Flat roof to pitched, raised collar, purlins, abutments

[Mr Chibs]

Hello all


Long time reader, first time poster

I'm looking at changing a flat roof (fibreglass) to a pitched roof and would like a little advice from you knowledgable types. The roof is on a single storey extension, (at back of house), that is sandwiched (at each gable end) between the house (on the right) and a brick store room to the left ... this store room is 5m tall with pitched roof at 30°.

I would like a raised collar type roof, preferably purlin free, if possible. With a couple of velux windows in, for added light. I'm trying to maximise head height. I would also like to use the same pitch angle as the store room.

The internal wall size is 4270mm wide x 4520mm deep, construction is double skin brick with cavity. Height to the ridge would be just under 4m.

Initial roofer seemed perplexed, but mentioned 650mm rafter centres. As I've been reading up I saw if this was reduced and larger rafters sized, then perhaps I wouldn't need purlins, which might interfere with the velux.

Roof tiles, concrete interlocking.

Any suggestions/advice please, as I'd like to tackle this project myself.

Sorry for the long post!

Chibs

 

[vinn]

You would get better informed replies if you posted photos of what you have existing.
All common rafters in traditionally cut roofs are centred at 400mm.
Or are you thinking of trusses which can have different centres?

 

[Mr Chibs]

Cheers, Yes, you beat me to it.

Roof-IMG_1434

• Mr Chibs
• 23 Jun 2016

Here's the idea, yes the furthermost window would be bricked-up to accommodate the roof.

*Fridge/freezer models' own

 

[vinn]

Thank you for the photo.

If you lower the pitch there would be no need to brick up the window - esp as it appears to be a bathroom window?

There's no need for purlins. Ridge height is taken from the wall plate level. Total span is taken from the outside edge of the wall plates - the ridge is centred at half span.

Rafters trimming the Velux's need to be doubled.

Any new plumbing (soil & wastes etc.) and guttering arrangements would have to be designed in when the project design is settled.

Any work to be done above the roof should be done now before any changes below.

To commence you might be best advised to remove all the flat roof including ceiling, joists and fascias.

Do you intend any future knock-through on either side eg. into the main house?

 

[noseall]

I'd be inclined to have a large front soffit to keep that fridge sheltered.

 

[garyo]

Do you intend building up both gable walls and then having a structural ridge beam? In that case your rafter length is only going to be 3 metres max so I'm not sure why your builder is talking about purlins or making a fuss - you should be able to simply use 150mm x 50mm timbers and make a vaulted ceiling. Doubling up/trimming around your veluxes as you go. No need for collars either unless you particularly like the look.

 

[Mr Chibs]

Gents, thanks for the replies so far.

I was thinking the raised collar would be a benefit to head height... and hadn't considered the vaulted ceiling/structural beam construction, this does seem a far better solution. Thanks.

Could the right-hand side of the beam, rest on the back wall of the house... equally the left into the store room wall, both of which are twin brick construction, the internal span, gable to gable would be 4270.
In the picture to the right of the green door, there is a twin brick cavity wall, running from front to back of the extension, if this was built up, could/would this aid spreading the load of the beam?
This wall is around 900mm from the left hand gable wall of the store room.

Sorry for so many questions, but you've all given me food for thought.

Especially on the large soffit for the fridge freezer! hahaha

 

[datarebal]

650 centres ? what size batten were you using? .. 400-450 i'd say and as garyo says structural ridge,

 

[vinn]

A structural ridge is not necessary. A ridge board will do.
Dont pierce either of the gable walls with your ridge, simply pin your end rafters to the face of the walls. Sheathe the back of the rafters in wall contact with DPC.
No building up of any kind is necessary to support your ridge - thats what your common rafters are for.
Given the above sketch your 8m calculation to ridge height must have been taken from ground level to ridge. Take it from wall plate level.
Any kind of gable knock-through will take you into cavity tray land.

For your purpose a sloping ceiling with 150mm x 50mm rafters will allow for insulation and the Velux's.

 

[datarebal]

knocking through has nothing to do with cavity tray implications.
they should be fitted anyway... sometimes you can be lucky and it not cause an issue. often it is.
no mention of spread then?

 

[catlad]

I would not put trays in what looks a newish rendered wall.

 

[garyo]

On a vaulted ceiling like that I'd want a structural ridge to prevent spread. You may well be able to do this by bolting the common rafters in to the wall at either end then using those to support the beam - calculations required.

Your other option could be a couple of nice feature joists running across the vaulted ceiling. A bit of stained wood on a contemporary backdrop can look quite attractive: https://uk.pinterest.com/pin/81346336989418623/

 

[vinn]

Given the information supplied, and unless specifically specified by BCO, cavity trays would not be needed.
Knock through's have everything to do with cavity trays.
Given the dimensions, and my suggestion, spread wouldn't be an issue - so why mention a non-issue?

 

[Mr Chibs]

Further to all your replies, I've had chance to look up your suggestions, thanks to all who have replied.

The gable walls are solid brick, no cavity on either, so I don't think I could use this 'cavity tray' method.
The rendering is old 30 years+, the house around 120 years old.

I could make a truss for each end of the gable walls and bolt to house & store room if I this would help?
Also, as I'm thinking of rooflights on the front of the roof, I could use a purlin on the otherside, if this would help with loading etc.

I hadn't thought of rafter sizing, but a rough guess was going to be around 150x50, c24 at 450.

In your collective opinions, what is the easiest way to go, the thought of steel is worrying me a little.... Anyone fancy a roofing job!