Please educate me on building a shed roof. What is a wall plate? How should I construct this roof?

FrankFalcon

Hi all

I have recently had a brick shed built by my pal. Being a Brickie he told me he knows nowt about roofs. So, I have decided to put the roof on myself. I watched a few YouTube videos and read up on a few articles and there are a couple of different ways of constructing the roof and I want to know which way to go and get some general advice. At this stage, I think it’s important that I tell you that I plan for the roof to be a Green roof. Yes, I plan to grow shallow-rooted plants on the roof. Therefore, I will put on a roof that has a slight tilt on it. Almost flat (from back to front) but with a slight incline for drainage. 

I would be grateful if you would look at the attached pictures. You can see that the shed is not square. It has 2 angled corners to run in line with the garden. You can see I have made provision for a window and a door. You can see that the shed is constructed of brick and block. So, whilst watching youtube etc in some of the videos they talk about a wall plate. I assume that a wall plate is timbers that sit on top of the brick. So, my questions about wall plates are:

What is a wall plate actually for? 

What do they do?  

What is their purpose?

Where do they sit (block or brick?).  In my case would I sit it on the course of Red Brick or the Blockwork? I saw in one video that it should be sat on a bed of mortar but in another it was sat on the dry brick.

In another video I saw some timbers that were sat directly on top of the brick so I’m guessing that you screw your roofing joints to these timbers. However, if this is the case why have I seen some videos using thin metal joist hangers?

I have seen a lot of videos using various methods and I'm confused which way to go.  Therefore, I would appreciate your opinions on what I should do.  I do have access to a joinery yard that can supply and deliver any timbers I require.

So, whichever method you guys advise me to proceed I would be grateful if you guys could advise me as follows:

What thickness timbers to use and how to fix them at what distance apart?

Is there need for a wallplate?

Which direction should I run the joists...Side to side or back to front?. 

Any advice would be greatly appreciated. 

Finally, I should say that because I am planning on constructing a Green roof what Ply should I use and is it OK to use a thick Rubber pond liner for roofing?

stuart45

I would have gone a bit higher and got some lintels in. Plates are usually 4x3 or 4x2 and bedded in mortar to get them level. Because of it's shape you need the joists to run from side 1 to 2 over the window.

stuart45

I missed the question on timber sizes and roof make up.
You would be best to Google span sizes as the timber size depends on whether you use 400mm or 600mm centres and whether it's C16 or 24 etc.
Also Google Sedum roofs for details. A friend of mine does them. Gets a lot of work in London as he said planning is much easier to get if you have a sedum.
PS forgot to mention that sedum roofs are quite heavy, so factor that into the joist sizes.

Section62

FrankFalcon said:

At this stage, I think it’s important that I tell you that I plan for the roof to be a Green roof.

If you are serious about having a green roof then you need to get an expert to draw up a roof design for you.

The loading on the roof will be greater than normal, and you need a design which enables the growing media to be permanently wet.  Although the construction could be done by an experienced DIYer, this isn't a job for a beginner to attempt.

The already complicated process of constructing a green roof has been made more complex in this case because you don't have a 'square' building.  And further complicated because the brickwork has already been completed without the provision of lintels over the openings.

Has any design work been done before your friend started laying bricks?  E.g. is the door opening large enough to fit a standard door?  For height, as well as width.

I'd also be concerned about the kind of foundations that have been put in.  The interior of the shed still appears to have the original patio surface inside it - so has he just used an angle grinder to cut away the slabs, or have proper foundations been dug and filled with a suitable concrete mix?

Also, what damp proofing arrangements have been used?  I can't see any weep vents, so has he constructed it as a solid wall, or is there a cavity between the leaves?

You've also positioned the shed very close to what appears to be a gabion retaining wall.  How much space is there between the shed wall and the foot of the retaining wall?  What are you doing to prevent debris from the top of the wall falling down and filling that gap?  Does the wall belong to you, and are there any restrictions on building near to the wall to allow future inspection/maintenance?

There's a lot of issues I've raised there, but many of them will have a bearing on how you proceed with the roof.  Getting the roof design detailed correctly could help overcome some of the problems which have already been built into the walls.  Get the roof design wrong and the problems may be compounded.

FrankFalcon

Wow. Some brilliant advice there. Thank you Stuart and section 62 for your extensive comments. 

To answer a few questions:

Re: Lintels. I was planning on literally sitting a Steel lintel (15mm or so) over the door and window (with a 120mm overhang each side). My pal who fits windows says this is perfectly sufficient.  

We DID take into account the door height and width. My pal told me to make the door at least 900 x 2100 but we made the opening both wider and higher to be safe. It won’t be a standard door. My mate works for a PVC Window Manufacturer so they will make me custom window and door. 

I did the footings myself. The bricky told me to cut the existing poured concrete and fig out to approx 1m deep. I then hired a reddimix concrete company to come in their huge wagon and pour proper concrete for footings. 

This is only intended to be a storage shed. However, the bricky did send me to screwfix to get a roll of DPC and I literally layed it down myself. 

There is a cavity (albeit small) between the brick and block. 

