# Roofing question

Discussion in 'Builders' Talk' started by Gerrofski, Aug 24, 2008.

1. ### GerrofskiNew Member

Hi

We have an extension built around 20 yrs ago, before we moved in. Unfortunately, the roof tiles used are large concrete ones which dont match the rest of the roof which has the 10 x 6 Rosemary type tiles.

I now want to replace the large tiles with matching 10 x 6's. I've checked the felt ( Tyvek I think ) from underneath and it appears to be in perfect condition. My quistion is therefore, do I need to replace the felt or can I simply re-batten and then re-tile, or do I need to re-felt?

Cheers

2. ### sam the builderNew Member

The problem you might have is that when you take off the old battens, holes will be left in the felt from where the clouts went in , leaving somewhere for rain to get in. its probably very unlikely that the new battens will sit in the place of the old ones.

One thing that you might need to check is that you have the right roof pitch for the tiles you are going to use. That might have been the reason that the roofer used the tiles he did at the time.

3. ### GerrofskiNew Member

Thanks for the quick response Sam,

You've confirmed my fears regarding the nail holes. I did wonder whether that would be a problem.

Also an excellent point you make about the pitch of the roof. It does look very slightly shallower than the main roof, so that will need to be investigated further.

Does anyone have any suggestions as to how to determine the angle of an existing roof?

Cheers

4. ### screwfoxNew Member

To calculate the angle of the roof (using a calculator!):

1) Measure the width of the roof (at the gable, say at gutter height) and divide by 2. call this A

2) Measure the height of the roof from a horizontal line accross the bottom of the roof (say at the gutter level) call this B

then do this calculation:

A squared + B squared = C

The square root of C call D (this is the length of the hypotenuse)

Divide B by D giving E

On a calculator do INV Sin E, this will give you the roof angle.

(as an example:

Say the width of the gable at the gutters was 6m then A would be 3m
Say the height of the roof above the gutters was 3m, then B would be 3m

3 squared is 9 + 3 squared is 9 = 18 (C)

The square root of 18 is 4.2426406871192851464050661726291

3 divided by 4.2426406871192851464050661726291 is
0.70710678118654752440084436210485

The INV Sin of 0.70710678118654752440084436210485 is 45, so the angle of the roof is 45 degrees)

Hope this helps and makes sense!

5. ### kaintheoNew Member

To be fair it is just easier to get a spirit level with an angle finder on it.

6. ### chip off the blockNew Member

or a quick square and a level or a roofing square

7. ### doing a bitNew Member

the shallower the angle the more overlap on the tiles to stop rain being blown under,local bco will probably help with that or roofing supply co

8. ### Mr. HandyandyScrewfix Select

With the felt and nail-hole problems, you may be able to leave most battens there, and just add more battens where they need to be set. You are sure to have to move some, but if carefully removed, the nail-holes(being on a rafter) can have a felt nail whacked into them..even a bit of tar if necessary.

Mr. Handyandy - really

9. ### chip off the blockNew Member

if going to the expense of new tile and batterns i would change the felt so the whole thing ages together

10. ### doing a bitNew Member

chip is right for the cost of the felt i would redo it all, main cost is tiles etc so ..

11. ### GerrofskiNew Member

Many thanks for all the replies, esp Screwfox for that detailed explanation. Will get out and measure up and then sit down with the old calculator.

As suggested, I could maybe get away with leaving the battens or plugging any holes or just bite the bullet and strip the lot. As you say the bulk of the cost is in the tiles.

Ok, once again, many thanks for the advice.

12. ### chippie244Super Member

Screwfox's answer was a little overcomplicated, divide the height by half the width then inv tan the answer.

13. ### busy builderNew Member

Don't use felt, use TYVEK (like existing) as its a breathable membrane unlike felt.

14. ### chip off the blockNew Member

i think i speak for all that have said felt to have meant tyveck or anyother beathable membrane. If you do this reroof you will have to insulate to current regs

15. ### gangmanNew Member

Screwfox's answer was a little overcomplicated, divide the height by half the width then inv tan the answer.
<u>_________________________________</u>
Chippie

Were you taught,
Some People Have, Curly Brown Hair, That Pulls Back

16. ### trenchNew Member

Rosemary tiles need have a min pitch of 35 degrees. DIY shops sell pitch meters/measurers?? (mine cost £1)which can be used either below or above the surface you are measuring.

If the roof to be re-covered is less than that, you'll need some sort of 'below pitch' tile spec. 2 layers of tyvek with staggered laps, or a waterproof sarking board like ondutile.

Alternatively, Marley do a big tile with a groove in it so that it looks like 2 rosmarys. Its called a Mockbond. If you google it you'll find its min pitch which from memory is well below 35 degrees