What size rafters?

[Margaret dailey]

Hi all,

 

Would really appreciate if somebody would give me some advice. 

 

I'm building a block shed at the back of my garden. 

 

It's dimensions of it's just under 5.97 m long and 2.6m wide.

 

The front wall is 2.48m high 

And the back wall is slightly higher at 2.71m

 

I know this isn't much difference in order to create a slope but this is the best we could do. 

 

I'm looking to know what size  rafters you would use for this mono pitch roof. 

 

Also what would be the space between each rafter. 

 

And lastly for the wall plate at the back on the higher wall what size timber would you use.

I have read for the wall plate, it should be 6x2 and then the rafter in front of this would be 4x2 

 

Please anybody can advise I'd appreciate it.

Edited September 30 by Margaret dailey

[Bonner]

It depends what roof covering will be and anticipated loading. Assuming you don’t want to engage a structural engineer, I would suggest looking at specifications of prefabricated garages and shed. The slope is more than adequate for a ‘flat’ roof, ie. felt or fibreglass covering. Wall plate should be the same width as the wall, 4” for a single brick/block.

[Margaret dailey]

Thank you for the response,

 

So the back wall is actually the boundary wall.

 

It was originally made out of bricks and then I added a few courses on top to raise the height of it and then the last course I made with blocks. 

 

The rest of the walls are block.

 

I want to use tiles as opposed to felt or fibreglass sheeting.

 

I want to create a slope in the form of a mono pitch roof. 

 

I understand there's not much slope but there is some so I was looking to know what size rafters I will need and what the space in-between needs to be.

 

And in respect of the back wall plate, when you say it has to be 4inches?

 

Do you mean a 4x2 timber  bolted to top course of concrete block.

Edited October 1 by Margaret dailey

[Ajm]

this might help

 

https://www.timberbeamcalculator.co.uk/en-gb/span-table/flat-roof-joists?load=1&class=C16

 

[Margaret dailey]

Thank you for your response.

 

I had to look at this calculator but I'm not a good person with numbers or maths.

 

It looks like it's telling me if I put the numbers in that I should use 4x2 rafters. 

 

I'm not sure if this is correct or not

Also this refers to a flat roof but I'm trying to do a pitch roof which I can put roof tiles on.

Edited Wednesday at 07:52 by Margaret dailey

[Gone West]

11 hours ago, Margaret dailey said:

Please anybody can advise I'd appreciate it.

Assuming your dimensions are external, your roof slope would be five degrees. There are very few tiles that can be used with such a low angle. There are some sheet tiles that would be suitable. The wall plates would be the same width as the wall.

[kandgmitchell]

Tiling at 5 degrees? I can't think of any that go that low, 12.5 degrees perhaps for one or two types on the market. If the minimum span requirement is ignored and tiles used then 100 x 50 over a 2.6m span is not enough. At a 0.5kN/m2 dead load (assuming interlocking tiles at say 50kg/m2) you would be looking at at least 150 x 50 at 400mm cntrs. 

[Russell griffiths]

As said above, you won’t be using tiles or slates. 
at that pitch you will need a metal sheet of some description or rubber or felt, all of these will actually help as the weight load will be less than tiles, you still need to calculate in snow load on such a flat roof. 

[Margaret dailey]

Wow thank you everybody for your input I'm learning a lot and I appreciate it every comment made. 

 

So obviously now reading these comments maybe tiles is not the best way because there's no much slope. 

 

One of the poster message me and said I should do use EDPM. 

 

It's a lot better than using tiles and a cheap option. 

 

He said on the back wall then you should bolt on a 4B2 along the last course of block. 

 

And then for the rafters to use 5B2 with 500 mil intervals. 

 

And then some plywood board 18 mil thick. And then the rubber.

 

You guys think this is a better idea and is his suggestion for the timber correct

[Roundtuit]

Hi Margaret.  Good to see you've got another project on the go.  How did the last couple turn out?  

[Margaret dailey]

The last one was the front drive 3 years ago but I didn't manage to put the flooring down still. 

 

So I've excavated the whole of the front which was done about 2 years ago or so I think and then built the retaining wall and then after that I just put a bit of hardcore down so I can park my car on it after that I need to eventually put something hard down. 

 

 

[Margaret dailey]

Any one can pls help in this. What size rafters would I need and what space. 

 

And what is the thickness of the wall plate 

[kandgmitchell]

For a "flat" roof like that with EPDM use a minimum of 125mm x 50mm at 400mm cntrs for a span of up to 2.67m. The ply will come in 2440 x 1220 sheets and 400 fits with that better than 500 so that as many of the edges are supported on joists and you aren't cutting sheets down. The wall plate, if it's being bolted to the face of the wall can be 100 x 50 as it's the bolts really doing the work as long as the spacing of the bolts is sensible at say 600mm.

[ETC]

[Margaret dailey]

Thank you all for your assistance I'll have a little read through this fingers crossed it makes sense to me.

[Margaret dailey]

1 hour ago, kandgmitchell said:

For a "flat" roof like that with EPDM use a minimum of 125mm x 50mm at 400mm cntrs for a span of up to 2.67m. The ply will come in 2440 x 1220 sheets and 400 fits with that better than 500 so that as many of the edges are supported on joists and you aren't cutting sheets down. The wall plate, if it's being bolted to the face of the wall can be 100 x 50 as it's the bolts really doing the work as long as the spacing of the bolts is sensible at say 600mm.

Thank you for this.

 

You have  explained it perfectly. 

