Building single story extension (oak joist info)

[trevorbayliss]

I am planning on tackling my extension this year.

I would like to have exposed oak beams in the sitting room area.

Because they are exposed i will not be packing them with insulation as this will go over the top of them and i will create a warm roof.

I plan on doing 18mm ply sheets over the joists, then 125mm insulation, then 18mm ply sheets over insulation, then membrane, then tile.

Questions are:

1) any idea on the sizes of oak beam i will need if 4.5m span. Are the beams wider rather than narrow and tall like a normal joist ?

2) Is it easy to tile onto membrane and ply, or do you need to use lats ?

3) My roof pitch is probably going to come in at 15 degrees, so i know this is an issue. Have found this product. http://easyroofs.co.uk as i want to use slate. That way i can do it all myself. I think fibre glass wouldn't look so good and zinc would be pricey. Roof area = 70sqm.

Got lots to think about and have an architect friend helping out with drawing, but doing lots of research at mo.

Thanks for any help.

 

[tomfe]

I'd get it drawn up by your architect friend and leave the thinking to someone who does this for a living.

 

[trevorbayliss]

What is this website about ??? Isn't it about exchange of ideas, tips and experience ???

I am wanting to get some info from other builders who have worked with oak and done roofs with low pitches.

The more input i get the better will be my understanding to see different perspectives.

This is not a case of an architect doing the thinking. I need to be thinking as well because i will be doing the work.

 

[tomfe]

If you have someone who you know and trust who will have the information you need, why would you ask a bunch of unknowns on the internet?
1. Are you using these just like normal cut/flat roof joists or are you going to do a proper oak frame, king posts etc.
2. If you have a low pitch many manufactures require you to board and lay tiles.
3. Same as 2.

 

[noseall]

Sounds to me like your terminology is a wee bit off. Do you mean rafters (sloping ones) when you say joists?

I have just built an oak porch (finished Friday) and it is a bu££er to work with. Everything needs pre-drilling and it is very expensive regardless. We had some oak matchboard machined up that we used as sarking boards prior to lath and felt, so that when you look up you see oak rafters and oak sarking.

You may be able to design say a pair or a trio of king trusses that will be able to support the entire roof but it will cost thousands. Alternatively you could have a steel beam supporting at the ridge but this would mean having a small ceiling section high up. A joiner may be able to make structural oak trusses with a raised tie, but again, be prepared to pay thousands.

 

[trevorbayliss]

Not looking into using a frame.

The idea is to just use oak rafters spanning 4.5m and resting on block walls. So i would get the beams in the ceiling effect. Basically i am swapping ceiling joists (rafters) for oak.

My roof will be coming off the back wall of the house and sloping downwards, so a simple flattish roof at a lowish angle (15 degrees).

The Frame bit i was thinking of having was on the wall into the garden where i will have french doors and windows. Would have a larger timber spanning across the width here and some posts holding it up, but this would actually be for effect because the block walls at left and right sides would be doing the structural work.

With regard to the laying of tiles. I will be boarding and the membrane will go over this boarding i think. Then the tile goes on top. Do i need to have a drainage gap between the board/membrane and the tile or do you just put the tiles onto the membrane and board. How do you set yourself out properly if not using lats as the guide ?

 

[tomfe]

So an oak mono pitch?

I would have though BC would want some form of design as it's not 'standard'. If you work off normal span tables it might not be correct for green oak and kiln dried will be too expensive.

 

[trevorbayliss]

Yes, but my rafters are going side to side, not front to back. Not standard i know. Instead of a frame i will have block work holding up the timbers. Makes sense to me as cheaper as don't need the frame, but i get the exposed beam timbers plus some frame work at the front window section which will be more for visual effect. Hope to speak to oak supplier tomorrow.

What about the tiles ? Do roofers tile onto ply board direct or do they add batons and tile. Do you need that 25mm odd gap for water to drain down. Air ventilation not needed as a warm roof. Just not sure about this detail.

 

[tomfe]

You'd counter batton the boards to allow any water to run off.
Some BCO ask for this when there it a low pitch others don't. The way you suggest is, I think, the best way for a low pitch tilted roof however I am sure may roofers would not like such a low pitch and would do a membrane or metal roof.

 

[trevorbayliss]

My pitch is going to be lower than 15 now after doing some calcs, so prob looking like a metal roof or fibreglass. I think a tiled roof would be too difficult to work with. Fibre glass i can do, but i think it is messy and not sure about such a big area. Metal is going to be expensive !