Does the joists run the same way as the fall?

 

[BubbleBlower]

Yes joists run same way as fall.

 

[masona]

Would be cheaper to put in an extra joist with firring to make it a standard width of the twinwall sheet.

 

[BubbleBlower]

1. How easy is it to put in extra joists?

2. Do you buy them with firring attached?

3. Anywhere you can recommend getting them from?

 

[masona]

1. How easy is it to put in extra joists?

Very easy once you have the ex-roof off and put in the same size joist from the top and screw or nail down to the wallplate.

2. Do you buy them with firring attached?

No, most timber merchants will have pre-cut and just cut the length off, normally 3.6m long or you could make one out of 4"x2" with a circular saw.

3. Anywhere you can recommend getting them from?

Any good timber merchants should have or they can make some while you wait or the roofing merchants might have them pre-cut, yellow pages should help you out.