With regard to the Gabion retaining wall I know of no restrictions. We only moved into the house 6 months ago so I am certain I would have noticed any restrictions on the purchase docs. I admit I never gave a thought to debris falling down the gap so so will do that ASAP. What would you suggest? Something spongy so any movement of the wall over time doesn’t affect the shed? The wall definitely belongs to us because it’s in the middle of our Garden and I have to admit I never even thought about inspections etc. It’s not mentioned anywhere in any purchase docs. 

stuart45

Do you mean a 15mm flat bar as a lintel, or angle iron? The downside of thick steel is that if it rusts it can expand to 4 times it's original size.
I had to remove some on a job where they had done a lot of damage to the brickwork.
Angle iron deflects less than flat bar. 

Section62

FrankFalcon said:

Re: Lintels. I was planning on literally sitting a Steel lintel (15mm or so) over the door and window (with a 120mm overhang each side). My pal who fits windows says this is perfectly sufficient.

As stuart45 suggests, you need more thought about this.  That window opening is very wide, so you'll need a lintel suitable for that span.  You also need to bear in mind that the roof joists will impose point-loads on the lintel, which means using a lintel design suitable for that.

At a guess, 15mm thick flat steel is not going to have the rigidity necessary to support the roof loads over that span - and if you are talking about angle then 15mm thickness means a very heavy (and not cheap) piece of steel.  Unfortunately window fitters are not great judges of what is needed in terms of structural support... far too often they get it completely wrong.

Unless you forget the green roof idea and go for something much lighter I think you might struggle to get a viable lintel solution for the window.  If it were me* I'd probably look to adding a pillar in the middle of the window opening to make it into two smaller windows - albeit that will push the window costs up.

The preferable solution would have been (one) taller and narrower window.  I'd also have put it in the back wall rather than Side2  (not clear from the pictures but it also looks like Side2 is very close to the trees so you probably won't get much light).

The problem with Side2 is that because it is at an angle compared to all the other walls, there is no sensible way of configuring the joists so none of them need to bear on a support above the window.  If the window was in the rear wall then the joists could span from solid Side1 to solid Side2 and probably avoid the need for lintels completely.

*So before putting the roof on I'd actually give serious consideration to bricking up the window as it is, and create a new window opening in the rear wall. That will make doing the roof so much easier.

FrankFalcon said:

We DID take into account the door height and width. My pal told me to make the door at least 900 x 2100 but we made the opening both wider and higher to be safe. It won’t be a standard door. My mate works for a PVC Window Manufacturer so they will make me custom window and door.

It would have been better to use standard off-the-shelf sizes.  Even if you can get mates rates for custom made sizes you may end up paying more than you would for standard ones.  And there's no guarantee you (or the next owner) will be able to get the same deal if you need to replace the door or window in future.

FrankFalcon said:

I did the footings myself. The bricky told me to cut the existing poured concrete and fig out to approx 1m deep. I then hired a reddimix concrete company to come in their huge wagon and pour proper concrete for footings.

Ok, that should be alright, but you have a lot of large trees very close to the shed, so there may be movement in future.  If you needed a whole truckload of concrete you've probably spent a lot more on the foundations (for a shed) than you needed to though.

FrankFalcon said:

This is only intended to be a storage shed. However, the bricky did send me to screwfix to get a roll of DPC and I literally layed it down myself. There is a cavity (albeit small) between the brick and block.

The problem here is that damp penetrating the outer wall may accumulate in the cavity and work its way down to the DPC. There should be weepers to allow that water to drain to the outside - without that the water may accumulate and start leaking through the inner wall.

Also, without good ventilation (through the weep vents) the inside of the cavity may never dry out properly.  I think you now need to make sure that the top of the cavity is kept open has good ventilation to the outside - although others might suggest better/alternative solutions.

Are the two leaves tied together?  If so, what kind of ties did he use?

FrankFalcon said:

With regard to the Gabion retaining wall I know of no restrictions. We only moved into the house 6 months ago so I am certain I would have noticed any restrictions on the purchase docs. I admit I never gave a thought to debris falling down the gap so so will do that ASAP. What would you suggest? Something spongy so any movement of the wall over time doesn’t affect the shed? The wall definitely belongs to us because it’s in the middle of our Garden and I have to admit I never even thought about inspections etc. It’s not mentioned anywhere in any purchase docs.

I'd recheck your documents carefully to make absolutely sure there are no restrictions before spending more money on the building.  Although if you own the wall and the land either side it does mean it is less likely that there will be restrictions on what you can do.

You shouldn't put anything between the retaining wall and the shed, but you need to do something at the top of the retaining wall to stop debris etc falling or being washed over the edge and down into the gap.  I.e. effectively raise the top level of the wall so it is slightly higher than the ground it is supporting.  Maybe something like 'railway sleepers' with a layer of membrane on the soil side to stop it washing through?  I think you also need to give some thought to whether there is a risk of people (especially children) falling into the gap between the two walls and becoming wedged  - the size of the gap isn't clear from the photos, but it could potentially be quite dangerous.

Digging a metre deep trench that close and parallel to the toe of a retaining wall really wasn't a good idea - too late for you now, but something for other people to be aware of if they are reading this thread in the future.  While the trench was open there was a risk of the wall collapsing, and now there is a (fairly small) risk of ground settlement causing the wall to move.  With a gabion wall any movement should hopefully be accommodated in the flexibility of the gabions, but with a brick wall there could well have been cracking.

When you come to sell you may have comments from a surveyor about the impact the shed has on the retaining wall in terms of stability and/or future maintenance.  If you took photos while you were digging the foundations and pouring the concrete then make sure you keep them safe and available to show to anyone who queries what you've done.