New porch.

1 Mar 2012
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United Kingdom
Hi all
New here, an internet search lead me to this website and seems handy. As the topic says a new porch, 2mx1.5m by 3m high. So this aviods planning permission. Looking at a double skin build rendered outside smoothish with double pitched roof and vaulted ceiling.

Looking at going 1m down in depth, herts is a chalky area, so should be ok.

At this point not too sure on the blocks to use for the best strength, U value and cost. Will be making scissor trusses and slate roof so some weight will be pushing down and apart.

Any help or guidance will be much appreciated. Im a very keen DIYer and nearly finished our semi-major renovation on the house.
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If its a porch, why be concerned about the U-value of the block, unless you are putting heating in it? In terms of load-bearing capacity, the dense concrete 7 Newton blocks are best, but on this scale I doubt whether this would matter. Your main problem is to make sure the roof doesn't put any undue sideways push on the walls.
Yes its going to have a radiator in it, the sideways pressure is a concern! As the vaulted roof has a tendency to push. On the case with looking how to avoid it.
With a vaulted roof, there is always the tendency for the rafters to spread sideways. However, with slate your roof will not be too heavy, which helps. Instead of putting a conventional ridgeboard in and nailing the rafters through from each side, why not do away with a ridge, and join each pair of rafters at the top with a triangular-shaped plywood gusset, glued and screwed to the sides of the rafters. If you use 6 x 2 rafters, you could try a 9" deep gusset, which should give enough rigidity on this span. You could make these 'trusses' up on the ground (you would only need 4 or 5). Put them up 1 by 1 and stabilize them with short timber blocking pieces at the ridge, between each one.
With such a short span, a light, slate finish, and 6" deep rafters, you shouldn't then have a problem with spread.
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Thanks for the input, the terminology is relatively new to me regarding roofing but will sketch something up. The idea of making them up myself one-bu-one was my feeling, final design needs finalizing.
Thanks again
The rafter would need a notch cut so it sits on a wooden block along the wall I believe. Was thinking of using a joist hanger on the house wall to stabilize the start of the trusses.
3Bowls";p="2323072 said:
The rafter would need a notch cut so it sits on a wooden block along the wall I believe.
Yes, you need to birdsmouth it. Make sure you get the angles right. The plumb cut needs to be the same angle as the cut at the ridge. You can use a rafter square for this.
Ill add that to the list of tools! Think it will be a learning curve but hopefully successful!
Hi all
If the house wall the porch will extend from has cavity wall insulation consisting of polystyrene balls and resin is there still the need of cavity trays?

Haven't seen info saying yes or no.

Fixing a cavity tray into new-build brickwork is relatively straightforward; trying to put one in an existing wall against where a new roof is to go is much more difficult, unless you plan on taking out a large area of the outer skin. This is why you don't see references to it in the books etc.
If the wall above the porch is in good condition, you will probably be fine without a cavity tray. Just make sure the brickwork/rendering or whatever is in good condition and re-point/repair & fill any cracks. You will, of course, need a suitable flashing on to your tiles, but that can simply be pointed into the mortar joints.
Thanks, would it be a good idea to use a bricking sealing liquid, then paint the front of the house? Was going to paint house after.

Was planning to use a cavity closer on the use block-work joining the house too, but they should be easier to fit, hopefully!

Sorry about the double post, my fault on entering it.

The selection of the blockwork for the porch is proving tricky, or maybe over thinking it. Was going to use standard bricks up to dpm then outside skin dense blocks and inner skin aerated. But read that aerated blockwork could be used from foundation surface up, but been told they could crack in frost?

The aerated blocks are easier to lift and cut.

Any help/advice would be great.

Thanks all
Looking at using 440mm long x 215mm high x 100 wide 7n celcon blocks above and below dpm for both internal and external skins.

Good choice I hope.

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