Uneven Flat Roof

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Finally, I have the old garage roof off and tarped in prep for OSB3 and EPDM.

Unfortunately, I notice that when I place a straight edge across different parts of the roof structure it seesaws. Over 3 joists I often see a difference of up to 10mm. I am assuming that a couple of mm wouldn't matter but this is way too much.

After researching this I think the unevenness may be caused by a lack of strutting between the joists. I can see that the joists are quite twisted in places. In fact, there aren't even any noggins at either end of the roof. Where noggins exist they are quite wobbly and don't extend much beyond the depth of the furrings.

The garage is just under 50 years old and I noticed that the old roof board was mismatched on removal, seems like a bit of a botch job.

How would you fix this? The best I can come up with (with my limited experience) is as follows:

Add wooden structure at the ends of the joist shaped to hold the existing structure in place and support the edges of the OSB3 board.

Add in strutting (2 rows at least) as the garage roof is 837cm X 582cm.

Use something like plastic Broadfix shims to fix the unevenness as I add on the OSB3.

I am now also considering the use of a larger 18 X 1220 X 2440mm OSB3 (not tongue and groove as was advised). I am assuming that this will be more forgiving, than the smaller tongue and groove board. I know I will have to support the long edges which will be a pain I wonder if someone has experience fitting to an uneven roof and can help here?

Any help/thoughts would be much appreciated.
 

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Finally, I have the old garage roof off and tarped in prep for OSB3 and EPDM.

Unfortunately, I notice that when I place a straight edge across different parts of the roof structure it seesaws. Over 3 joists I often see a difference of up to 10mm. I am assuming that a couple of mm wouldn't matter but this is way too much.

After researching this I think the unevenness may be caused by a lack of strutting between the joists. I can see that the joists are quite twisted in places. In fact, there aren't even any noggins at either end of the roof. Where noggins exist they are quite wobbly and don't extend much beyond the depth of the furrings.

The garage is just under 50 years old and I noticed that the old roof board was mismatched on removal, seems like a bit of a botch job.

How would you fix this? The best I can come up with (with my limited experience) is as follows:

Add wooden structure at the ends of the joist shaped to hold the existing structure in place and support the edges of the OSB3 board.

Add in strutting (2 rows at least) as the garage roof is 837cm X 582cm.

Use something like plastic Broadfix shims to fix the unevenness as I add on the OSB3.

I am now also considering the use of a larger 18 X 1220 X 2440mm OSB3 (not tongue and groove as was advised). I am assuming that this will be more forgiving, than the smaller tongue and groove board. I know I will have to support the long edges which will be a pain I wonder if someone has experience fitting to an uneven roof and can help here?

Any help/thoughts would be much appreciated.

Hi, First if the roof is as you say 8370mm x 5820mm then yes you are correct there should be rows of strutting or solid nogging between the joists, and this could be a reason some of it has twisted.
If you have limited carpentry skill you dont need to go down the road of traditional herring bone strutting, but could simply fix solid niggins between the joists, but dont leave out the very last joists where they abut the wall, they need nogging between the side of the last joist and the wall as well. This also makes the whole roof stronger with less bounce.
That done how do you correct the unevenness on the top?
Not seeing any pictures I can only suggest different approaches. One being add ply strips to the top of the joists. you could cut strips of 4 and 6mm and 12mm ply at 50mm wide. fix 12mm ply to the good staight joists and build up the other uneven joist as necessary with the 4 and 6mm.
As you say a couple of mm wont make a difference to the boarding out on top.
The other option is, assuming with a 6m span you have something like 200 or 225mm joists, laminate an additional joist of say 100, x 50mm to the side of all the existing joists. Fix the new 100 x 50 with bolts but fix them so the tops are to the level you want, then you can ignore the original joists when fixing the top boarding. With this method you would do the nogging after fixing the 100 x 50 additional joists.
 
Thanks for your insights stevethejoiner,

The joists are 20cm X 6cm (60cm centres), the furrings start at the deeper end at 15cm high.

Using a metal straight edge spanning 3 joists some 5 or so joists seem to be higher than the others. These joists seem to be off down their entire length (because of the tarp, I'm limited to what I can easily check).

