Lean-to roof rafter replacement

[battenburg]

I I've this query on another forum, but reading through some stuff on here since makes me think that there could be some useful knowledge as this is a bit more specific.

My local BC have also proved very difficult to just call and have an informal chat with, because that would be a great option if available! I will try that again on Monday anyway.

Lean-to utility room on the back of a Victorian terrace. Single skin brick, no insulation in the roof and damage to the ceiling. The utility room isn't original, but its probably been there more than 50 years, perhaps longer.

I'm finally getting a builder to come and install insulation and a Velux window to let in a bit more light. Because the existing rafters are quite shallow and the area of the roof is small (3x3m ish) they advised just replacing the woodwork and membrane which is all pretty old, which will allow a better depth of insulation. My understanding is that it is effectively like for like and there is no structural need for it, other than they might be able to make the spacing match the Velux window. They will also replace the plasterboard ceiling and an electrician will put a couple of lights in. The same tiles will go back on. The ceiling inside is attached to the rafters and follows the same pitch so there is no void space.

When I asked way back when about BC, the builder said it wasn't normally something they would worry about with this sort of work, but they would help me put the documents together if needed. My reading of stuff as a layperson is that, because the timbers are being replaced, then officially I should get BC involved.

I thought that this level of of minor work could be covered by a Building Notice and I would just write a narrative description of what was being done and submit that to the council. It could be submitted a few days before the work starts. Yesterday evening I asked ChatGPT to turn the quote from the builder into some sensible text I could send to BC if needed, and then was getting a bit more info on the process. It suggested that because replacing rafters is effectively structural, I might need do a full plans application! I'm hoping this is one of those instances where a human knows better.

I think the insulation bit is fairly easy to meet the required regulations on for thermal retention, so I'm not really worried about that. What I'm more concerned about is the structural element. For this kind of work, I can see there are span tables for the spacing of rafters based on the pitch of the roof and the span. Hopefully that's fairly bread and butter stuff and could be submitted as narrative i.e. span is x, pitch is y, roof tiles are made of z therefore 200x50mm rafters will be spaced at xyz.

Edit: I'll add to this that I've just done some quick measurements and the angle of the roof is about 25 degree, the span is 2.1m and the width is approx. 3m (didn't actually measure that one). It sounds like the proposed 200mm rafters are fine, potentially excessive in terms of sturcture. And potentially 600mm spacing.

Is anything else expected in terms of things like foundations? This is where I start to get myself tied up in knots - if this question was asked, the answer would be that I have no idea what the foundations are like. And then does my whole project get scuppered or they tell me I have to rebuild the room with proper foundations?!

Can any help advise? Other than the obvious "BC don't need to know about this". Does the above sound like it is appropriate to just submit as a Building Notice, and might a BC officer want calculations signed off by a structural engineer or is this so run of the mill to somebody that works in the industry that quoting those span tables would be sufficient? Tbh i'd ditch the Velux window at the point if it made my life easier!

 

[mrrusty]

Can any help advise? Other than the obvious "BC don't need to know about this"

is I suspect the answer that 95% of people would give for a repair to a 3x3 tacked-on utility room roof.....

 

[stevie888]

As a general rule I always advise people to go for full plans unless they really know what they are doing as it avoids the risk of having to redo stuff BC decide they don't like and the fees are usually a little less. I woudn't class this as structural per se so Notice would be ok.

With what you've described I'd probably just do it TBH: you could wind up having to look at other stuff. Are you aiming to insulate the walls? You are normally expected to do things to code if you do anything, but even 25mm of Celotex on the walls would make a difference.

2x4 C16 400crs will do for your rafters you certainly don't need 8s!

 

[freddiemercurystwin]

Can someone summarise, can't be arsed to read that essay, tbh I stopped at the bit where you expect BC to be an advisory service.

 

[Deluks]

Is this lean to open to the rest of the house or separated by a door?

 

[battenburg]

As a general rule I always advise people to go for full plans unless they really know what they are doing as it avoids the risk of having to redo stuff BC decide they don't like and the fees are usually a little less. I woudn't class this as structural per se so Notice would be ok.

With what you've described I'd probably just do it TBH: you could wind up having to look at other stuff. Are you aiming to insulate the walls? Yu are normally expected to do things to code if you do anything, but even 25mm of Celotex n the walls wiuld make a difference.

