Roof overhang limits

[Mad Moose]

I have enjoyed many hours reading loads of posts here over the past week. About to start groundworks for my new workshop. First time build of a bespoke structure - my only previous experience is putting up prefabbed sheds 6'x8' / 8'x10'. So my concrete pad should be getting poured next week. The footprint of the workshop will be 4.8m x 4.2m which will be huge in comparison - I am so excited!

The roof will be single plane pent with 1:40 fall (2.4M at front down to ~2.3m at the back internally). Boarded with T&G 18mm OSB and covered with EPDM. My question relates to how big I can have my overhangs. Planning on using 6x2 timbers at 400mm centres for the rafters so front and back are straight forward enough to create - but not sure about any limits. Thinking specifically about the sides, how wide can I make the laddering on each side. Hoping to get 60cm to allow some protection from rain and to have some storage for timber under the eaves. once I understand the constraints I will know the size of the roof and how much EPDM I need to order.

I can guarantee that there will be lots more questions as I get started. I use the University of YouTube extensively but there is no substitution for the ability to get answers to specific questions from wise and experienced woodworkers.

I plan to document the whole thing and will post on here if anyone is interested. Currently trying to learn to use SketchUp to do the design and get a clue as to how much timber, how many sheets of OSB, insulation etc. I need to be buying and work out just how much is is going to cost - or more realistically working out what the initial budget needs to be. . .

 

[Jameshow]

If your using 400mm centres why not make it 400mm overhang seems obvious to me...land a joist on the wall? One bay out each side. Noggins every 1.05m

Just my inexperienced thoughts!

 

[Bingy man]

I have enjoyed many hours reading loads of posts here over the past week. About to start groundworks for my new workshop. First time build of a bespoke structure - my only previous experience is putting up prefabbed sheds 6'x8' / 8'x10'. So my concrete pad should be getting poured next week. The footprint of the workshop will be 4.8m x 4.2m which will be huge in comparison - I am so excited!

The roof will be single plane pent with 1:40 fall (2.4M at front down to ~2.3m at the back internally). Boarded with T&G 18mm OSB and covered with EPDM. My question relates to how big I can have my overhangs. Planning on using 6x2 timbers at 400mm centres for the rafters so front and back are straight forward enough to create - but not sure about any limits. Thinking specifically about the sides, how wide can I make the laddering on each side. Hoping to get 60cm to allow some protection from rain and to have some storage for timber under the eaves. once I understand the constraints I will know the size of the roof and how much EPDM I need to order.

I can guarantee that there will be lots more questions as I get started. I use the University of YouTube extensively but there is no substitution for the ability to get answers to specific questions from wise and experienced woodworkers.

I plan to document the whole thing and will post on here if anyone is interested. Currently trying to learn to use SketchUp to do the design and get a clue as to how much timber, how many sheets of OSB, insulation etc. I need to be buying and work out just how much is is going to cost - or more realistically working out what the initial budget needs to be. . .

I see no issues with a 600mm overhang given your 6”x 2” joists I’d treat any exposed timber with a suitable wood preserver eg sika 5 star complete as any rain driven by the wind will find any weak points. You can work out how many sheets of plywood/ osb to buy as they are 8’x4’ or 2.4 x 1.2 .. good luck

 

[Mad Moose]

Thanks for the input guys - much appreciated. I have dimensioned the workshop to utilise full sheets and minimise cutting - hence the long walls are 4.8m by 2.4m.

 

[Jake]

You'll have some small cuts with that - wood panel sheets tend to be 2440 x 1220 not precisely 2.4 x 1.2 (although plasterboard and insulation are the latter, and chipboard and plywood T&G floor is 2.4 x 0.6).

 

[HOJ]

Planning on using 6x2 timbers at 400mm centres for the rafters

I know its a workshop, but worth looking up the span tables for your roof timber lengths @ 6" x 2" (Regularised C24 will be 145 x 44mm) is fleet.

 

[rob1693]

If they're osb3 tg4 dimensions are 2400mm x 590mm , it's what I used on my workshop walls and roof

 

[Fergie 307]

I know its a workshop, but worth looking up the span tables for your roof timber lengths @ 6" x 2" (Regularised C24 will be 145 x 44mm) is fleet.

No mention of which way the joists are going, ie are they spanning 4.8 or 4.2?
Seem a bit light to me, especially for a low pitch pent roof.
My shed has 8x2 spanning 4m at 600 centres, that is on either side of a 15° pitched roof.

 

[Mad Moose]

You'll have some small cuts with that - wood panel sheets tend to be 2440 x 1220 not precisely 2.4 x 1.2 (although plasterboard and insulation are the latter, and chipboard and plywood T&G floor is 2.4 x 0.6).

Thanks for the heads-up on the oversized OSB panels. I will adjust the concrete bas accordingly. I could use the extra 40mm of height to tap the OSB down over the edge of the concrete to ensure runoff.

 

[Mad Moose]

I know its a workshop, but worth looking up the span tables for your roof timber lengths @ 6" x 2" (Regularised C24 will be 145 x 44mm) is fleet.

Thanks for the input - I was going for 6x2 on the basis of 'The DIY Guy' on YT. His build was 4m span and he says 6x2 at 400 centres was more than enough. Granted, mine is 4.2 span - so I have checked the span tables (here) an maybe I need to be looking at C24 7x2 (47x175).

