Rebuild lean-to conservatory with polycarbonate roof

[alton]

We currently have a lean-to extension at the rear of our house: the dining room and kitchen make an L-shape and the lean-to sits between these and the neighbour's wall. Therefore the construction of the lean-to is simply one door and a panel at the end with a corrugated plastic roof.

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..............................|.....................|....Kitchen..|
Neighbour's house | Lean-to ......|..................|
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..............................| Dining room.....................|
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Its dimensions are approximately 150cm wide by 330cm long.

The lean-to is not well constructed and was here when we bought the house. It lets a lot of rainwater in and the timber supports along the walls are rotten.

We plan to dismantle the lean-to and rebuild it.

A soil pipe goes through the current corrugated plastic roof and this is the source of many leaks as it is not well flashed, and so I assume that a large timber beam will be attached to the wall past the furthest point of the soil pipe and the roofing to start here, so that the roof does not have to be cut, and then the timber will be flashed.

We have had some extortionate quotations from windows and conservatory companies, but this may be because they want to build it in the style of a conservatory with uPVC etc. All we are looking for is something that is waterproof and keeps the heat in. And thus I plan to do this myself.

I plan to attach two long pieces of timber against the walls and one against the dining room wall. I will then use 25mm polycarbonate for the roof but all the diagrams and help guides I've seen on this forum and on the net show the polycarbonate running vertically downwards. With the set up we have, we won't have vertical rafters, we will instead have three horizontal purlins. Now because of this, how will the roof be fitted? Will the roof run vertically? If condensation builds up, it will run down the inner then leak through at the purlin. If we fit the sheets horizontally, any condensation won't run off at all. Should we use timber purlins at all: can self support bars be used?

Another concern I have is that the roof will need flashing. Of course against the dining room wall the roof sits flush and can be flashed easily (albeit with what kind of flashing? And how?). But where the roof slopes downwards, how will this be flashed against the length of the wall? The flashing can't be inserted into the mortar as it slopes downwards at an angle.

Any help would be appreciated. Or indeed any builders in the Great Yarmouth area who would be able to do this work for us at a reasonable price, do get in touch. Thanks.

 

[RedHerring]

You've asked a lot of questions, some of which may be difficult to answer without a site visit.
My contribution is a few points: one about the Party Wall Act.
Your work will involve work on the party wall so you ought to be aware of it:
http://www.communities.gov.uk/documents/planningandbuilding/pdf/133214.pdf

Another point I would make is the question of Building Regs, to which I don't know the answer. If it was a new build I'm sure BC would be involved. Ask in the Building Regulations & Planning Permission forum.

Additionally, You do not mention if you intend to use single skin polycarbonate, double or triple skin. Triple skin will reduce condensation and improve insulation. It may even add rigidity and strength.
You'll need to consider access for maintenance.

Across a 1.5 metre width, I would think that purlins only would be sufficient but I'm no expert. It's all down to sufficient suport and rigidity of the panels so a targetted question on this topic might be better.
I don't know what self-support bars are. I'm tempted to be whimsical but it may be misinterpreted. I can't comment on the preference of glazing bars V timber.

The flashing on the sloping part of the roof is no problem, just look at the side of a chimney, any abutment, etc. Flashing is normally done in lead or a replacement such as Ubiflex.This'll give you an idea: http://www.leadsheetassociation.org.uk/html/1401.html

Finally, in responding, a couple of pictures may help, also where exactly is the soil pipe, and the proposed placing of your timber is not absolutly clear.

 

[alton]

Thank you for your reply.

I have attached to my profile some sketches of the area from above, from the side and my proposed work.

The lean-to is an existing structure for which agreement had already been made in respect of the party wall act. The work we intend to do is maintenance which will improve rainwater flushaway and insultate/waterproof the area. With regards to building regulations, the structure satisfies these as there is still an external door between the kitchen and the lean-to, and its structure of plastic roof, glass panels, etc. make it exempt.

I intend to use triple-skin polycarbonate sheeting. The third sketch shows my intention to install two purlins (I have read that the sheeting should be supported every 100-150cm and these purlins are attached at 110cm intervals). My concern is, however, how to install the sheeting, i.e. if it is installed vertically, when the sheeting comes down from the dining room wall and hits the first purlin, does the sheet have to be cut here and attached with a glazing bar (e.g. http://www.mrplastic.org/index.php?cPath=1102_1103)? The diagram on this website shows the glazing bars being used vertically, and so the polycarbonate sheeting "tubes" continue to flow downwards. If the bars are used horizontally, does the sheeting have to be cut and sealed at each purlin? I don't think this is ideal as I would prefer the "tubes" to continue to the gutter: in case any condensation does build up in them, it can then flow into the gutter.

The self-support bars I mentioned are shown here: http://www.master-plastics.co.uk/contents/en-uk/d69_Polycarbonate_Self_Supporting_Bars.html, the advantage being no need to install timber purlins for the bars to be screwed down into. But again, how do I assemble this so the polycarbonate "tubing" runs downwards vertically?

Thank you in advance.

 

[RedHerring]

I think I understand what you are asking.
The bars do not have to be installed across the shortest span. They can be installed along the longest span.
I don't know how much support is required for the glazing bars. I assume continuous support.
If you're using self-support bars why not run them at right angles to the purlins, i.e. with the flow of surface water/condensation towards the gutter? You'll only need to join sheets if they are not long enough to span the whole length. Then you can either use glazing bars or self-support bars. But you can get sheets 4 meters long.
You could even put noggins between the purlins to support glazing bars.

On the Party Wall issue, it is not just new building/work that it covers, nor do improvements count as an exemption. Check out para 4 to assure yourself that it does not apply.

 

[alton]

Thank you for the heads-up with regards to the party wall act. Your comments are noted.

I can indeed purchase a sheet of polycarbonate that would cover the entire area in one sheet, but I am under the impression that it would need to be "bolted down" somehow, which is why I suggested the purlins.

Ideally, as you can see from my sketch, the shape of the lean-to would have the horizontal purlins as opposed to vertical rafters. Ideally I would not want to install the bars at right angles to the purlins (even though this would achieve what I am seeking, I fear there would be too much "going on" on the roof). Secondly, I would not want to use a vertical support bar as the distance between the two points where it would rejoin the main structure of the walls is too far to provide enough support.

So thank you for your help so far, but can I ask for any more suggestions to the above?

 

[RedHerring]

Your leanto is 3.3 meters long and the self-supporting bars will span 4.15 metres. (at 600mm centres) You don't need purlins.

I wasn't suggesting you try to cover in one sheet, only the length in one sheet but the width in multiple sheets, e.g. 3 off at 600 mm gives 1.8 metres.

 

[alton]

Forgive me if I'm being quite a novice with this question, but surely three 1.5metre purlins going across the width of the structure would give better support than the 3.3metre rafters (or support bars) going along its length?

 

[RedHerring]

I think it's all about making choices. I would agree that 3 purlins will give more support than 3 self-suport bars as rafters but I have no experience with these self-support bars. At the end of the day if they don't 'do what it says on the tin' you have some comeback against the vendor.

You decide on vertical bars or horizontal purlins bearing in mind the 'business' of the roof space and the problem of condensation dripping at a junction.

I know which way I'd go.
You could mix & match 1 or 2 purlins and vetical bars.

 

[Jandiy]

You can get polycarbonate in a single sheet 3.5M long x 1.2M wide thus negating the need for a rafter or self supporting bar. I would recommend using specialist screw buttons to fix the poly onto the cross purlins you have. If you want good heat insulation I would recommend looking at fivewall 25mm or 7 wall 35mm polycarbonate.