Flat roof between two hip bay roofs

[tufty]

Hi all

Firstly thanks for any replies.

I am part way through extending my porch , all done via local B.C.O as part of a larger extension to our property.

I am moving my porch forward by some two metres between two hip bay roofs on a bungalow, by constructing a flat roof between the bays.

Untitled

• tufty
• 5 Apr 2011
• 1

I have joisted across between two walls on the bays, and have firrings with the deck boarding on.

Now the next stage, weather allowing, is to remove the bay roof tiles around the area, as i believe i have to provide some additional boarding up the hip bay rafters to accomodate the felting across the flat roof.

Can someone enlighten me as to how far up the rafters i need to go from the flat roof decking, and what is a sensible thickness marine ply to use for this purpose ( or am i barking up the wrong tree even).

The ply on the decking is 20mm, but this would be too thick to go up the rafters, so i was thinking about 9mm?

Any guidance appreciated, many thanks

Tufty

 

[SurreyRoofingEstimator]

9mm sounds a bit floppy to me, 12mm might be better.

As far as taking up under the tiles is concerned, you need to take it the equivalent of 150mm vertically, as an example if the pitch of the tiled slopes is 35 degrees you need to take it a minimum of 260mm up the slope

 

[tufty]

Many thanks for the reply SurreyRoofing Estimator.

I'll go with 12mm then !

And thanks again for the advice on the height, just need a slot with some good weather.....looking reasonable from Thursday.

Also if i may, is there any other advice where the boards laid on the rafters meets the flat roof deck board, ie any sealing required or lead work ?

The plan was to just run the felt across and up, but thats being done by someones else.

Again thanks for the prompt reply....much appreciated.

 

[noseall]

First strip off enough tiles so that you can work without interference form overhanging tiles.

You will need to cut the ply layboard (use 18mm) so that you end up with the board tucking up to the last available roofing batten. This will mean shaping the ply cut so that it gets wider as the firings fall towards the low end of the slope.

This will mean showing a bit more flat roof at the low end than the high end.

Obviously across the slope at the back it will be a straight piece.

You will also need to shape the the layboard as it meets the valley.

You will also need to fix a piece of cocking fillet along the layboard so that the eaves tile has something to sit on. It is important that this cocking fillet is covered with the flat roofing material to stop it rotting.

You will need to extend the roofing membrane so that it lips over the cocking fillet as you would a fascia eaves detail.

Also, if your roof previously relied upon fascia vents at the eaves detail you will need to insert roof tile vents as the flat roof layboard detailing has removed this option.

 

[tufty]

Thanks Noseall, all clear and understood.

Especially the bit about the cocking fillet, makes sense now you have mentioned it.

Whilst on the subject of lifting the last tile row, i did think about trying to get some sort of flat bottomed guttering to sit around the edges of the flat roof under the last row of tiles, to at lest try and take most of the water away and help the flat roof so to speak.

Yes, i appreciate its probably not needed, as the flat roof should do the work if properly sealed but is there a way of doing this ?

I have height on the flat roof to be able to pick the existing gutter arrangement at the front edge which sits lower down.

There were no fascia vents at either bay when i removed the fascia and soffits, although when i get to replace the existing around the rest of the property i plan to fit a few.

The loft area has very good ventilation, with all areas in the roof void open to each other, and i should have good cross flow between the bays for the flat roof as well.

Again Noseall many thanks.

 

[Tuck1]

Agree with Noseall & SRE for all points.

For the fillet, use off-cuts from your 18mm ply, good lengths obviously not a jigsaw puzzle.

You could have several options if you really had to have a 'gutter' under the tiles: Form lead or even run a batten as a gutter about 3'' from the tiles around the roof edge and felt over. But youre creating a problem area for me. Just leave without id advise.

Main thing bothering me is your main weather protecting material- felt. I wouldnt touch it with a barge pole. Have you considered GRP (fibreglass)? All i ever take off flat roofs is felt, some only been on 8-10 years! Never been called to replace a fibreglass one. Plus you get pre-made fibreglass fillets which go on the deck & under the tile. No seams/joints, no grit, look clean. Sealed!

 

[tufty]

Thanks Tuck 1 for your reply.

All points noted and yes i agree with your views on felt, i have recently helped a friend fit a rubberised roof covering to his garage and was suitably impressed with its ease of use, finish, and looks.

I have also considered fibreglass as an option as per your suggestion but having just near completed a 4.3metre square hip roof conversion at the rear, plus doing this porch move.....moneys a bit tight.

If the flat roof last ten years i'll be happy, and then will look at fibreglass when i have some pennies in the pot.

Thing is, felt is the traditional way, and people often overlook other methods because of it......having seen the rubber, and used it, its quite good i think, used widely across the pond ( America), although how long it lasts only time will tell.

Thanks again for your advise Tuck1, i will leave as is re the gutter and see how it performs without....but may concider the formed lead idea if needs be.

 

[tufty]

Sorry guys, i know i keep asking but after fitting the ply up the rafters do i need to re batten across for the tiles to hang too, only its in my mind that should any water get behind the tiles onto the sarking any battens across the ply will stop any water running down the roof pitch.

Thanks for being patient with me !

 

[noseall]

You don't fit any tiles or battens to the ply lay board. The ply lay board finishes at the last available tile batten. You then position the cocking fillet so that it acts as a kicker for the last course of tiles.

The sarking membrane then drapes over the cocking fillet as will the bottom course of tiles so that any water finding itself on the membrane is discharged onto the flat roof.

The flat roof material is fitted up and over the cocking fillet so that the tiles, sarking membrane and flat roof material all have adequate lap.

think about it.......you can only fix the lay board up to the bottom roofing batten (tile lathe) wherever you decide it will be.

 

[tufty]

Many thanks for that nosell, it makes sense now.

Well, what can i say.......what a wonderful forum this is.

My sincere thanks to Noseall, Tuck1 and Surrey roofing estimator.

Ply is ordered should be delivered tomorrow, have stripped back the tiles today, should be a nice warm day tomorrow.....all i have to do is carry out the excellent advise.

Will re post a piccy when done later, then you can rip me to shreds

Brilliant stuff guys, best regards.

 

[ANGLER]

Who supplies cocking fillets and what size?