Flat roof falls GRP roof

[Canski]

Hi Guys

 

I am currently in the process of re designing my flat roof which will be covered in GRP. I note from NHBC that the roof should be designed with a 1:40 fall to give a finished 1:80.  
 

the build up (top to bottom) is  

GRP

18mm T&G OSB3

200mm PIR

Alutrix 600 VCL

18 mm ply or OSB ( unsure about which to use)

massive firring strips

225 mm pozi joists

vapour barrier

15mm plasterboard

The rectangular roof is sandwiched on one long side of the rectangle with the first floor cavity wall, the opposite long side is a cavity wall parapet and one short run also has a cavity wall parapet the other short run I’d like to run straight off into a gutter (at the top end of the drawing )

anyway the rectangle is 4.7 m long and needs firings perpendicular to the joists which run across the short length. If I make these at the 1:40 fall that NHBC recommend then they will be from 50 mm up to a whopping 167 mm. This would give me a total of 628 mm at the ‘high’ end. I have 675 mm to the cavity tray outlet leaving 47 mm upstand from the roof to the cavity tray and weep vents instead of 150 mm. I know this not viable. 

 

Here comes the problem ….I’m late to the game in planning this and it seems that if I do this then the high end won’t have a 150mm upstand against the house wall due to the cavity tray already  being positioned too low. Is it really necessary to have a 1:40 design to achieve 1:80. I spoke to my designer today who suggested (if it’s possible)reducing the pozi joist depth to 150 mm and slightly reducing the firings to 114 mm giving a designed fall of 1:73. 
 

I would like to keep the runoff to the short end rather than have unsightly penetrations and down pipes to the long side. 

 

Any suggestions ?

 

Edited November 9, 2023 by Canski
CrP spelling due to cold hands 🙄

[SteamyTea]

Properly layed up GRP can handle standing water for a very long time. If you think about what those sewage tanks put up with, wee and poo one side, ground water the other. They survive.

 

So reducing the fall a bit should not be a problem.

 

Use ply against the GRP as it is easier to lay up onto. Takes less resin as well.

[joe90]

Loads of boats are made of GRP and last a very long time 🤷‍♂️

[SteamyTea]

5 minutes ago, joe90 said:

Loads of boats are made of GRP and last a very long time 🤷‍♂️

Until the killer whales get them.

https://www.newscientist.com/article/2378796-why-have-orcas-been-damaging-and-sinking-so-many-boats/

[nod]

19 minutes ago, joe90 said:

Loads of boats are made of GRP and last a very long time 🤷‍♂️

Good point 

I’d no experience of GRP when I laid ours A friend who works in a boat yard came round to advise me He saiid you will finish with an upside down boat 

[SteamyTea]

5 minutes ago, nod said:

A friend who works in a boat yard

Keep them as a friend, there could be lots of work to be done in here.

[Canski]

1 hour ago, SteamyTea said:

Properly layed up GRP can handle standing water for a very long time. If you think about what those sewage tanks put up with, wee and poo one side, ground water the other. They survive.

 

So reducing the fall a bit should not be a problem.

 

Use ply against the GRP as it is easier to lay up onto. Takes less resin as well.

Thanks ST it just seems daft that we have to design a 1:40 fall to end up with 1:80 when GRP is flat. I could understand it if there were lumps like with felt. Especially at the run off point. Is there a lump where they join the preformed run off thingies with GRP?

[SteamyTea]

2 minutes ago, Canski said:

Is there a lump where they join the preformed run off thingies with GRP

A single layer of 450 GM/m² mat is about 1mm thick. I would always recommend 3 layers on a flat roof. Where it overlaps you can get a ridge, but if all the layers have the staggered overlaps then it is reduced. You can also get rolls of mat that has one cut edge and one torn edge, overlapping the cut and torn by about 50mm, and some decent consolidation makes it virtually bump free.

If it really is an issue, then 5 minutes careful angle grinding takes the ridge out.