Best to do it properly first time - but 8x2 (Fergie 307) seems like overkill. . .or maybe not. . .

Cheers.

 

[Dabop]

Thanks for the heads-up on the oversized OSB panels. I will adjust the concrete bas accordingly. I could use the extra 40mm of height to tap the OSB down over the edge of the concrete to ensure runoff.

It pays to check carefully, as they can be (in Australia at least) EITHER size (our 'local' Bunnings carries them in both sizes for example)- basically the smaller 1200 x 2400 is the 'metric' sized, while the 1220 x 2440 is the old 'imperial' sized sheets in metric measurements

(it's a right pain when its an hours drive each way to the hardware store, and you grab the metric size when you wanted the imperial size... grrr... at least with the imperial you can cut the excess off...)

 

[Fergie 307]

Thanks for the input - I was going for 6x2 on the basis of 'The DIY Guy' on YT. His build was 4m span and he says 6x2 at 400 centres was more than enough. Granted, mine is 4.2 span - so I have checked the span tables (here) an maybe I need to be looking at C24 7x2 (47x175).

View attachment 188741

Best to do it properly first time - but 8x2 (Fergie 307) seems like overkill. . .or maybe not. . .

Cheers.

Maybe slightly over the top, but the extra cost is pretty insignificant. And you have that confidence that they are definitely up to the job. Think of your relatively flat roof carrying a good load of snow for example, heck of a lot of weight.

 

[HOJ]

on the basis of 'The DIY Guy' on YT

Yet another one that should be banned/barred and definitely ignored, I am aware of him in the real world.

The table you link to is for ceiling joists you need look up flat roof, slightly different.

 

[Mad Moose]

Maybe slightly over the top, but the extra cost is pretty insignificant. And you have that confidence that they are definitely up to the job. Think of your relatively flat roof carrying a good load of snow for example, heck of a lot of weight.

I agree - hence my "... maybe not".

As for the relatively flat roof, I was working on 1:40 which is apparently acceptable. I could do 1:20 which would lose me another 100mm or so on the short wall.

EDIT: This is the info I used for the slope: "The finished EPDM flat roof should have a fall of at least 1 in 80 (1.25%) as recommended by Single Ply Roofing Association (SPRA) to ensure water runoff. " (from here). I effectively doubled that to 1:40 as I thought it was too shallow!

 

[Fergie 307]

Thanks for the heads-up on the oversized OSB panels. I will adjust the concrete bas accordingly. I could use the extra 40mm of height to tap the OSB down over the edge of the concrete to ensure runoff.

What I have always done is to have the membrane on the walls hang 25mm or so below the joint between walls and base. Then use an extra wide batten at the bottom of the wall, so it also spans the gap over the membrane. Once your cladding is in place any water coming down the walls falls clear.
I also put a shallow step in the base where the door is going to be, and use a length of stainless angle embedded in the base as the actual threshold. Saves water blowing under the door, and the edge getting damaged. The stainless also very useful anchoring point for any bolts.

 

[Mad Moose]

Yet another one that should be banned/barred and definitely ignored, I am aware of him in the real world.

Oh - please tell me more! Pm me if you don't want to go public.

 

[Mad Moose]

What I have always done is to have the membrane on the walls hang 25mm or so below the joint between walls and base. Then use an extra wide batten at the bottom of the wall, so it also spans the gap over the membrane. Once your cladding is in place any water coming down the walls falls clear.
I also put a shallow step in the base where the door is going to be, and use a length of stainless angle embedded in the base as the actual threshold. Saves water blowing under the door, and the edge getting damaged. The stainless also very useful anchoring point for any bolts.

Great tips. Thanks.

The previous owners put a full width extension on the back of the house and left the existing UPVC patio doors in place. Looks odd internally so they were coming out and getting replaced with proper internal folding doors. I will be repurposing the patio doors from the house as the doors for the workshop. They are in great condition, seals good all round and low threshold.

 

[Fergie 307]

Ignore the very wide threshold. That was designed to allow cars to drive in easily. It is also because the opening is 12 foot wide with two sets of bifold doors. The frame is all in I beams. Because I wanted the internal wall to be completely flat, the uprights on the front wall are 6x2, so the steelwork is contained within the walls. The doors swing out flat against the side of the building when open on big parliament hinges.
On the one I have just built the step in front of the ss angle is the the same depth as the door is thick.

 

[Fergie 307]

I agree - hence my "... maybe not".

As for the relatively flat roof, I was working on 1:40 which is apparently acceptable. I could do 1:20 which would lose me another 100mm or so on the short wall.

No criticism of the pitch, just that it won't shed snow for example, so important that it is up to the potential weight. As you say saving height is a very good reason for choosing a pent roof.

 

[Fergie 307]

Yet another one that should be banned/barred and definitely ignored, I am aware of him in the real world.

The table you link to is for ceiling joists you need look up flat roof, slightly different.

A lot of these people have great confidence in what they are doing, however stupid it might be. So they can come across as being really knowledgeable.
I have not seen any of this guys videos so can't specifically comment.
But I have seen many where you can't help thinking god help anyone who follows this example thinking it's a good idea.
There are very knowledgeable people on here, many much more so than myself, who will give you decent advice.