 

I just want to clarify that when you say 2.67m span,  that 2.67m is from the back wall to the front wall. That is not the measurement from left to right of the shed. 

 

From the left wall to the right wall it's 5.67 m wide. 

 

Sorry if I did not make that clear. 

 

So knowing this does that change your advice because you refer to it as 2.67m span

 

And my intention is to go and fit each rafter from the back wall plate to the front wall and then go across from left to right fitting  the rafters one my one.

Edited Wednesday at 19:53 by Margaret dailey

[ETC]

Use the table I posted for flat roof joists.

Install them at 400mm centres.

Use a 100mmx50mm treated softwood wall plate strapped to the wall at maximum 2.0m centres. Straps should be 1000mmx30mmx5mm.

Strap the end walls across three flat roof joists with the same straps and timber noggings between.

Any joiner worth his salt will have that roof on in a morning.

[Margaret dailey]

Thank u for the advice.

 

Due to limited funds, me and my grandson are going to try to fit the roof ourselves.

From the information you said, I don't quite understand everything you said and I'm sorry for that.

 

If possible can you explain again step by step In layman terms please 

[Margaret dailey]

9 hours ago, ETC said:

Use the table I posted for flat roof joists.

Install them at 400mm centres.

Use a 100mmx50mm treated softwood wall plate strapped to the wall at maximum 2.0m centres. Straps should be 1000mmx30mmx5mm.

Strap the end walls across three flat roof joists with the same straps and timber noggings between.

Any joiner worth his salt will have that roof on in a morning.

Thank you for this. Much appreciated.

 

When you talk about straps, I believe you are talking about the wall plate that is mortured  and fixed flat on top of the top concrete block of the front wall.

 

So I think I understand that one now. 

 

But what is the going to be the size of the timber that you bolt to the back wall so you can fix the rafter to it via brackets.

 

That is a piece of timber that is bolted to the far back wall and the back of the rafters sit on this timber piece.

 

I'm trying to determine what thickness this timber would be...

 

My neighbour has suggested that this has to be 4x2. So as the wall is just under 6m wide, he said to get 2 prices of 3m 4x2. And bolt to wall.

 

And then fix top of rafters to this 4x2 along back wall.

 

He said rafters should be 5x2 thickness.

 

Is he correct.

 

I apologise that I forgot this before but I use this forum before I know got the best advice for a rather follow you guys then my neighbour lol. 

 

 

[Ajm]

Have a look at this thread on another forum:

 

https://www.ukworkshop.co.uk/threads/help-how-to-fix-a-wall-plate.133457/

 

it shows how the wall plate rests on top of the wall (bedded on mortar) and straps go across it and down the wall to tie it all down.

Then rafters are fixed to wall plate using brackets.

In my experience 4x2 for wall plate is fine when used in this way.

Looking at the span tables I would say 5x2 a bit small, I would use 6x2 (145x47)

 

[Margaret dailey]

Thanks again.

 

As I mentioned before you are referring to the wall plate that is fitted to the front wall. 

 

I've managed to work out from your previous instructions that that would be a 4B2 laid flat and mortared to the top of block wall and then strapped down. 

 

But I'm asking what type timber would you use as a wall plate for the back wall that's slightly higher to create a slope. 

 

Or are you saying that even the timber used on the back wall should not be bolted to the wall but laid flat on the top block and mortared just like the wall plate on the front block. 

 

My understanding has always been the back wall plate is bolted to the wall.

 

I've asked my grandson to send a pic. You can see from the arrows there's a front wall which is where your instructions about mortoring the 4B2 flat will be done.

 

But if you can see the other red arrow and the back wall which is higher. The wall was made out of bricks but the top course we used concrete blocks.

 

Are you saying then on this back wall I also lay a 4x2 flat on top of the block and mortor it down and add straps to this as well. 

 

Or do you bolt a piece of timber along the side of the block.

 

Really sorry to be a nuisance about this but I think there was some confusion about whether I was talking about the front wall or the back wall.

Edited Thursday at 08:13 by Margaret dailey

[Ajm]

For a flat (or slightly sloping roof like this) I would always lay the wall plate flat on top of the wall, both front and back the same.

Both strapped down.

 

The only times I'ver bolted a plate to the face of the wall is when putting a single storey flat roof extension on a two storey house, so the 'back' wall extends above the flat roof.

[Margaret dailey]

Oh ok I got you now.

 

I think every thing I've ever read about bolting a timber to the back wall for the rafters to sit on has always been referenced to sloping roof. 

But obviously this one is not going to be much sloping so I understand what you're trying to say now. 

 

 so I understand when the rafter comes down and sits on top of the flat wall plate on the front wall and then you bolt this down using the straps and even brackets just like the guy did on the link you sent me. 

 

But how do I do that to the rafter sitting on top of a flat plate on the back wall. 

Because that rafter is going upwards so will not sit flush with the flat timber on top of the block. 

 

I understand you're saying that I have to strap that timber down as well. 

But what am I doing to secure the top part of the rafter to that flat plate because they are both mismatched. 

 

Rafter will be going upwards so just the bottom piece of the rafter will be touching or resting on the back wall plate.

[Margaret dailey]

Edited Thursday at 09:07 by Margaret dailey

[Margaret dailey]

Sos for the poor drawing 

But I hope you understand what I'm trying to say. 

 

That back rafter will be going towards the sky and normally you would just cut it so it sits flush and then fixed it to the the wall plate you have bolted to the blocks.

 

But here the rafter is going up and the wall plate is been laid flat.