Garage Corrugated Roof to Flat Mineral Felt Roof

[Sonic70]

Hi All,

Our separate garage which was probably built in the 70s has corrugated asbestos sheets on it. We are intending to get the roof replaced soon and have had a few roofers round to quote. The roof as it stands is not watertight and needs replacing. Some of the roofers have said they will reuse some of the existing roof joists and others have said they will fit all new ones. The joists have been there since the 70s, what are your thoughts on whether they should be reused. Also some have said about fitting fascias all round where others have said the felt will cover the joist ends where it is folder over and so no fascias are needed. Also the existing joists are on 900 centres approx and the max spacing I believe is 600. Joists are 6x2. I'm not sure on all that needs to be done to make the wooden structure for this new roof and so If anyone has any more information on this it would be helpful. The joists currently sit on the wall but does it need to be done differently for the new roof. Any help, advice appreciated, also any questions I should be asking the roofers on what they intend to do. Ive attached some images, hopefully it helps.

Victor

 

[foxhole]

If the joists are sound then will be fine, add additional to reduce spacing , then ply or osb before felt .
( I would prefer EPDM )

 

[Alastairreid]

As Foxhole said additional joists to reduce span, also you will need facings to attach the trims from the flat roof.

 

[Sonic70]

Thanks for the info both of you

 

[Sonic70]

I have narrowed it down to two quotes, one is using ply the other osb boards. What is the better option out of these two materials and why.

 

[Alastairreid]

Osb 3 for epdm.

 

[Sonic70]

Update, I am thinking of changing the slope of the roof from lengthways as it is now to slope across the width instead which will give me more headroom in garage. So the way I think it needs to be done is to remove the two rows of bricks under the copings at the front and at the side the bricks also will need to be Brought down to this level and to the right of the side door a row of bricks added. Joists c24 47x145 spaced max 600 apart but preferably less. Where the side door is I'm not sure what needs doing there as the door gap is more than 600 and there is no lintel. Joists to rest on wall with straps every 2m minimum. Noggins fitted halfway across span. Firrings to be fitted to give 1 in 60 slope. Any thoughts on what I have said so far would be appreciated and anything I've missed.

 

[^woody^]

You will need a fascia of some sort, this would normally project about 50mm from the the wall face, and then make sure the roofers fit a further 25mm batten at the top edge to form the drip from the felt perimeter. Any flat roofer that just bends the felt down onto fascia is not a roofer.

With flat roofs you should really go for 400 or 450mm spacings not 600. This prevents the joists sagging over time and causing ponding. Also increase the furring fall (or joists angle) more than the recommended minimum for the same reason.

You can reuse any existing joist if its not rotten or soft at the ends.

You can fit a timber trimmer across the top of the door.

 

[Sonic70]

Thanks woody I really appreciate your reply. I agree that a 450 spacing is better and that's what I want now. I'm thinking of doing the beams myself now so I get it done how I want. I will be fitting noggins halfway across joists. At the joist ends the ones that are not strapped to the wall every 2m are they secured in some way, bricks between joists, more noggins at ends or something else. Also above the garage door do I fit two beams screwed together there with the inner one strapped to the wall at either end. Hope that's not too many questions, if I'm not clear in my explanation it's because I'm not well at the moment.

 

[^woody^]

Its common for the joists to be built in with bricks to help secure the roof and stop the joists twisting.

If you don't want to do this, the fascia will stop any twisting, and increase the staps to 1.5m centres or fit long straps (1.2m) at the ends and every four joists, and an intermediate short strap (0.6m) between them.

 

[Sonic70]

Thanks Woody, will brick in between. Your advice has been really helpful.

 

[Sonic70]

Woody the trimmer joist you said about putting across the side door. I had not heard of that before but I've done some reading on it now. So correct me if I'm wrong, this trimmer joist lays across the top of door opening where a lintel would normally go and is then fixed to the next adjacent joist at each end. Now I'm not sure if they are just nailed together or joist hangers used here. Then finally a trimmed joist is fitted on the trimmer with a joist hanger and the other end resting on opposite wall like all the other joists. Hope that is explained well enough as just starting to feel a bit better today.
Victor