Lateral force of Roofing sheets on a pitched flat roof.

As per usual - difficult to describe the problem without actually seeing it, but here goes:

I have an attached garage which is 6m long and 3.3m wide and it runs along the side of the house so that one of the 6m walls is the house wall.
Joists run across the width of the garage - with a very shallow pitch away from the house. The drop is 10cm (4") over that 3.3m span and the existing roofing is corrugated plastic sheets, so is very lightweight.

I would like to replace these plastic sheets with 2 x layers of 18mm OSB and a waterproof layer of either GRP, lightweight shingles or however many layers of felt as is deemed necessary.

I will need to use 16 sheets of OSB, which will weigh approximately 550kg's --- and that will be aside from any of the waterproofing materials.
Additional joists will also be fitted, as the existing joists are 6x2 whish are spaced 1.2m or 4ft apart and I will be putting at least one new set in each gap, reducing the joist spacing to 60cm or 2ft.

Is there a formula or calculation that could ascertain what sort of pressure/force would be applied towards the wall opposite the house which is at the lower end of the 10cm drop in pitch?

If this is going to be a concern, then I am considering bolting 5 x steel beams (perhaps 80mm x 80mm steel tubing with a plate welded to the bottom as a foot) to the floor and supporting each of the 5 x new joists on top of these so that the new OSB sheets are supported directly by these steel beams?

Thanks

Steve

You should be putting 175x50's at 400mm centres for the 3.3m flat roof.. your current joist sizes are inadequate unless they are ridiculously close together..

As for lateral loading from the roof well IF you fully connect the joists to the wall then youll get a small lateral loading.. something like this:
Pitch = Tan-1 x 0.1 x 3.3 = 1.73 deg
Load = 1.5 x 3.3 x 0.4 / 2 = 1kN (angular) based on 400mm centres
Lateral = 1.0 x Sin 1.73 = 0.03 kN per joist (or 3kg)
So probably as bad as a fat guy leaning on the wall..

If you increase the pitch and the joist centres.. then you might one day have a problem.. maybe around 30deg
Course if you dont fully fix the joists and simply support them then your get no lateral loading..

Basically boils down to what type of wall your taking the support onto? If its some flimsy single skin brick wall thats really high then maybe youll need to do things

Thanks for that.

Yes, the existing rafters are spaced way too far apart and I can only assume this was because of the original notion of supporting a lightweight plastic roof.

And yes, it is a single skin 4" block wall, which is why I am concerned.

I think I might look at 2 x new joists in between each of the existing 1.2m spaced joists (bringing it down to 400mm centres) with each of these new joists being supported by a steel beam rather than the wall alone.

If I did as my above thoughts, then there would be a total of 10 of these beams supporting the 10 x new joists, plus 550kg of OSB and the waterproofing --- with the existing 5 x joists that are tied into the wall being in the equation as well.

Any suggestions as to what size steel "beams" to use - and should I use square tube, channel, I-beam etc etc?

Thanks for any advice.

Steve

turnitup said:

And yes, it is a single skin 4" block wall, which is why I am concerned.

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If the existing wall has piers at about 3m spacings then it might be adequate.. what was/is the wall used for? was it an old garage wall?

I think I might look at 2 x new joists in between each of the existing 1.2m spaced joists (bringing it down to 400mm centres) with each of these new joists being supported by a steel beam rather than the wall alone.

Click to expand...

Like i said the 150mm deep joists are inadequate and you should be putting in 175mm deep ones at 400 centres.. or least to compy with building regs.. (well i suppose 150mm deep at 300 would be fine)

Any suggestions as to what size steel "beams" to use - and should I use square tube, channel, I-beam etc etc?

Click to expand...

Depends on your choice really.. could even do it with a couple of 50x200mm timbers bolted together so long as they are supported on a post at 3m intervals.. I beam or PFC of about 150 depth would be way sufficient.. but if you want something to run the full 6m then it will be chunkier..

You would be better off using the wall if you can, would avoid having to fix to the foundations with your new posts.. might prove easier..

In the end im guessing its still a car port and not some conversion or garage?

Static said:

could even do it with a couple of 50x200mm timbers bolted together so long as they are supported on a post at 3m intervals.. I beam or PFC of about 150 depth would be way sufficient.. but if you want something to run the full 6m then it will be chunkier.

