Advice Please: Parapet wall

8 Mar 2023
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United Kingdom
Hi guys,

I've had trouble with builders as of late, and you guys have proved extremely helpful, and I would like some advice please. We want to build a parapet wall to complete the roof of this extension. The extension is around 4.5m x 4m.

I have attached a drawing below, the blue line signifies our current position.

We have considered the following options:

1) Traditional Double skin: Brick and Block.
Unfortunately, we cannot do this because a wallplate has been applied across the blockwork, and the OSB boards sit on top. So this cannot be done

2) Double Skin: Brick and Timber.
This is the option that has been put forward in the drawings below, however, I'm concerned about the longevity, and then considering the cost and effort required if we have to re-do/repair this regularly.

3) Single Skin: Brick
Build up the outer skin brick wall by 4 courses (including cap stone). However, my main concern is that the wall will not tied into anything.

The single skin seems like the most straightforward option but is it feasible?

Thanks for your help guys.


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A mate of mine is about to 'do' the roof on an extension. I've been giving him a hand here and there.
The inside is blockwork and outer is brick, similar to yours.

Lintels are installed and he now wants to have a parapet wall, as he doesn't like the look of front of flat roofs.
I've not built a parapet wall before, and when he mentioned this my initial thought was that the rafters would not sit on a wall plate, but directly on the blockwork inner, pocketed in and carrying on the blockwork to the top of the parapet, this can then be tied in through the insulation, and into facing bricks.

Is the wall plate actually sat on the block work, or is it just the camera angle? also can't see the cavity insulation??

I wouldn't do No.2 in plywood, I'd use cement board.

I wouldn't like to have a single skin of brick (No.3) as a strong gust of wind could weaken it over time.

I'd be interested to hear how our resident experts would normally do this.

PS are you having a lantern? if so have you got steel or doubled rafters either side? Curious again, as my mate will be putting 2 in!
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In regards to the rafters certainly slot them into the blockwork, I think that's what we should have done.

Yes, the wallplate is onto the blockwork

I'm not too fond of the single skin either particularly given the length, but it would be the easiest...

We have a flat skylight (1m x 2m) - our architect designed triple rafters due to the span of the extension 4.5/5m across.


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No2 is fine build your stud wall first with osb and breather membrane to outside then use timber frame ties as you build you're brick work up it won't go any where so don't worry you're only concerne is the felt detail as them to courses will be floating
also capping detail we normally use alu cappings you can use stone but would need stainless dowels to connect to stop movement
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Thank you Gonch69,

How about this option, this will be more solid and allows for us to do the following:

4) Traditional Double skin: Brick and Block
Coated mild steel plate, 200m width across the length of parapet, bolted to the joist - and then brick on it. With DPM to cover steel.
You don't want the roof deck or covering going through a parapet. A timber wall is not at all ideal in this location.

As per below, but the exposed blockwork on the back should be brickwork, suitably frost rated.

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Thanks for the above woody, three questions though.

1) Are you suggesting that we put the blockwork directly onto the roof deck? Or should we line it with a plate of some sort, and then cover it with DPC.

2) By frost-rated, would engineering bricks suffice?

3) Also, above the DPC level, I assume we don't need insulation in the cavity of the parapet?

Appreciate the input
In these situations, the wall is be built between and over the joists, and the deck stopped at the wall abutment. Any joist parallel to the wall is kept back from the wall line.

Yes, engineers are frost rated, but whatever facings bricks you are using may be too and may look better.

Current thinking is that the insulation is continued up the parapet, the reason escapes me.

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