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 Post subject: flat garage roof advice
PostPosted: Tue Aug 01, 2017 3:38 pm 
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Hi,

I'm unsure on the best design for my garage roof replacement and would very much appreciate some advice on possible solutions. I'm leaning towards an epdm roof, although I do have experience working with GRP too.

I will attach pictures of the current state of the roof(s). The garage is in two sections. One side (smaller), has relatively new joists (although they look rotten now), and is covered with metal sheeting. This side drains into a gutter on one edge. The other half is very rudimental, consisting of what are rotten joists at incorrectly spaced intervals, covered by damaged plastic sheeting. The plastic section is much longer and is pitched towards the centre of the garage and drains into a gutter which runs through the middle. The entire roof section is surrounded by a parapet.

I am considering a warm/cold roof well insulated as I often use the garage as a workshop for woodworking. In order for there to be adequate height along the parapet to allow for the various boards, insulation, covering and possible flashing, I will need to lower the install of the new joists at a slightly lower level. I am considering a wall plate (ledger?), and then joist hangers. 150x50mm joists @ 400 centres as the span is only 2.6m. I have measured the lowest height of the joists, and in some places I won't have enough room for >100mm insulation and the correct space (>150mm) for the flashing under the coping stones and one of the joining walls. Would it be better to stick to a cold roof and insulate from underneath?

As for the design, I can either leave the current longer pitch draining into the middle, or perhaps change this with a box gutter along the parapet?
I would like to remove the coping stones altogether and attempt to, using the correct detail, lay epdm over the edges. The problem is that the neighbours wall is awfully close and so have limited access to fix onto the side of that wall. I was considering pre fabricating the detail, then hang over the edge and screw into the top of the parapet, with the epdm leading straight over this?

The approximate size of the roof 5m x 2.5 (metal sheeted side) and 8m x 2.6 (plastic sheeted side).

I would greatly appreciate the best design suited to this perhaps oddly arranged roof.

Thanks,

Vips

Photos: https://www.icloud.com/sharedalbum/#B0Z5yeZFhCrqDn


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PostPosted: Tue Aug 01, 2017 5:36 pm 
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Its certainly a cobbled together roof :-)

My thoughts, but bear in mind its difficult to visualise the construction.

could you just remove the middle parapet? -if I understand it there isnt a wall below just a lintel.

If it works, Id be tempted to lower the wall along the edge so that you can have a gutter the whole length.

If you want to remove the coping stones, could you raise the roof so it is higher than the top of the painted blockwork and then form a flat roof with raised edges on 3 sides and a fall to the gutter. It would then be similar to your neighbours felted flat roof and similar height.



For this message the author Notch1 has received gratitude : Vips
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PostPosted: Wed Aug 02, 2017 12:01 pm 
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Hi Notch,

Thanks for taking the time to reply. It really is cobbled together!

I did want a warm roof over the entire structure, but with there being no space to lower the joist further (various door frames fouling) to allow for adequate room below the coping stones for the insulation, I have given up on the idea.

Somebody suggested building up on the neighbours side creating a mono pitched roof. Unfortunately, the mono pitch at the front of the garage would only allow for a height of a further 400mm, so the pitch would be a measly 9 degrees, unless vaulted, which again creates issues with lowering joists. Aside from rebuilding the front mono pitched roof and the entire structure a fair amount higher, at least another 750mm (15 degrees), that solution seems like a lot of work, for not a lot of pitch. In the future, if we're still around at the property, tying in the utility room roof with the garage to create a decent pitch and height would be best I think, but for now, that's way past my technical know how.

Feeling like I'm considering defeat, I'm going to repair the rotten joists with adequate sister joists in the steel section of the roof, and create a similar wall plate and joist sequence with 2x6 at 450 centres for the longer section ensuring I get a decent fall. A box gutter at the end will take care of the drainage.

As for the coping stone issue, the current ones have no drip detail and are loose. So I have two options, cut in a drip detail in every single one, paint and relay, or buy new stones. I have found an epdm detail which shows the epdm laid 2/3 under the coping stones. This should sort out my issue of not having the minimum 150mm for flashing.