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I'll jump in here as I had a chance to check out the building in question yesterday afternoon, and marks my first face to face with a fellow Diynot member. Not as traumatic an experience as I had envisaged


100mm lightweight block wall, which had been used to infill between the 4" timber posts of a carport on the side of the house. Wall is about 2.4 high and roughly 6 long
One end is restrained by a trench block wall, a couple of blocks wide, which the timber door also swings from.
Other end is a wobbly door frame so the only other restraint comes from the roof joists which are notched into the blockwork (the blocks have been cut to accomodate the joists) Didn't get a chance to measure the joists but they appear to be deeper than 6" probably close to 7 and only holding up corry pvc sheets.
No pillars in the centre of the wall, also I suspect no proper foundation as the the single course below the dpc are mortared straight onto the concrete slab, which in itself appears sound.
I suggested a wind post type affair bolted to the concrete floor in lieu of a block pillar, as room width is at a premium.

Your idea seems like an obvious solution, fix rhs centre of the wall, block infill where the dodgy door frame is to take one end of steel/timber, other end to rest on the trench block wall at the front.

The only issue here is that the wall might be too lightly loaded so would it be wise to keep the existing joists in the wall to keep it restrained at the top? So roof loading is shared between 'new purlin' and existing wall. Or would bolting the rhs to the wall midway suffice?

Sounds like it would fall over in a stiff breeze without the roof joists on it.. personally i wouldnt want that coming down on my car..

I would be tempted to frame up the whole thing and brace it back to the building.. most costly and time consuming but end result would be a stable wall and carport that will last.

As Deluks says posting up the corners of the new carport seems best, can then strap in the wall to the new carport and fix it all back to the house and stop things moving around in future.

@ Static and Deluks --- thank you both VERY much, you have been a great help.

If I may ask one final question before I leave this thread in the clutches of the archives:

I would be tempted to frame up the whole thing and brace it back to the building.. As Deluks says posting up the corners of the new carport seems best, can then strap in the wall to the new carport and fix it all back to the house and stop things moving around in future.

Click to expand...

Are we still talking about the concept of supporting a horizontal member on the corner posts (additional vertical support at 3m midway point - or perhaps at 3 x points, therefore every 1.5m) - and ensuring that the entire wall is strapped to this member (thru-bolts?) with the 10 x new joists at 400mm centres (7"x2") that will be tied directly between the house and this member sufficing as a reasonable brace as well as a support for the new roof?

If not, then I will assume a more robust framing and strapping concept is what you are saying and I will discuss this with the able (and hopefully willing) contractor who may be assisting with this job ... hmmm, now I wonder who that might be

Thanks
Steve

Basically yes, im saying by the sounds of it that way is a pretty small for its height, even single brick thick walls (9") upto 2.5m can be dangerous. I would expect staggered wall or 13" piers every 3m or so.. So as your making a proper job of the car port tie the top of the wall to your new frame and stop it toppling over in a gale ontop of your Bentley..
Heres what 10mins at lunch can produce:
View media item 21743Can tie the support beams (taking the new roof) to the top of the wall or the top of the posts.. whichever is easier..

Gotcha - once again, thanks for the time and advice.
Steve

Resurrected, again!

Closer inspection has revealed that wall may not be taking any roof loading at all. The joists sail through cuts in the blockwork and rest on a purlin (prob 6x2, will confirm) which runs full length and is supported by four 3x3 posts which are metposted into the concrete floor. Standard carport construction.

The wall, having been built 'around' the existing carport between the posts is thus only restrained by the render on the outside, and the snug fit of the blockwork up to the underside of the joist. No idea if expamet was used under the render Will dig out the Bosch DMF although being lightweight block and with the render looking intact, probably.

So back to Steve's closing paragraph in the opening post:

"I am considering bolting 5 x steel beams (perhaps 80mm x 80mm steel tubing with a plate welded to the bottom as a foot) to the floor and supporting each of the 5 x new joists on top of these so that the new OSB sheets are supported directly by these steel beams?"

What's a suitable product that will eat into the room space as little as possible?? (Straight onto the existing conc floor using a plate to spread load)

Possibly even a 3x3 with bolt down metpost per joist?