Flat roof reconstruction - advice needed

[tintin99]

My Dad has a 40 year old free-standing Banbury sun room in his garden with a sagging and leaking flat roof. The roofing structure is built correctly according to the original plans but the design looks completely inadequate and I'm amazed it's stayed up as long as it has.

I'm intending to strip the whole roof off including the structural timbers, redesign the structure and renew the plywood deck and felt covering.

I've not done as big a job as this before, although I'm a pretty competent carpenter and diy-er and it's well within my capabilities. What I really need is some info on how to design the structure (rafter size, spacing etc) how to fit the soffits, fascias etc...and of course what sort of covering to use. For the covering I'm looking for something that won't be too difficult to install (and I'm assuming hot bitumen might be a bit beyond me!) and that will last about 10 years (I imagine eventually the sun room will be demolished so the roof doesn't have to last forever).

I'd be really grateful for any advice or if anyone can recommend books, websites or leaflets.

Many thanks,

Bob.

 

[masona]

Any photo ? or room size ? When you say a sun room do you mean a conservatory type ? Is the room part of a extension with no inner back door or patio door ?

 

[tintin99]

Thanks for the reply. No photos at the moment although I could take some tomorrow and post them if that would be useful.

It's actually a completely free-standing building, about 5m x 3m. The front (one of the 5m sides) has double doors in the middle with windows on each side of the doors that take up the whole of the rest of the front.

The other 3 sides are solid with no doors or windows. The back, where the gutter is, is right up against the garden fence, so the gutter can only be accessed from the roof. I'm thinking of changing the construction so the gutter's at the front for easier access.

The building actually came as a kit and the idea was that it could be assembled by someone with basic skills only. It was called a "sun room" back in the 60's when it was sold but it doesn't bear much relation to a modern conservatory.

Bob.

 

[Johndug]

You sun room sounds very similar to the workshop I built fifteen years ago at the end of my garden, except I used concrete blocks. On the roof, I used 4x2 timbers at 600mm centres, with 18mm chipboard on top. I then nailed down two layers of underfelt with a mineralised top layer fixed with cold bitumin. The edges were made up from 12mm marine ply for the facias with felt covered 2x1 battens round the edge of the roof. The slight slope falls to the rear with a gutting along the back. As you say, the gutter can only be reached from the roof, but is best for appearances sake. If 4x2 timber sounds too heavy for your roof, I would use the same size as is already there.

 

[tintin99]

Thanks for the info Johndug - that's very helpful. I have to admit I'm not totally clear how to handle the bit where the roof timbers join the walls, and it's rather difficult to explain without drawing diagrams!

Do the 4x2's sit on top of the concrete blocks and extend beyond the walls or did you fit timber along the top of the walls and then hang the roof timbers off that using joist hangers? If the former how did you seal the gaps between the top of the walls and the 18mm chipboard?

Hope all that makes sense!

 

[masona]

This is the nearest drawing I can find at the moment but not quite right for you. When you say you have concrete blocks, are they pre-slab concrete type, if so they will have hole in the centre of the slab for nut & bolt the 4"x2" wallplate onto the top of the wall. Then use ceiling joists flush at the back if you want to, On the front let the ceiling joists overhang say 300mm for the soffit vent underneath and fascia board on front. On top of the ceiling joists use firring strip (a thin wedge approx 50mm x 50mm sloped down about 5mm thickness at the front end for the rain fall) then exterior plywood on top of the firring. If you don't want to use roofing felt, you can use glazing bars on top of the firring strip and use smoke twin-wall polycarbonate sheeting (cooler and non-glare compare to clear sheeting) and remember the overhang at the front for the gutter. I would use 6"x2" for ceiling joists at 400mm centre for the 3m span and noggins in the middle to prevent joists twisting.

 

[Johndug]

Do the 4x2's sit on top of the concrete blocks

That's right. 4x2 laying flat and fixed on top of the 100mm block wall. 4x2's standing upright and skew-nailed to the wall plates with the 18mm chipboard on top. The 12mm facia was cut from 8x4 sheets and fixed to the joist ends and wall plates, overlapping the blocks by 25mm. So each facia strip is around 200mm deep. Since the roof was only on a workshop, I didn't use Masona's firrings, but instead raised the joists by 50mm at the front.

 

[tintin99]

Thanks - it's all starting to become a bit clearer. I still have a couple of questions though:

How long are firring strips? Presumably they need to be as long as the joists or there will be gaps between the joists and the plywood? Or am I missing something?

Is there any advantage to using firrings or is it just as good to raise the joists at one end by using a thicker wall plate?

On the subject of felt, I see that Wickes do an Easy Seal self-adhesive system which they reckon will last for 20 years...do you know if it's any good?

 

[masona]

How long are firring strips?

What ever length you need and timber merchant will cut some for you. My local timber merchant sell them at 3.6m long then just cut the end off.

Is there any advantage to using firrings or is it just as good to raise the joists at one end by using a thicker wall plate?

The beauty of firring is that the ceiling joists will lay flat on the wall plate and will give you a level ceiling if needed plus the fall of the roof.

On the subject of felt, I see that Wickes do an Easy Seal self-adhesive system which they reckon will last for 20 years...do you know if it's any good?

I don't like flat roof because there're prone to leaking problems and for 20 years I doubt it ! You would be better off having a low pitch roof for maintenance free, your choice really.

 

[tintin99]

Couldn't the Wickes system be used on a low pitch roof too? I guess its main advantage is that it's self-adhesive.

 

[masona]

Couldn't the Wickes system be used on a low pitch roof too?

Don't know, best to read the manufacter details to see if it's suitable for your angle of your flat roof. Not sure what the self-adhesive are made of, a hot pour bitumen would be better.

 

[tintin99]

Just wanted to thank you Masona and Johndug for your help - things are a lot clearer now. I'll post again if I get stuck!

Cheers,

Bob.