Lintel selection

[MadAlicesDad]

I need to replace the windows and door in an extension. It was constructed about 40 years ago and uses a single piece of 4x2 timber across a span of 3.5m. Above this are joists at approx 400mm spacing supporting a tiled roof at less than 15deg pitch.

Untitled

• MadAlicesDad
• 28 May 2010
• 1

I want to install concrete lintels and raise the wall height by three brick courses to get a proper door frame in. I need to build a pier to the left hand side of the door to fix the frame to and another pier tied to the wall of the house to reduce the span of the lintel over the window to 2100.
I've done roof load calcs and estimate the linear load (dead + snow) is 1.5kN/m. The wall plate will rest directly on the lintels (ie no brick course above).

Using the lintel selector http://www.naylorlintels.co.uk/lintelsselector.asp I need a 70x100 (or 70x140) lintel. This feels right as despite the shorter span it is a bigger cross section than the piece of wood that has been in place without problems for all these years, but have I missed something or made any wrong assumptions?
TIA
Mike

 

[Static]

Your loading seems a little light.. dont know the overall width of the roof but guessing its around 2.5m to the party wall?

Anyways, would get about 2.2kN/m or so from that.. dead+imposed..
Bear in mind you will probably be getting the Naylor economy range so go for a 100x140 as the 100x70 only has a 0.84kN/m allowable

Might want to consider a catnic style to cover both wall leafs..

If your increasing the wall height wont the roof need redoing?

 

[MadAlicesDad]

Thanks for the quick reply.
It's about 2.1m front to back. The back wall is being raised also but that's the easy bit as it's all block with no openings, and the roof will be replaced (see other conversation in 'roofing' forum).
I calculated the total roof load (for the new roof inc thicker rafters and sarking boards, insulation etc) and assumed it would be split 50:50 with rear wall. I forgot to apply an additional safety factor so your value of 2.2 (50% factor) is preferable.
The Naylor Hi-spec range 100x70 has 1.96kN/m allowable which is too low with the new loading, but as the piers will be thick enough, then a 190x70 is possible which has 3.41kN/m.
I thought about a catnic, but apart from the piers upon which it would rest there won't be wall leafs to cover as the wall plate will be directly above the lintel. (or have I misunderstood you?)
Mike

 

[Static]

My loading was unfactored, as the naylor listings are for unfactored allowables .. you seem to be sellecting shallow lintels im assuming you have height issues?
I can only assume you have solid walls or something so need a wider lintel?

You might wanna check prices on the Hi-spec lintels compared with the econ ones..

Would normally design a roof for 2kN/m2 assuming its clay tiles, pitched and has a loft void that has an access panel.. can reduce that by 0.25kN/m2 if the loft area is unaccessable and a further 0.15 if its a vaulted ceiling..
Should increase if using concrete tiles..
So on that a line load of 2.1kN/m is about reasonable..

Eaves lintel; http://www.catnic.com/lintels/CGE50-100.aspx

Ill hunt down your roof forum post may shed some light

 

[MadAlicesDad]

Height is the issue. I want to get high enough at the front for a door frame, but higher at the back to increase the pitch. I'm already a block higher than next door's extension and don't want to go much higher. There's no loft space, and the walls are solid (single brick).
My loading calcs were for 13m2 of:
Redland 50 tiles @46kg/m2 =598kg
12mm OSB sarking @7.4kg/m2 =96.2kg
12mm plasterboard @13kg/m2 =169kg
75mm Kingspan @2.4kg/m2 =31.3kg
plus 14 off 2.5m rafters (125x50mm) @ 7kg each
This gives dead load of 993kg or 9.74kN
Plus snow load of 5.2kN (13m2 @ 0.4kN/m2) gives 14.94kN.
This is spread evenly over two 5.2m long walls giving load of 1.44kN/m.
Does this seem right?

Mike

 

[Static]

Snow load should be around 0.75-1.0kN/m2 depending on your altitude (1.0 for over 100m).. apart from that pretty much there.. just make sure you get clay tiles as concrete ones are more like 80kg/m2

 

[MadAlicesDad]

Sorry for the slow reply, I've been away.
I'm at about 80m ASL just north of Bristol and I got the snow loading from a contour map (can't remember the website) that shows 0.4kN/m.
The weight of Redland 50 Double Roman came from the Redland website for min gauge (ie worst case). http://www.monier.co.uk/products/product-catalogue/product-range/product/50-double-roman.html
Is there an additional factor to apply to get to your values?

Mike

 

[Static]

Chances are you got the basic value from Building Regs Part A or BS6399 Part 3 which you then apply coefficients to get the loading.. but simple method is to take from Trada guidelines which simplifies the above to anything under 100m alt is 0.75kN/m2, (exept in cornwall/devon) anything over 100m is 1.0kN/m2 with the exception of higher areas around the sheffield/hull/newcastle where the BS should be used..

 

[MadAlicesDad]

Notwithstanding the snow load calculation method, the problem is that I am limited to a 65/70mm concrete lintel because I need height for the wall plate, but these concrete lintels are borderline for large spans. Having done some more thinking, is it acceptable to use timber for a lintel? That way, the lintel also forms the wall plate and effectively doubles the height of the lintel.
Two 47x125mm C16 joists side by side will support 2.2kN/m over a 2.1m span with deflection of 6mm which meets the 0.3%L criterion for lintels.
Is there any reason why I can't use timber?
TIA
Mike

 

[Static]

Sounds good..

Only that the lintel becomes a maintenance item and needs good weather/insect protection.. but nothing that that shouldnt be considered for a wall plate or rafter..

Just make sure the timbers are bolted together at staggered centres..

 

[noseall]

Just make sure the timbers are bolted together at staggered centres..

......including dog tooth washers betwixt the timbers.

 

[MadAlicesDad]

Damn, you've just reminded me what else I meant to order from Toolstation...

Anyway, thanks for the help and advice. Looks like I've something to do this weekend.

Mike