2x4 C16 400crs will do for your rafters you certainly don't need 8s!

This is what I'm worried about. The original plan, some time ago, was to insulate the walls as well but I was concerned that if there was still an issue with moisture in the room, going all out straight away might end up causing other issues. So the plan is to insulate the ceiling and an an extractor fan and then some other minor plaster repairs etc., and see how things go.

I think the depth of the rafters that have been suggested is so that they can put a good chunk of insulation in but also leave an air gap.

Is this lean to open to the rest of the house or separated by a door?

Its separated by a standard internal door (which doesn't actually close very well). You walk through the kitchen, step down into the utility room and then out of the upvc external door into the garden. Someone just tacked this onto the back of the old external wall so I guess that the door through the kitchen was once the external door.

Now you ask this, it does ring some bells that many moons ago I read something about what was classed as a 'room'. Is that what you might be getting at? It has a sink, washing machine, fridge/freezer and boiler. But it isn't heated in any way. I think someone just decorated it like the kitchen and put the same style of fitted units in, but only having lived here for a while did I realise that its just an add-on.

 

[Dereekoo]

Can someone summarise, can't be arsed to read that essay, tbh I stopped at the bit where you expect BC to be an advisory service.

Yes he asked a long question

 

[noseall]

This is what I'm worried about. The original plan, some time ago, was to insulate the walls as well but I was concerned that if there was still an issue with moisture in the room, going all out straight away might end up causing other issues. So the plan is to insulate the ceiling and an an extractor fan and then some other minor plaster repairs etc., and see how things go.

I think the depth of the rafters that have been suggested is so that they can put a good chunk of insulation in but also leave an air gap.



Its separated by a standard internal door (which doesn't actually close very well). You walk through the kitchen, step down into the utility room and then out of the upvc external door into the garden. Someone just tacked this onto the back of the old external wall so I guess that the door through the kitchen was once the external door.

Now you ask this, it does ring some bells that many moons ago I read something about what was classed as a 'room'. Is that what you might be getting at? It has a sink, washing machine, fridge/freezer and boiler. But it isn't heated in any way. I think someone just decorated it like the kitchen and put the same style of fitted units in, but only having lived here for a while did I realise that its just an add-on.

You are perfectly entitled to dress up a conservatory as a room i.e. blinds, sofa, sockets, lamps, tv etc, but it will never be classed as a habitable room as long as it is kept thermally separate from the house and it will not invoke B'reg's.
Things become cut and dried the instant you remove the external door and pipe the domestic heating in there.

 

[stevie888]

"I think the depth of the rafters that have been suggested is so that they can put a good chunk of insulation in but also leave an air gap."

Shallow rafters with insulation between, topped up as necessary will be easier to do and more thermally efficient.

 

[battenburg]

You are perfectly entitled to dress up a conservatory as a room i.e. blinds, sofa, sockets, lamps, tv etc, but it will never be classed as a habitable room as long as it is kept thermally separate from the house and it will not invoke B'reg's.
Things become cut and dried the instant you remove the external door and pipe the domestic heating in there.

Would a standard internal door count as keeping the room thermally separate? The door separates the double skinned, heated part of the house from the single skinned lean-to that is unheated. But its just a standard door. All this discussion makes me think that getting a decent thickness door between the kitchen and utility room would be a good thing to do and then it would be very much separate.

"I think the depth of the rafters that have been suggested is so that they can put a good chunk of insulation in but also leave an air gap."

Shallow rafters with insulation between, topped up as necessary will be easier to do and more thermally efficient.

Having got myself a bit caught up in everything yesterday, I'm actually leaning towards this as being a better option. There is just about enough head room. The only issue is that the pipework coming out of the boiler (not the flue, but the water pipes) go upwards towards the wallplate before entering the main house, which makes space a little bit tight at the very top. But I think this could be boxed in. That would leave me with say 50mm of insulation between the rafters *to keep the air gap) and then bulk up with insulated plasterboard on the inside. That approach would mean I don't have to change anything to do with the roof structure and meeting the insulation requirements should be ok.

 

[noseall]

Would a standard internal door count as keeping the room thermally separate?

No.
It needs to be external spec (44mm+) door and frame.