Flat Roof Angle - Firrings v 'Angled I Joists' v Offset Wall

[Saul]

I have read all the previous chains on flat roofs and know pitch angle and methods has been covered to some extent already but trying to wrap my head around why you would use Firrings as shown in the first picture when you could just create I Joists that have a slope manufactured within them ie by cutting the OSB Web such the bottom edge is flat and the top edge is sloped. In effect the I Joist would then have one end being 450mm tall and the other 300mm tall overall. (I know in the below example I have run Firrings front to back. If I used them this is what I would do as would rather have the gutter at the back)

 

As well as custom angled I Joists I know I could also build up one side wall to be taller by adding a row of 2 bricks which would achieve the same fall off with guttering placed on the side of the garage rather than the back. I just don't like the gaps that this creates and think an angled I Joist allows everything to be nailed together square.

 

As this is a single skin 100mm Block 7.3 build with Piers and no heating it won't be insulated and will simply have an 18mm OSB roof with EPDM Rubber, Fascias and Guttering. STS board will be attached the timber work and the sides will be rendered. Roller/Double Door.

 

Appreciate any comments. Ultimately I need to build this as cheaply as possible as people seem to think £45k for a double garage is normal. I want to achieve this for max £20k in total including base finished and ready to store my classic car.

 

Cheers

Saul

 

 

[Mr Punter]

Probably cheaper with conventional joists.  Run the firrings in the same direction as if you run then perpendicular they need to still be 40mm at their thinnest.

[Saul]

29 minutes ago, Mr Punter said:

Probably cheaper with conventional joists.  Run the firrings in the same direction as if you run then perpendicular they need to still be 40mm at their thinnest.

Thanks. I've pretty much abandoned the idea of a gutter at the back of the garage. As clean as that would be I'm not putting a steel across a door gap that wide as it will be super expensive. The issue with a 5.83m Firring is that it would be approx 150mm down to Zero. I just feel that those would be impossible to cut consistently.

 

Edited October 26, 2023 by Saul

[Mr Punter]

I buy the firrings from a local timber merchant.  You can put them on top of 4 x 2 or 2 x 2 to make longer lengths.  You could also have a ridge down the centre and fall either side.  More gutter but it will be symmetrical.

[Saul]

27 minutes ago, Mr Punter said:

I buy the firrings from a local timber merchant.  You can put them on top of 4 x 2 or 2 x 2 to make longer lengths.  You could also have a ridge down the centre and fall either side.  More gutter but it will be symmetrical.

Presumably you would add one firring per joist. Just found a site where they are £3.25 per m up to 6m. For my build with 5.83m this would mean 15 x 5.83 x 3.25 = £285. 2 rows of bricks on one side of garage would be around £50 excluding laying labour. I don't like the idea of offset walls but don't know why that wouldn't work unless there is a stress issue on the I Joists from not being square if they were ever subject to heavy load from snow etc. At the same time at the high end of the firring ie 150mm above Joist I'd prefer to know that the OSB was attached to the joist where in reality it will be nailed to the Firring which will be nailed to the Joist. The wall ties won't offer any protection against wind strain. Who would have imagined that 3 walls, a floor, a roof and a door could be so involved! 

 

[Mr Punter]

Compare the cost of your special I joists with solid timber joists.

[Saul]

10 minutes ago, Mr Punter said:

Compare the cost of your special I joists with solid timber joists.

Will do though not sure I can manage the weight of 5.83m long standard wood joists. I'll most likely go standard I Joist and Ferrings. Not seen anyone sitting I Joists on top of the block work or if they have how these would be secured to the blockwork...perhaps upside hangers. The house has Web Joists sitting on hangers within the walls but don't want to do this for the garage as would need another row of blocks and not keen on that design for something that doesn't have a floor above it. 

[FuerteStu]

3 hours ago, Saul said:

Will do though not sure I can manage the weight of 5.83m long standard wood joists. I'll most likely go standard I Joist and Ferrings. Not seen anyone sitting I Joists on top of the block work or if they have how these would be secured to the blockwork...perhaps upside hangers. The house has Web Joists sitting on hangers within the walls but don't want to do this for the garage as would need another row of blocks and not keen on that design for something that doesn't have a floor above it. 

Even a 6m I joist will be around 50kg if mine were anything to go by.