It really comes down to the skill of the laminators.

 

[Gus Potter]

2 hours ago, Canski said:

I am currently in the process of re designing my flat roof which will be covered in GRP. I note from NHBC that the roof should be designed with a 1:40 fall to give a finished 1:80.  
 

the build up (top to bottom) is  

GRP

18mm T&G OSB3

200mm PIR

Alutrix 600 VCL

18 mm ply or OSB ( unsure about which to use)

massive firring strips

225 mm pozi joists

vapour barrier

15mm plasterboard

Good to see you making progress.

 

Hope this helps.

 

As you are doing this yourself you are probably doing a good job.. getting levels etc right. I like your roof build up spec apart from "the massive firring pieces" and a vapour barrier to the underside of the joists. Your joists are in the warm envelope.. why the vapour barrier.. maybe best to let any water vapour that condenses in the joist depth zone just re evaporate and come back into the room? Anyway by the time you paint the ceiling it will be failrly impervious anyway so will resist most say steamy bouts that may occur in a say kitchen area?

 

Ok now the structural side.

 

These types of roof are pretty light weight so don't deflect that much under normal conditions.. so under these loading cases your 225 Posi joists won't deflect / creep that much. Practically under their self weight they will dish a bit in the middle as you would expect. When you lay them examine each one as the won't be straight.. put the bendy side upwards.. like a pre camber.

 

On the top side of the drawing this will be fine but you may trap water behind the kerb at the bottom of the drawing. An easy way of sorting this out is just to add some extra GRP layers on the top / high side (bottom of the drawing) to give you local fall so the water / snow melt does not get trapped behing the kerb at the bottom of the drawing. Once you build it and put the spirit level on it you'll see where you need to build up locally... this you can do with GRP but not say as easily and well with  EPDM or PTFE membranes.

 

It looks like you have potential for snow drift loads, snow being driven against a higher parapet wall and falling onto the flat roof or falling off a higher roof onto the flat roof etc. I think your SE may have clocked that hence the joist size.. so you can go back to BC if asked and say my SE has accounted for the loads so can use a shallower gradient... 1:80

 

For all it's not just the extra weight of the snow but the fact that snow blocks the drainage routes up that causes the problem.. it's that wet snow / rain you get.. it clogs everything up and is heavy.. not like powder snow you ski on... it's just nasty.

 

Now it looks like your SE has sized everything up properly in terms of joist size thus you can use the finished fall of 1:80.

 

Roof length / 80 fall  is 4940 / 80 = 62mm fall.

 

But this requires a configuration of firring pieces that will be 62mm thick at one end tapering to zero..! This means that you have to cut each firring piece to a different depth, to a close tolerance and fix it to the top of each joist.. which the merchants are not keen on supplying as each firring piece is bespoke.  Where the firring pieces are thin you use longer fixings and glue... the glue is important as is allows transfer of the forces in the roof to the walls. called diaphragm action.. long fixings tend to bend so we need the glue to properly bind the firring to the tops of the joists.. like a Glulam beam.

 

The thin firrings are just packers. You need 40mm screw penetration into timber so as soon as the firrings are less than 40mm thick you need longer fixings that go all the way and at least 40mm into the posi joists flanges.

@Russdl has done something similar as has height and thickness constriants. I hear he cut the firring pieces on site with a Skill saw, got them spot on and made a saving... even put a bevel on them as the top side of the firring piece needs a scuff to follow the plane of the roof as it slopes perpendicular to the firring piece.

 

In summary.. maybe @Russdl will let you know how he did it... with varying depth firring pieces tapering to effectively zero.. which could solve your dilemma. In other words you roof fall will be 62 mm.

 

At the high end the roof will be 62mm higher. If you end up with only 100 mm upstand at the bottom of the drawing this should work if you seal it all up well as you have not got potential snow drift build up or wind driving the rain up the wall as it is on the leeward exposed side?

