Freestanding Fibreglass Flat Roof Advice

4 Jan 2006
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United Kingdom

I am building a block and timber workshop in my garden and intend to put on a fibreglass flat roof.
I am hoping to fit the roof joists today / tomorrow and before I do I need some advice on the structure to ensure that I can fibreglass it properly later. All of the guides I have seen usually have one side of the roof butted up against a wall however mine will be sitting on top of the tall wall on the right of the pic below, this will be referred to as the back of the building.

Her is the partially completed frame (minus noggins and additional wall plates):


On top of the frame in the pic I will be fitting an additional top plate. A wall plate will also be fitted to the top of the back wall so that all the plates are level.

I will then fit 4 x 2 roof joists running from right to left in the pic above e.g. from the tall wall at the back. This means the drop of the roof will be towards the path which you can just see in the left of the pic. My intention is to have a gutter running along the front of the building (in line with the path) with a down pipe either depositing water down the steps or into a small water butt on the RH side of the building (looking towards the door opening).

1. When fitting the roof joists on top of my wall plates should they end flush or overhang all around slightly?

2. I am using 18mm OSB3 as my base board for the roof, I understand I need a 3mm gap between any joins but should the board overhang the roof joists or be flush?.

3. At the front of the building where I intend to fit the gutter should the roof joists and or board overhang to allow for a downpipe?

4. Do I need to fit an arris rail all around?

Looking at this page everything is flush?

I understand that I need to fit furrings on the tops of the roof joists to give a 1:50 drop.
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1. Overhang all round to allow for a drip.
2. Flush
3. The downpipe will be underneath the gutter, so this becomes immaterial.
4. Would be advisable, but not desperate at all, it's only a small shed.
5. exactly which part of that page am I supposed to be looking at.
6. It's not a great depth front to back, so you could raise the rear level to give you a fall.
7. Have a look on Youtube at EPDM, and you may find it easier than fibreglass.
1. Overhang all round to allow for a drip.
2. Flush
3. The downpipe will be underneath the gutter, so this becomes immaterial.
4. Would be advisable, but not desperate at all, it's only a small shed.
5. exactly which part of that page am I supposed to be looking at.
6. It's not a great depth front to back, so you could raise the rear level to give you a fall.
7. Have a look on Youtube at EPDM, and you may find it easier than fibreglass.

Many thanks for the reply.
1. How much overhang, 10mm? Not sure how I will be able to overhang at the sides?
2. OK
3. DOH!!
4. OK
5. Sorry - the line diagram under the sub heading "Building a Flat Roof".
6. OK - by adding another wall plate or packing. I could cut out the front wall plate where the joists intersect it but that would more work and affect the strength ? Just need to work out the correct height to get the correct drop. According to this page it should be 1:80. In this thread it suggest designing a steeper fall of 1:40 - 1:60 to allow for sagging?
7. EPDM - just took a look - seems easier to install just need to work out what bits are needed and cost it up.
1. 20mm would be better, but it's personal really, just remember to take into account the side coverings.
5.This ones tricky to advise on. As you say, everything looks flush, but you wouldn't want that on the final build. You need overhangs to push the water away from the walls, and you need to seal the gap where the wall covering meets the blocks, or have the wall coverings drop down over the gap, so the uprights would be flush, but the wall finish wouldn't be. But if you intended to render the blocks, then you'd need to take that into account as well.
6. You have a support on the front walls on either side of the door, but are you intending to put a wall plate across the whole of the front section. If you are, then you just need a slightly higher wall plate at the rear to give you your fall. Once the roof joists go on, you then need to add noggings to stop any twist, and to provide support. You don't really need to start cutting out any of the joists, and certainly not the wall plate. Setting the joist stright on to the wall plate is quick, but requires a bit more wood for the noggins, so it's your choice. It looks like you have about 2m depth front to back, and on a 1:50 fall, you need 40mm difference in height, so you could take out a 40mm notch at the front, or make sure the rear wall plate is 40mm higher.
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1. So my joists will overhang the timber frame by 10 - 20mm + any extra for wall coverings (this will be membrane, battens and cladding).
On the sides I will need to add this extra by screwing more timber to the side of the roof joists, I guess it does not have to have the same depth as the roof joist but will need to take into account how the facia boards will fix to it.

