Flat Roof Support

[wjm]

I am building a concrete block shed in the garden, and I am considering what to do regarding the roof.

The plans are to have a flat roof that is sloped to allow easy drainage. The problem I have is the building is an irregular shape but the biggest area is 6mx4m, and the beams supporting the roof would be running the 6m direction.

So the wooden beams would have to be quite hefty, so they wouldn't bend over that distance, what sort of size would anyone recommend?
What about using steel supports instead, sort of H shaped the other way round:
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Is this a viable cost alternative? presumably they wouldn't have to be that thick because the weight of the roof will be fairly minimal? Any thoughts/experience? any idea on cost of such an item?

Thanks in advance

 

[masona]

the building is an irregular shape but the biggest area is 6mx4m

Can you do a drawing of your roof layout and what angle do want the roof to be ?

What sort of roof do you want ? pitch roof,flat roof with pour bitumen felt , roofing sheet, polycarbonate sheeting with glazing bar or slate tiles etc.

It would cheaper to make the roof all out of timber.I can tell you what to do with more info on your type of roof.Don't worry about the weight of the roof etc, as it can be supported with purlin which go under the rafters.

 

[wjm]

Hi masona,

Thanks for your help.

Here is a rough diagram (birdseye):
The little diagram on the bottom right is supposed to be a side profile!

I was thinking of having a flat felt roof that slopes , higher front lower back, probably using roofing felt i.e cold, on plywood covered rafters.

The rafters would be running from front to back, meaning they would have to be around 6m in places, and I am worried that they will bow, so will need to be very thick, therfore heavy and expensive .

Any thoughts?

 

[masona]

Done a quick calculation on Wickes handbook price guide,

14 sheets 18mm 8' x 4'exterior plywood £336
3.6m 150mm x 47mm x 32 £256
Easy Seal Capsheet 6m(cold applied) £24.99 x 7 £175
+ parts £40
£807 + the cost of 6m RSJ (which I don't know how much they are.)

3 felts hot bitumen pour approx £ 10 - 15 per metre depending on your area.
As a guess you're looking at approx £1100,and if you use hot bitumen pour instead of cold felt you're looking at approx £ 1400.

I personally would not use cold felt or hot bitumen etc as they are prone for long term problems ( I should know, I work in the bitumen market !)

I know you want to keep the cost down,but can you try this for me.It will be a lot cheaper and looks better if you have roof truss frame made to your plan (I'm hoping I'm right!) as it is economical, and can be up to 40% less timber used than in a normal 4"x2" rafter roof and competitively priced and basically nailed down on to wallplate which come with drawing instruction and they are a lot stronger than you think.Depending where you're base at,see this for info.When I did my extension with a pitch roof I've saved approx 60% instead of making it! You can have a very low pitch roof approx 20 degree(pitch in the middle or back to front slope etc) then metal roofing sheet with a tile effect or whatever appeal to you.I have got a tile effect metal roof which work out approx £11 per metre to whatever length you want and a lot lighter.(there are cheaper one instead of tile effect) Done a quick calculation on your roof using metal tile sheet approx £460 + the cost of truss frame.
See this and have look at difference type of roofing system. This is a picture of my summer house roof tiles,

If you do it this way you can forget about long term leaking problem as with flat roof.If you feel you still want to use a flat roof but I'm not sure about the cost difference,then I will write back again.Hope there're others carpenter's on this forum with some more idea.

 

[breezer]

Masona, thats a tin roof? it doesn't look like it, but it does look really nice, not at all what i thought i was going to see.

that is something you should be proud of

 

[masona]

The roof is a metal sheet 1m wide to whatever length you want and consist of starting at the bottom,Backing Coat,Primer,Pre-Treatment,Steel,Metallic Coating,Pre-Treatment,Primer then top coating of your choice !

Believe it or not it come with a 40yrs warranty !!!!

 

[Scoby_Beasley]

Believe it or not it come with a 40yrs warranty !!!!

That would see me out then

 

[wjm]

Hi,

I've been thinking long and hard about your suggestion masona.

Have you any idea of the spacing between the trusses? Should the roof trusses overhang or is it just the roof that overhangs?

I think it may be the way to go. I need to find some quotes for roof trusses etc now.

With the metal tile sheet, presumably you overlap at joins? how much do you overlap normally, any idea?

Thanks for your help.

 

[masona]

The truss frame,you can have any shape you want ! overhang or no overhang,the choice is yours.I would have the roof square shape and let it overhang on the side where the 2 walls rebated in.Quite easy to do,at the front of the building you put a 6m 4"x2" wallplate on top of the 4m wall which overhang 1.5m and 0.5m on the other side so it will pick up the roof truss frame which will make it easier for you if you're putting roof sheeting or whatever.

Normally the roof trusses are 400mm (16") centre,but that is for the weight of concrete roof tiles which can take heavy weight,so you properly can have a wider centre if you're use roofing sheets as it's lighter.The roof truss company will calulate that for you.

The metal tile sheet overlap 1 roll length section like you do with corrugated sheeting