 

 

 

 

 

 

 

 

 

[Russdl]

As @Gus Potter mentioned we’ve cut many many many meters of firings to go on top of C section steel joist, diminishing in size front to back so that the roof deck for our new garage has the required fall. I shopped around and no one was really interested in doing it for me and ultimately I’m pleased about that, quality control when you’re doing it yourself is so much easier. It took time, but that was mine to give. 
 

With the drawings from a certain SE and good old SketchUp I knew exactly how much scuff (or taper) I needed on each firing so that the boards laid flat on the firings instead of just sitting on the high point of each firing. For us it’s was 0.7mm - we settled for 1mm as it was easier to measure!
 

So, for example, say one of our firings needed to be 45mm thick on the uphill side, it needed to be 43mm thick on the downhill side. I measured the 44mm and 43mm on either side of the timber, drew a line and then lined up the skillsaw with the pencil line, did a few test cuts close to the line on the waste side of the timber to confirm the angle of the saw and then set the guide and cut away. The set angle remains the same for all firings. 
 

Our joists are equally spaced and each firing was 7mm bigger than the previous so I just moved the skillsaw guide by 7mm for the next firing strip. It worked really well, each sheet of OSB sits squarely on all the firing strips it touches and has the required fall. 
 

Hope that helps. 

[Gus Potter]

@Canski

 

Check out @Russdl how he cut the firrings and set up a basic Skill saw.. gold dust!

 

Now you have my techy bit and someone who has been there and worn the tee shirt on site.. will this work for you?

 

 

[SteamyTea]

8 hours ago, Gus Potter said:

An easy way of sorting this out is just to add some extra GRP layers on the top / high side

You can use corrugated cardboard to thicken up a laminate, or get some Cormatt XF if you want to do it properly.

I would lay down two layers of 450 g/m² mat, a bit resin rich, pre-wet the cardboard on the rough side, lay into place, then one layer of 450 g/m² mat over the top in the usual manor.  Same with Cormatt.

Edited November 10, 2023 by SteamyTea

[Canski]

Sorry guys I should have stated that I'm handing the GRP over to the professionals. I still have nightmares after the first and last time I did it. I looked like the abdominal snowman after an hour.

 

I thought that if the firings were running perpendicular to the joists then the minimum depth needed to be 50 mm to support the 18 mm sheets ?

 

Also if my joists are at 600 mm centres shouldn't I be using 22 mm sheets like in floors ? I know that the load isn't great on flat roofs but as Gus has kindly pointed out in his very informative reply there are snow loads to consider.

 

Would it be wise to use diminishing strips directly fixed on to the joists to create the fall rather than firrings ? I'm not kitted out with the fine equipment for cutting firings to the standards described on here. I thought that if I started with zero at the outfall position and fixed a 62 mm timber to the high end I could string a tight line and infill with pretty accurately infill timbers to suit. This would save me some depth having negated the need for a minimum 50 mm firing ran the opposite way to the joists.

 

Regarding the kerb at the bottom of the drawing I had already thought about that and was considering packing the firings / diminishing strips up slightly at the middle of this to create a gentle sideways fall to each side of the lantern. This obviously will entail getting a good fix between the top and bottom sheets to pull them up tight.

 

I was thinking of these Fixings to use and hoped I would be able to sink the 'mushrooms' in to the sheet slightly to avoid seeing them afterwards.

 

 

 

[Mr Punter]

Yes, the firrings need to be 40-50mm minimum when perpendicular.  Run the firrings at 400 centres and you can use 18mm sheets.

 

I have bought firrings at 1:60 and this seems fairly standard.  You could always have a gutter either side and a ridge in the middle.

[scottishjohn]

recomendations for a grp walk on balcony area

no  of layers?

[markc]

7 minutes ago, scottishjohn said:

recomendations for a grp walk on balcony area

no  of layers?

Grp thickness g/sm really depends on what is supporting it, larger spans are better done as a sandwich with light weight core