I am still unsure on how a wall plate and joist construction ventilates? If I was to board with insulation backed plasterboard at a later date, there would be no ventilation, or do I simply attach between the joists to the OSB underneath?

Let me know if the above sounds good.


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PostPosted: Wed Aug 02, 2017 2:23 pm 
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Would it not be possible to remove the coping stones and build a flat roof flush to the top of the walls?

Basic process as per
1. remove existing coping stones and roof.
2. fit a 4 x 2 flat on top of wall
3. fit an 8 x 2 wall plate with the top edge flush to the top of the 4x2 plate fitted
4. fit joist hangers to new wall plate
5. fit new joists so top of joist is flush with top of the 4 x 2 laid flat
6. insulate over, then 18mm osb t and g decking
7. form upstand around 3 edges to make drip
8. fit epdm so it goes over the edge detail and down beyond brick work.

If you have the height, you could just fit your joists on top of the wall and close off the end with a fascia board

You will end up with a conventional flat roof like the neighbours avoiding any parapet walls or flashing needed.

it would be a warm roof no need for ventilation at all.

It would be much easier than keeping the existing cobbled box gutter and parapet walling with flashing.


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PostPosted: Wed Aug 02, 2017 2:40 pm 
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For most of the roof, that is good option, but raising the wall and even with a decent fall, the section where the utility room is connected would cause an issue. The room has a roof that terminates with a gutter that is barely 200mm above the top of the current garage parapet. The joists would have to be heavily skewed (vaulted?), and even if they were, I wouldn't have enough room to allow for the insulation. This section would probably need a box gutter section too.

I spoke with an epdm distributor who claimed Firestone epdm and classic bond is designed to terminate 2/3 under the coping stones, and where this is not possible, a termination strip will suffice. https://www.rubberroofingdirect.co.uk/training/cat/duoply-edge-tim-terminations/post/duoply-how-to-install-coping-stones-with-epdm-membrance/

This does solve the most of the issues, as there will be minimal, if any flashing needed.


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PostPosted: Wed Aug 02, 2017 4:23 pm 
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Ah ok! I can see the white gutter in the first image.

You could still come higher, just finish the top of the roof below the parapet and dress over with EPDM. If you eliminate the flashing then you dont need 150mm drop

The new roof could finish just below the white gutter, and weather to the roof.

The fall on the flat roof only needs to be 1:40, so not much over that span.

Its difficult to see a solution from the images, but it seems very doable to me.

If the span of the joists isnt very big, then you can have 150mm joists with 50mm insulation over with 18mm osb. additional 50mm in between.

Ideally you dont want to fix celetex insulation directly over joists, but you can use 11mm osb first to prevent the joists digging in.

it is in quite a mess, so definitely worth sorting out before it starts leaking :-)


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PostPosted: Wed Aug 02, 2017 5:34 pm 
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starts leaking... lol

Thank you notch again for taking the time. It is difficult to see a solution, especially from only the photos. I'll measure to see if I can get 50mm in there. As I understand it, this will still require another 18mm of OSB on top in order to be able to walk on it comfortably for maintenance/repair?

I shall keep this thread updated with my progress.

Thanks again.


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PostPosted: Wed Aug 02, 2017 5:49 pm 
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You can do 50mm celetex in between the joists if it helps with height

fix tile batten along the joists 50mm down, then put in celetex to finish level.

tape over every joist with foil tape to create vapour barrier. 18mm osb over top.

We make orangeries so every job involves a flat roof, often with quite awkward falls or weathering details. They can be a head scratcher!


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PostPosted: Fri Aug 04, 2017 9:53 am 
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That seems like a decent solution. Providing there is no gap between the insulation and osb, there is no chance of condensation problems?


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PostPosted: Fri Aug 04, 2017 11:15 am 
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Provided foil of celetex is continuous that will act as a vapour barrier.

There will be slight thermal bridges at the joists if insulation is in between as timber is not as good insulator as celetex. But its a possible compromise

OSB must be tight -a gap here could allow condensation, although provided there is no route for passage of warm air from inside room to underside of osb, there isnt a risk of interstitial condensation.