This is the kind of look I am going for, albeit mine will have a part block walls and the roof will fall from rear to front. It would be nice to fall the other way but it sits against a boundary wall (mine) so I cannot have guttering on the rear. A 10mm - 20mm overhang at the rear will be OK.


6. Yes a wall plate is about to go across the entire front. I would probably add the 40mm on the rear wall plate as I have a spare length of 3 x 2 which I could mount on top of the 4 x 2 wall plate. When you say or take a 40mm notch at the front do you mean out of the underside of the joist?
Yes noggins were planned but not sure how many? The longest span is about 2.24m and the joists were to be spaced at 400mm centres (I seem to recal)
I fiberglassed 35sq metres after watching these videos and I am very pleased with the end product.

6. If you're going to add a higher wall plate at the back, then ignore cutting out the 40mm from underside the joists. The noggois will fill the gaps between the joists.

Um, which videos Andy.
Mmm I have just checked the roof joists that I bought based on a response in this forum. They are 4 x 2 (45mm x 95mm) C24 graded timber which according to some span tables that I have found seems they are inadequate so some advice please :)

In this table flat roof joists start at 95 x 125mm but according to the third table down on this page for C24 timber (Which I am guessing is the same as SC4) is good for spans up to 1.94m

Now that I have all but the rear wall plates on I have just been out and measured the spans (outside of block wall to outside of wall plate) at 400mm gaps. I will need 8 joists in various lengths (due to the shape of the roof). The shortest span is 1.35m and the longest is 2.10m (Only two joist spans exceed the recommended maximum of 1.94m). Does this mean I need to change all of my roof joists to 47mm x 125mm or would I be OK?

With 97mm deep joists, if I decide on a cold flat roof construction, I will just manage to insulate with 50mm Celotex leaving a 45mm ventilation gap just under the required depth.
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This is a good introduction to DIY GRP roofing. Hope it helps
Thanks for the video link - I remember watching these before I started the build. My only concern with DIY is the cost of the disposables and materials and getting it wrong v getting someone in? Winter is coming!!

Looking at the roof build itself / overhang to keep dripping away from the walls. I was going to overhang my joists to aid this however, in the mock roof I see the Facias had been fitted before the Fibreglass and Trim and the battens holding the side and drip trim seem to be screwed into the Facia Boards? As the Facia Board is fitted to the side of the outermost roof joist it would be proud of the wall therefore with the added width of the battens, side and drip trim everything will be well proud of the wall.

Before I cut and fit the roof joists I need to confirm exactly how I am going to construct the roof. A quick search of Google here shows some examples. Most of them look like the roof finishes flush with the walls and the overhang is created by the facia and the drip trim and raised edge trim. What I do need to consider is that if this is a flat roof then I think I need to have an overhang at the front with a soffit providing ventilation?
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Just done some sketching ref roof design. If this is to be a cold flat roof then an overhang on the front to incorporate a 25mm air vent into the roof space will need a corresponding 25mm air vent on the opposite side. Most designs I have seen is where the roof is butted up against a wall.

The problem I have is that structure is built up and onto a boundary wall (It is my wall). To accommodate an air vent I would need a 50mm overhang or thereabouts which I don't think I would get away with this overhanging into the neighbours property?

I had not planned to install any form of heating in this work space but when I am in there breathing will I guess cause warm moisture which would condense on the ceiling if it is not built correctly? So ventilation will be needed if I go with a cold roof.

Maybe I could build a raised section at the back of the roof to vent out from each side? But maybe I am over engineering this?

Other option is a warm roof but that will take the build higher and the roof would be softer?
Just found this thread, it seems that if I omit the void in the roof and fill it with insulation then it becomes a composite roof and no ventilation required.
So the joists are 95mm deep so if I fill the joists with rockwool and put on a ceiling covering all should be fine and dandy.

I'll still overhang the front to provide a run off and protection over the door way.
I think you've basically worked everything out, but there's be so many holes in the place, you'll efectively have enough ventilation.

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