3 sided parapet or fascia overhang

[cs21]

Hello, long time lurker first time post.  Small scale builder and ill be building a small flat roof extension soon for a client soon. The extension will be 8m long, 5.8m wide and the clients have gave me the option of a fascia soffit overhang or a parapet for them for the roof. The roof will be a warm roof and will be covered with EPDM. They much prefer the look of the parapet or maybe a 3 sided parapet with one side opened to a gutter.

 

Myself and the tradesmen i work with all have some experience with overhang flat roofs but none yet with a parapet. Would anyone from this forum have a parapet detail for capping it off with either a coping stone, concrete coping or other. My first thought would be aluminium coping would be the better option. The cavity will be full fill insulation at 150mm wide and closing it off with an insulated cavity closer on top. I would like to know the detail above this and below the coping to help keep damp away from entering the inner skin of block. If the parapet isnt to high of the roof, what i had on mind was i can fold the EPDM into the corners of the and up the parapet. The aluminium coping can overhang this, however with my limited knowledge of the parapet, would there be any other detail i would need to do for better water/damp proofing. Maybe i could run the EPDM up and over the cavity wall then install the coping on this but this could turn out tricky unrolling the rubber roofing out. In theory everything is simple but in practice, thats a whole different ball game.   Originally the engineer has a single ply roof with an overhang and a secret guttering but the clients not to keen on this. 

[Dave Jones]

parapet done right look really good, depending on the size of it you may need two outlets for the water to comply with building regs. Good call going for a warm room, i'd pay the extra and buy tapered insulation so no buggering about with furring strips. It's not much more expensive and kingspan plan it out with crossfalls etc. 

 

Make sure you have at least 150mm from the rubber to the underside of the copers and lap the rubber right up tight to them. The copers need to overhang both sides of the parapet and have a drip groove underneath both sides to shed the water.

 

Spend the money and get a really good vapour barrier something like Alutrix.

[cs21]

2 hours ago, Dave Jones said:

parapet done right look really good, depending on the size of it you may need two outlets for the water to comply with building regs. Good call going for a warm room, i'd pay the extra and buy tapered insulation so no buggering about with furring strips. It's not much more expensive and kingspan plan it out with crossfalls etc. 

 

Make sure you have at least 150mm from the rubber to the underside of the copers and lap the rubber right up tight to them. The copers need to overhang both sides of the parapet and have a drip groove underneath both sides to shed the water.

 

Spend the money and get a really good vapour barrier something like Alutrix.

 

 

I had planned on using the cheaper version of vapor barrier. What benefits would the more expensive version have. I thought even thou there is cheaper brands, they should still do.

 

My method of a warm roof is 18mm OSB, vapor barrier, insulation at 150mm, 11mm OSB all screwed through with large screws with plastic caps then the EPDM on top. 

 

If i go with the parapet, i was going to use 2 outlets. What i had in mind was set the back wall, wall plate level, then the facing wall plate set this level with the back but have the furthest side of this wall plate slightly lower. This way when i put the firrings and the rain falls to the lowest point and stops at the parapet, it will also have a slight fall to one side so there will never be any sitting water pooling on the roof. 

 

Have never used the tapered insulation. Is it much more expensive! 

Edited April 29, 2021 by cs21

[tonyshouse]

Parapet guaranteed to leak heat out and water in, 

[cs21]

13 hours ago, tonyshouse said:

Parapet guaranteed to leak heat out and water in, 

 

 

How does parapets leak heat! If done correctly they shouldnt let water in. Surely if the EPDM is laid correctly there will be no breaks in it to let water in. 

 

I could go with an overhang but what bothers me is one will be able to see the slope with the firrings. if the width of the roof is 5.8m and the firrings are 4.8 with 100mm taper, and with the extra taper for the other 1m needed, this will be noticeable. That is unless i run a 4inch length of timber where the last firring strip is needed to create a sq look rather than the slope.  This will create a 4inch drop thou to one side of the fascia board.   

[Dave Jones]

100mm isnt enough fall over 4.8m you need min 1:40 to comply with building regs. 

[cs21]

6 hours ago, Dave Jones said:

100mm isnt enough fall over 4.8m you need min 1:40 to comply with building regs. 

 

 

Then why is it firrings are cut with this slope.They are actually  95mm on one end and tapered to 0mm in the 4.8m lengths.  It isnt far off the 1:40 and for a flat roof, i think thats efficient. Even 1:80 should let the water flow off the flat roof ok. 

[cs21]

On 29/04/2021 at 19:12, Dave Jones said:

 

 

Spend the money and get a really good vapour barrier something like Alutrix.

 

 

Would this be similar to alutrix?

 

 

 

SuperFOIL SFTV is the ultimate all-round building membrane for roofs, walls and floors. Water & vapour proof SFTV is also a foil radiant barrier that will reduce solar gain in summer and improve insulation in winter. SFTV's reflective properties add to the overall uValue of the building fabric. To achieve the maximum energy performance in new build and retrofit projects, SFTV makes an ideal 'airtight' layer suitable for the entire building envelope.

Technical
Layers : 3 - polypropylene reinforced heavy laminated foil.
Width : 1.5m.
Length : 50m.
Coverage : 75m2.
Colour : Silver.
Weight per roll : 10.7kg.

Thermal insulation values including 2x air gaps
Roof : 0.95 R.
Wall : 1.42 R.
Floor : 2.8 R.

Features

• Waterproof.
• Vapour proof.
• Air tight.
• Radiant barrier.
• Helps to prevent mold & damp.
• Enchances thermal resistance.

 

Product Documents

• Brochure
• Data Sheet
• Range Brochure

Specifications

Dimensions

Width1500mmLength50m

Details

PermeabilityVapour Barrier
Air TightRoll Size75m2