Front extension roof and general feedback required!

[garyo]

Hi Guys,

I'd welcome your feedback on my front extension design. I'm especially interested in what you thinik of the roof, and how it meets the wall plates, and the fact that the soffits are so low that they meet the top of the door. It's a scale drawing, so I'm going to have to make damn sure that the lintel I use will clear the roof. Am I being over the top using a steel cavity lintel rather than two concrete lintels?

Due to the small scale of the project (1.8M x 4M foot print) I'm considering getting an engineer to design the roof trusses, then make them myself. Does this seem silly?

porch cross section

• garyo
• 14 Feb 2010
• 3

Thanks

Gary

 

[noseall]

There is nothing wrong with the design, that a little bit of tweaking here and there won't solve.

For starters it is fairly common for the the door and widow heads to terminate directly under the soffit, i.e. but up against it. Just ask the window manufacturer to fit a head knock-on to the windows/doors should you wish to have any (side hung) openers - also helps with the trickles vents.

You will not need any external lintelling (front elevation) in this instance as it will be supporting nothing and will actually get in the way. All you will need is an internal box lintel. You will however require a standard cavity catnic for the side gable elevation window.

You are showing a course of bricks above the soffit on the front elevation, this is not needed. We make our own sturdy cradling above the door or widow heads prior to fitting the soffit. This will often consist of a piece of 4" x 2" laid flat across the head of the door or window secured with (pitch-cut) sprockets fixed to the rafter ends.

Make sure you carry on the cavity insulation so that it meets the roof insulation to avoid cold spots. Incidentally why are you not using the ceiling void as the insulation location, i.e. why celotex betwixt the rafters?

As for building the roof, a lean-to like yours is probably the easiest to build.
I would have thought 125mm x 50mm would eat it maybe even 100mm x 50mm.

 

[noseall]

By the way, what is the width of the front elevation window/door?

If it is particularly wide then you will need to look at how deep the lintel will be as this may be prohibitive to the wall plate location.

It could mean you using a piece of 152mm x 89mm steel u beam instead of a proprietary pressed steel lintel. We have had to do this in the past. It will depend upon the location of the wall plate, your chosen wall plate thickness, the depth of the birds-mouth cut and the steepness of the roof.

 

[garyo]

Thanks noseall. The door is 1.05m wide - the front elevation is shown below. As you can see there isn't a gable end, so I guess there's not strictly a need for a lintel on the external leaf there either, although do you think it'd be advisable on the side elevation window so that it ties the top of the front elevation wall into the rest of the house, rather than allow the roof to push the front wall away?

Untitled

• garyo
• 14 Feb 2010
• 3

I've already bought the doors and windows, and have 40mm of frame between the top of the door and top of the door frame, and 30mm of frame between the top of the window opener and top of the frame.

I like the idea of replacing the bricks over the soffit on the front elevation, with some kind of wall plate equivalent, and then bolting this down using anchor plates. I'll modifiy the plan to suit.

Re: insulation, my wife is keen to use the roof space to store bits and pieces, as the main house loft is only accessable by a fairly high ceiling. We're pretty realistic about the fact that it's going to be a tiny space, but even so I'm trying to maximise it, and my understanding was that if I wanted the insulation in the ceiling void then I'd need to have 20cm of it? Or can I use 10cm of kingpsan/cellotex in the ceiling void, rather than 20cm of rockwool?

Thanks

Gary

 

[Blagard]

Hi Guys,
I'd welcome your feedback on my front extension design.

You probably already know, but that porch is almost certainly going to need Planning Approval as well as Building Control due to it's size.

 

[tufty]

Hi garyo

Just interested to know what you used for you drawings, assume some CAD program

Nice drawings by the way.

Tufty

 

[garyo]

Hi Blagard,

Yep you're correct. Planning approval has been sought and granted. I'm about to submit for full-plans building control, which is what has lead me to where I am today. Having to think through the construction in detail is proving to be a very useful process though.

Just interested to know what you used for you drawings, assume some CAD program

I'm using Visio. It's all pretty straight forward as it's all to scale, and pretty capable when you're used to grouping shapes and sub-assemblies together. The biggest drawback is that every elevation needs to be produced individually, and there's no intelligent linking between elevations, so each change I make has to be made three times. I get it free from work so that's the main driver for using it.

Gary

 

[noseall]

By the looks of it then you have a hip end so it will be soffit all round, so there is only a need for internal leaf lintels. These will provide some degree of arch stability.

Regards the soffit:

You build the outer brickwork so that it finishes level with the intended window/door tops, usually 2.1m. You then build the inner masonry to the same level and fit the lintels (inner leaf only), then square the blockwork up to wall plate height.

Bed the plate and build the roof, leaving the rafter ends longer than necessary. You then level off the external brickwork and draw a line on the rafter bottoms allowing for the thickness of soffit board then trim the rafter ends to suit, cutting the plum cut for the fascia also.

The soffit board then rests upon the brickwork and is fixed to the rafter bottom. You then trap the soffit down onto the brickwork with pitch cut sprockets, fixed to the rafter ends within the soffit space.

Where there is no brickwork i.e. windows and doors you lay a piece of 4" x 2" flat on top of the soffit spanning the window opening by say 100mm each side and trap this down the same using pitch cut sprockets fixed to the rafter ends.

 

[noseall]

Top tip; You will need to feed in the 4" x 2" prior to installing the soffit board or else you won't get it in. This can be crudely fixed to the rafter underside, within the soffit space until you need to release it and fix it in place.

 

[garyo]

Thanks Nose, that's an excellent explanation. There are a couple of things I don't quite follow:

1) What's the difference between 'build the outer brickwork so that it finishes level with the intended window/door tops' and 'You then level off the external brickwork' Are they the same thing in this case, because my outer skin soffit height happens to be exactly the same as the window height?

2) 'pitch cut sprockets' - What are they!? I can't find any trace of those on Google! I guess it's some kind of vertical noggin with an angle cut on the end to match the roof pitch, placed vertically above the outer skin?

Cheers

Gary

 

[noseall]

1) What's the difference between 'build the outer brickwork so that it finishes level with the intended window/door tops' and 'You then level off the external brickwork' Are they the same thing in this case, because my outer skin soffit height happens to be exactly the same as the window height?

Correct. The bottom of the soffit, the top of the outer skin brickwork and the tops of the door/widow frames are all at 2100mm. You let the brickwork dictate the cut on the rafter end (less the thickness of the actual soffit board - say 10mm) and balance a level on this brickwork to give you the cut line.

2) 'pitch cut sprockets' - What are they!? I can't find any trace of those on Google! I guess it's some kind of vertical noggin with an angle cut on the end to match the roof pitch, placed vertically above the outer skin?

Absolutely spot on! I think we are going to get along.