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PostPosted: Fri Aug 04, 2017 11:57 am 
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That sounds like plan. I will use 72mm foil tape (http://www.screwfix.com/p/aluminium-foil-tape-silver-72mm-x-45m/50629) so there is some overhang, and make certain there are no gaps in the barrier. The thermal bridging is unfortunate, but I'm not expecting great thermal efficiency from the workshop as the walls are single. A log burner from offcuts will have to do.

Thus far, I have researched the following:

M12 bolts with square washers at 450 centres when making sister joints to the joists to be repaired. I've made certain two need repairing, the rest have no wet/dry rot.
M12 through bolts (http://www.screwfix.com/p/rawlplug-xpt-throughbolts-m12-x-180mm-10-pack/87397) for the wall plate, one in between each joist (@ 450 centres). Will these need any chemical fixing too?
Noggins running through the middle attached with 5x100 screws (http://www.screwfix.com/p/screw-tite-pz-countersunk-screws-5-x-100mm-100-pack/37146?_requestid=153878)
5x50 screws to attach OSB to joists with a staggered arrangement aiming for joins and edges to supported by joists.
Termination bars under coping stones where flashing is not possible.
EPDM to run 2/3 under coping stones.

Still unsure on whether to go for treated or untreated timber for the joists and furrings.
For the coping stones, I have read I can either cut in a drip detail in each, or apply a bead of sealant. If either of the two options aren't worth it, I'll have to buy all new coping stones.
I'll be making the box gutter section from OSB. This can then be attached to a joist placed at the end and encapsulated in the insulation.


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PostPosted: Fri Aug 18, 2017 9:29 am 
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An update as promised.

The smaller section of the garage is almost complete. Joists were repaired with treated equivalents, and sections were added to 'sister' to the joint.

Unfortunately I did hit a few problems as I progressed, so in the interest of possibly helping others, I shall provide my thoughts here.

The firring strips bought were marginally slimmer than the ones I had removed, so a ridge was to form near the front of the garage. At this point my 'tradesman' brother in law appeared who in the end caused a real headache. As I was contemplating a solution, he stated we should cut some shims for each joist to bring them up to the same level. This seemed like a reasonable solution, so I ripped the appropriate pieces. Whilst doing so, he had removed all the OSB boards, applied the strips and placed them all the boards in the incorrect places. My precious 3mm gap around all the boards was gone, he told me the foil tape was a 'daft' idea and had placed the shims above the foiled firring strips. As I had originally marked out my joists on the board to make sure each joist was correctly screwed into, he had attempted to use the same lines, and low and behold, I had continuous lines of screw holes popping out either side of the joists! To make matters worse, he drove the screws in at least 10-15mm in an 18mm OSB board!

We went onto apply the the last boards near the fascia which he freehanded with his circular saw, even though I have a Festool tracksaw which should have been used, so now I have a wavy front fascia. The final straw was when I heard a massive thud to find he had tried to remove the remaining mortar under the coping stones with a large sledgehammer and had hit the wall towards its unsupported side!. Now I have a two courses of mortar to somehow repair as it seems dangerously loose. :angryfire:

Once he left, and had somehow ruined all my work in the space of a few hours, I thought about how to repair it all. I am not so sure why some people aim for speed, and leave absolutely no quality in their work. In this particular case it was only dangerous! One whole board of OSB was loose!

My old man had popped round to help me fix and level everything. We spent hours digging out screws he'd driven in so deep and almost horizontally. I did consider removing it all, as he had ruined the edge of most of the boards, and I had extra holes everywhere, but I am on a strict budget :(

For the covering layer, I chose to go with the 3M LS657 system. The points to note on this were to not span too large a space with duct tape. I had edging around the wall to reduce the angle at which the OSB meets the parapet wall. Here the duct tape spanned the entire edging and some of the parapet wall. After the first coat, we noticed the duct tape sag. Having spoken to 3M, he recognised that this area was one for the 3M reinforcement mesh, not duct tape as advised by the distributor. He suggested using a gap filler for this side, and overcoating with mesh if any problems arise. I did just that and applied another coat. The roof is now watertight, and I have learned lots for the second larger side of the garage roof. The great thing about this system is that it can be applied in the wet, close to 0 degrees Celsius and is immediately waterproof.


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