Flat roof repair

[i_am_fubar]

Hi Folks.

So me and the missus are buying our first house, it has a single story extension on the back of the house with a sliding patio door that has been there at least 10 years and the flat roof is accessable from the master bedroom as a roof terrice.

These are all great things, but it has a leak which needs fixing and the survey report raises questions as to whether the roof is actually load bearing. Other signs of dodgy DIY in the house further back this up.

The plan as it stands is as follows:

-Remove existing roof.
-Fit new lintle (if needed) over the sliding door.
-Ensure joists are of suitable strength and replace if too small.
-Clad with 18mm WBP ply.
- Fit EDPM roof covering.
- Replace hand rail and privacy screens.
- Fit some form of loose lay paving.

My questions are:
1) what should the joist size and spacing be for a 4m span?
2) Would it be worth having 2 layers of ply?
3) Due to the property being a reposession, we have no knowledge of planning permission or builing reg approval. What possible implications could there be if we re-roof it in accordance to building regs?

Cheers.
Fubar.

Also, I've been nagged about putting a hot tub up there, but that's a long shot

 

[i_am_fubar]

Further to the above.

There is not much gap between the bottom of the access door and current surface of the roof. So over the top insulation is out of the question. If I use a vapour barier under the joists, can I insulate between them without worrying about damp, or would an air gap still be needed?

Also, could I lay these pads directly onto EDPM:

http://www.wallbarn.com/rubber-paving-support-pads/?

Cheers,
Fubar.

 

[i_am_fubar]

And another one.

The existing hand rails are bolted down onto what I can only be describe as blocks beneath the existing felt. How would I go about doing similar with EPDM?

Cheers.

 

[cotswoldbuilders]

Firestone do proper walkway pads.

When you bolt the railings down fit some form flash around and over the foot.

 

[pinenot]

All things considered, I think you first have to ascertain the question of whether the roof is load bearing. Although if that is the way the surveyor has described it in their report, I wonder at their competence!

22mm WBP would be better that 18mm, in my opinion, but all depends on what it's laid on (re above statement)

Re insulation, it will all depend on the depth you have from the bedroom/patio doors to play with??, as you could conceivably combine the insulation with the walk on surface, if there was enough room...pinenot

 

[i_am_fubar]

They look good Cotswold, cheers.

Pinenot, agree with 22mm over 18mm. Though chatting with a builder friend, who happens to be the other halfs dad (handy). The idea of using pre-stressed concrete beams was raised. Allowing for a thinner roof for equivalent load bearing, giving more room for warm deck insulation. Currently, there is only about 150mm to bottom of door, just enough for flashing.

Also, survey was only home buyer, not structural. Though the local search has come back to say that the extension does have planning permission.

Anyone have any idea on what they would be priced at? (or a supplier who publishes price on website).

Cheers all.

 

[Xenon]

Code of practise requires 150 up stands from finished roof surface. Snow loadings need to be in calcs too.

 

[pinenot]

Can I make an alternative suggestion to your query -

Instead of thinking joists/beams and decking, are you able to get hollow concrete panels craned in?...pinenot

 

[i_am_fubar]

Interesting idea Pinenot. I can see appeal of having fewer components to work with. Though it would require far more planning and logistics (making sure walls are ready for roof on delivery day. No ability to move them again once placed...etc).

I was looking at the beam and block systems that use insulation blocks for infill between the beams. I imagine it would have a better U value that hollow concrete? Though that's down to how thick the panels are, is there a usual thickness?

Also, in any method like this, would the beams/concrete panels run in line with the pitch or across the pitch, and in either case, how would one build the pitch in? I believe these technologies are usually used for interior floors rather than roofs and are thus laid flat.

Fubar.

 

[pinenot]

Point taken on the wall heads, 4" would suffice for your intended load, span of units would be side to side i.e. 2 walls support, allow overhang each side and ditto to front.
The units go in level the screed is laid to a fall - EPDM - reinforced insulation/deck.

What are the dims??...pinenot

 

[i_am_fubar]

I'll need to confirm the dimms once we've taken posession. At an estimate, it's 4mx4m. There is some complexity due a chimney breast that I will detail when I can. Is there a minimum screed thickness, could I taper from 0mm to whatever is needed for the correct fall?

Also, how could the change in load effect the demands on the foundations? I imagine that while beam and block is heavier, there is a large saftey factor on foundation loading.

Fubar.

 

[pinenot]

These hollow precast's won't really impact on the founds, assuming they are OK at present.

Yes you can run the screed at the fall required.

Here's another, poss simpler way forward replace concrete slabs with reinforced woodwool slabs, get a quote for cutting PIR insulation at the fall required - apply EPDM - slab corner blocks - slabs, the fall can be minimum and you could live with an ever so slight slope on the slabs?...pinenot

 

[i_am_fubar]

Not totally following Pinenot.

Are you saying:

----------------------------patio slabs------------------------
------------------------------EPDM-----------------------------
-----------PIR Insulation (cut to provide fall)-------------
---Beam---Woodwool---Beam---Woodwool---Beam---
--------------------------Plasterboard------------------------

If that is the case:
- EPDM directly onto PIR without screed?
- If no screed, is there not the risk of having low points (PIR joints)?
- Could a layer of ply be fitted between EPDM and PIR as opposed to screen?
- Can you hot wire cut PIR?
- How to fit PIR to B&B? grab adheasive, foam or do you need to use ties?
- Why woodwool rather than breeze blocks?
- Would a vapour barrier be needed between plasterboard and beams?
- 1:80 or 1:60 fall considering the (hopefully) insignificant sagging?

Cheers,
Fubar.

 

[pinenot]

I took this from the 'Ruberoid Blue Book' -

WOODWOOL DECKS
Woodwool is an open textured board composed of
wood fibre bound together with Portland cement.
Slabs are produced in 600mm widths, in a variety
of thicknesses and surface finishes. Plain edge units
are available for use over short spans and channel
reinforced slabs for spans up to 4 metres.
When channel reinforced slabs are used, there is a
possibility of cold bridging and condensation along the
channel edge. Slabs are available with a thermal
break formed by setting the metal channel
reinforcement within the thickness of the slab or forming
a rebated edge to accept an infill strip of insulation.
Woodwool slabs for roof decking should be type SB
to BS 1105:1981. Type SB has a minimum nominal
thickness of 50mm and sufficient strength to be used
without attracting the requirements of Regulation 36(1)
of the Construction (Working Places) Regulations,
1966. These regulations apply to working surfaces
which are not considered to provide a safe working
platform and restrict the use of woodwool to type SB
slabs.
Type SB slabs are supplied with a plain finish, prescreeded finish or a pre-felted finish. Where a plain
finish slab is used a sand and cement screed or slurry
should be applied to give a suitable surface for
application of the waterproofing.
Taping the joints of the woodwool will prevent
subsequent applications of hot bitumen penetrating the
joints, and in the case of pre-felted slabs will complete
a temporary waterproof covering. On no account,
however, should the pre-felted finish be considered as
part of the final waterproofing specification...pinenot