In terms of the solid timber strutting from what I have read online, you can add 38mm thick timber x ¾ depth of joist so 15cm X 3.8cm. I plan to add 2 rows of strutting plus similar structure to both ends. Would that be sufficient in your view?

Other than my existing method is there a better way to check what joists are out and by how much?

If you need to see specific pics or info let me know, I did include some in my original post?

Thanks again for your help.
 
To see how bad the joist are out you will have to strip the tarp off.
As you won’t have a straight edge long enough use a taught stringline.
You will need 4 nails and two lengths of stringline. Fix one nail at the ends of both the first and very last joist. Leave the nail sticking up an inch or two.
Tie the end of one line to the first nail and walk across all the joists to the last joist and tie the other end of the line to that nail. Do the same to the other side.
Get four small blocks off 1/2 ply or similar and place one block under the string beside each nail.
You will then have two taught lines running 12mm above all the joists.
You can measure down from the line to the top of each joist to see exactly what the gap is. You could use this method to work out the thickness off each packer required for every joist and get the tops of all the joists level accordingly.
 
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I don't see the problem. Screw the OSB to the joists (it will follow any uppy downy bits), then do the EPDM. 10mm is not going to throw anything out. As long as the roof drains.
 
To see how bad the joist are out you will have to strip the tarp off.
As you won’t have a straight edge long enough use a taught stringline.
You will need 4 nails and two lengths of stringline. Fix one nail at the ends of both the first and very last joist. Leave the nail sticking up an inch or two.
Tie the end of one line to the first nail and walk across all the joists to the last joist and tie the other end of the line to that nail. Do the same to the other side.
Get four small blocks off 1/2 ply or similar and place one block under the string beside each nail.
You will then have two taught lines running 12mm above all the joists.
You can measure down from the line to the top of each joist to see exactly what the gap is. You could use this method to work out the thickness off each packer required for every joist and get the tops of all the joists level accordingly.

Got it ;)

Would the addition of 2 rows of 15cm X 3.8cm strutting be sufficient in your view?

I don't see the problem. Screw the OSB to the joists (it will follow any uppy downy bits), then do the EPDM. 10mm is not going to throw anything out. As long as the roof drains.

I may be over worrying (wouldn't be the first time).

I can see what you're saying but what about the plastic edge trim, wouldn't it throw that out of wack?
 
For up to half an inch here and there? I think you're over-thinking, and potentially making huge amounts of unnecessary work for yourself. It's a garage, not a piece of precision engineering.
 
For up to half an inch here and there? I think you're over-thinking, and potentially making huge amounts of unnecessary work for yourself. It's a garage, not a piece of precision engineering.

Hi geraldthehamster I see what you're saying, my wife is giving me an I told you so stare.

I do tend to overdo things sometimes, do you think I could also get away with the Tongue and Groove OSB3 and avoid adding supports for the larger boards?

What about the 2 rows of strutting is this also overkill in your view?

I will still add structure to the end of the joists to support the boards if nothing else.

Justin
 
Speaking purely personally I wouldn't bother with adding struts. Once you've screwed the OSB to the joists you have a fairly solid structure anyway. I prefer to use the full sheets because it's less faffing about. If you lay them with the long edges along the joists (cut if necessary so the edges fall mid-joist), the only upsupported joins will be every 2.4m, and the width of the gap between joists where edges butt together. You could add noggins there if you felt it was necessary.

Please bear in mind I am just a DIYer who has done this sort of thing, and don't do it for a living.
 
Speaking purely personally I wouldn't bother with adding struts. Once you've screwed the OSB to the joists you have a fairly solid structure anyway. I prefer to use the full sheets because it's less faffing about. If you lay them with the long edges along the joists (cut if necessary so the edges fall mid-joist), the only upsupported joins will be every 2.4m, and the width of the gap between joists where edges butt together. You could add noggins there if you felt it was necessary.

Please bear in mind I am just a DIYer who has done this sort of thing and don't do it for a living.

You have more experience than me and what you say makes sense.

I can see that the alternatives are a massive amount of work, what you suggest will be tons better than what was there.

Thanks for your help.
 

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